Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Engineering
Dependable
Protection
For An
Electrical
Distribution
System
Bulletin EDP-1
(2004-1)
Part 1
A Simple Approach
To
Short Circuit
Calculations
Bussmann
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
Basic Considerations of Short-Circuit Calculations
Why Short-Circuit Calculations
Several sections of the National Electrical Code relate
to proper overcurrent protection. Safe and reliable
application of overcurrent protective devices based on
these sections mandate that a short circuit study and a
selective coordination study be conducted.
Sources of short circuit current that are normally taken
under consideration include:
- Utility Generation
- Local Generation
- Synchronous Motors and
- Induction Motors
These sections include, among others:
110-9 Interrupting Rating
110-10 Component Protection
230-65 Service Entrance Equipment
240-1 Conductor Protection
250-95 Equipment Grounding Conductor Protection
517-17 Health Care Facilities - Selective Coordination
Capacitor discharge currents can normally be
neglected due to their short time duration. Certain IEEE
(Institute of Electrical and Electronic Engineers) publications
detail how to calculate these currents if they are substantial.
Asymmetrical Components
Short circuit current normally takes on an asymmetrical
characteristic during the first few cycles of duration. That is,
it is offset about the zero axis, as indicated in Figure 1.
Compliance with these code sections can best be
accomplished by conducting a short circuit study and a
selective coordination study.
The protection for an electrical system should not only
be safe under all service conditions but, to insure continuity
of service, it should be selectively coordinated as well. A
coordinated system is one where only the faulted circuit is
isolated without disturbing any other part of the system.
Overcurrent protection devices should also provide shortcircuit as well as overload protection for system
components, such as bus, wire, motor controllers, etc.
To obtain reliable, coordinated operation and assure
that system components are protected from damage, it is
necessary to first calculate the available fault current at
various critical points in the electrical system.
Once the short-circuit levels are determined, the
engineer can specify proper interrupting rating requirements, selectively coordinate the system and provide
component protection.
C
U
R
R
E
N
T
TIME
Figure 1
In Figure 2, note that the total short circuit current Ia is
the summation of two components - the symmetrical RMS
current IS, and the DC component, IDC. The DC component
is a function of the stored energy within the system at the
initiation of the short circuit. It decays to zero after a few
cycles due to I2R losses in the system, at which point the
short circuit current is symmetrical about the zero axis. The
RMS value of the symmetrical component may be determined using Ohm`s Law. To determine the asymmetrical
component, it is necessary to know the X/R ratio of the
system. To obtain the X/R ratio, the total resistance and total
reactance of the circuit to the point of fault must be
determined. Maximum thermal and mechanical stress on
the equipment occurs during these first few cycles. It is
important to concentrate on what happens during the first
half cycle after the initiation of the fault.
General Comments on Short-Circuit Calculations
Short Circuit Calculations should be done at all critical
points in the system.
These would include:
- Service Entrance
- Panel Boards
- Motor Control Centers
- Motor Starters
- Transfer Switches
- Load Centers
Normally, short circuit studies involve calculating a
bolted 3-phase fault condition. This can be characterized
as all three phases “bolted” together to create a zero
impedance connection. This establishes a “worst case”
condition, that results in maximum thermal and mechanical
stress in the system. From this calculation, other types of
fault conditions can be obtained.
3
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
Basic Considerations of Short-Circuit Calculations
Interrupting Rating, Interrupting Capacity and Short-Circuit Currents
Interrupting Rating can be defined as “the maximum
short-circuit current that a protective device can safely
clear, under specified test conditions.”
Interrupting Capacity can be defined as “the actual
short circuit current that a protective device has been
tested to interrupt.”
The National Electrical Code requires adequate
interrupting ratings in Sections 110-9 and 230-65.
To accomplish this, study Figure 2, and refer to Table 8.
IP =
115,450A
Ia
Ia =
66,500A
IDC
Is =
50,000A
C
U
R
R
E
N
T
TIME
Is
Section 110-9 Interrupting Rating. Equipment intended to
break current at fault levels shall have an interrupting rating
sufficient for the system voltage and the current which is
available at the line terminals of the equipment.
Ia - Asymmetrical RMS Current
IDC - DC Component
Section 230-65. Available Short-Circuit Current. Service
Equipment shall be suitable for the short circuit current
available at its supply terminals.
Is - Symmetrical RMS Component
IP - Instantaneous Peak Current
Figure 2
Low voltage fuses have their interrupting rating
expressed in terms of the symmetrical component of shortcircuit current, I S . They are given an RMS symmetrical
interrupting rating at a specific power factor. This means
that the fuse can interrupt any asymmetrical current
associated with this rating. Thus only the symmetrical
component of short-circuit current need be considered to
determine the necessary interrupting rating of a low voltage
fuse. For U.L. listed low voltage fuses, interrupting rating
equals its interrupting capacity.
Low voltage molded case circuit breakers also have
their interrupting rating expressed in terms of RMS
symmetrical amperes at a specific power factor. However,
it is necessary to determine a molded case circuit breaker’s
interrupting capacity in order to safely apply it. The reader
is directed to Buss bulletin PMCB II for an understanding of
this concept.
Figure 2 illustrates a worst case waveform that 1 phase
of the 3 phase system will assume during the first few
cycles after the fault initiation.
For this example, assume an RMS symmetrical short
circuit value of 50,000 amperes, at a 15% short circuit
power factor. Locate the 15% P.F. in Table 8. Said another
way, the X/R short circuit ratio of this circuit is 6.5912.
The key portions are:
- Symmetrical RMS Short Circuit Current = Is
- Instantaneous Peak Current = Ip
- Asymmetrical RMS Short Circuit Current
(worst case single phase) = Ia
From Table 8, note the following relationships.
Is = Symmetrical RMS Current
Ip = Is x Mp (Column 3)
Ia = Is x M m (Column 4)
For this example, Figure 2,
Is = 50,000 Amperes RMS Symmetrical
Ip = 50,000 x 2.309 ( Column 3)
= 115,450 Amperes
Ia = 50,000 x 1.330 (Column 4)
= 66,500 Amperes RMS Asymmetrical
With this basic understanding, proceed in the systems
analysis.
4
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3 ø Short-Circuit Current Calculations –
Procedures and Methods
3Ø Short-Circuit Current Calculations,
Procedures and Methods
To determine the fault current at any point in the
system, first draw a one-line diagram showing all of the
sources of short-circuit current feeding into the fault, as well
as the impedances of the circuit components.
To begin the study, the system components, including
those of the utility system, are represented as impedances
in the diagram.
The impedance tables given in the Data Section
include three phase and single phase transformers, current
transformers, safety switches, circuit breakers, cable, and
busway. These tables can be used if information from the
manufacturers is not readily available.
It must be understood that short circuit calculations are
performed without current limiting devices in the system.
Calculations are done as though these devices are
replaced with copper bars, to determine the maximum
“available” short circuit current. This is necessary to
project how the system and the current limiting devices will
perform.
Also, current limiting devices do not operate in series
to produce a “compounding” current limiting effect. The
downstream, or load side, fuse will operate alone under a
short circuit condition if properly coordinated.
To begin the analysis, consider the following system,
supplied by a 1500 KVA, three phase transformer having a
full load current of 1804 amperes at 480 volts. (See System
A, below) Also, System B, for a double transformation, will
be studied.
To start, obtain the available short-circuit KVA, MVA, or
SCA from the local utility company.
The utility estimates that System A can deliver a shortcircuit of 100,000 MVA at the primary of the transformer.
System B can deliver a short-circuit of 500,000 KVA at the
primary of the first transformer. Since the X/R ratio of the
utility system is usually quite high, only the reactance need
be considered.
With this available short-circuit information, begin to
make the necessary calculations to determine the fault
current at any point in the electrical system.
Four basic methods will be presented in this text to
instruct the reader on short circuit calculations.
These include :
- the ohmic method
- the per unit method
- the TRON ® Computer Software method
- the point to point method
System A
3Ø Single Transformer System
System B
3Ø Double Transformer System
Available Utility
S.C. MVA 100,000
Available Utility
S.C. KVA 500,000
25’ - 500kcmil
6 Per Phase
Service Entrance Conductors
in Steel Conduit
1500 KVA Transformer
480Y/277V,
3.5%Z, 3.45%X, .56%R
If.l. = 1804A
30’ - 500 kcmil
4 Per Phase
1000 KVA Transformer,
480/277 Volts 3Ø
3.45%X, .60%R
If.l. = 1203A
2000A Switch
Copper in PVC Conduit
1600A Switch
KRP-C-2000SP Fuse
Main Swb’d.
KRP-C-1500SP Fuse
Fault X1
Fault X1
1
400A Switch
400A Switch
LPS-RK-400SP Fuse
20’ - 2/0
2 Per Phase
Copper in PVC Conduit
50’ - 500 kcmil
Feeder Cable in Steel Conduit
Fault X2
MCC No. 1
2
M
1
LPS-RK-350SP Fuse
225 KVA
208/120 Volts 3Ø
.998%X, .666%R
Motor
Fault X2
2
Note: The above 1500KVA transformer serves 100% motor load.
In this example, assume 0% motor load.
5
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Ohmic Method
3Ø Short Circuit Calculations,
Ohmic Method
Step 9. The symmetrical motor contribution can be
approximated by using an average multiplying factor
associated with the motors in the system. This factor varies
according to motor design and in this text may be chosen
as 4 times motor full load current for approximate
calculation purposes. To solve for the symmetrical motor
contribution:
Most circuit component impedances are given in ohms
except utility and transformer impedances which are found
by the following formulae* (Note that the transformer and
utility ohms are referred to the secondary KV by squaring
the secondary voltage.)
Step 1.
Step 2.
†X utility Ω
X trans Ω =
=
1000 (KVsecondary)2
S.C. KVA u tility
•I
sym motor contrib
Step 10. The total symmetrical short-circuit RMS current is
calculated as:
(10)(%X**)(KVsecondary)2
KVA trans
Itotal S.C. sym RMS = (IS.C. sym RMS ) + (Isym motor contrib)
††
(10)(%R**)(KVsecondary)2
Rtrans Ω =
KVA trans
Step 11. Determine X/R ratio of the system to the point of
fault.
Step 3. The impedance (in ohms) given for current
transformers, large switches and large circuit breakers is
essentially all X.
Step 4.
= (4) x (Ifull load motor)
X/Rratio =
Xcable and bus Ω.
Rcable and bus Ω.
Xtotal Ω
Rtotal Ω
Step 12. The asymmetrical factor corresponding to the X/R
ratio in Step 11 is found in Table 8, Column M m . This
multiplier will provide the worst case asymmetry occurring
in the first 1/2 cycle. When the average 3-phase multiplier
is desired use column Ma.
Step 5. Total all X and all R in system to point of fault.
Step 6. Determine impedance (in ohms) of the system by:
Step 13. Calculate the asymmetrical RMS short-circuit
current.
ZT = √(RT)2 + (XT)2
IS.C. asym RMS = (IS.C. sym RMS) x (Asym Factor)
Step 7. Calculate short-circuit symmetrical RMS amperes
at the point of fault.
IS.C. sym RMS =
Step 14. The short-circuit current that the motor load can
contribute is an asymmetrical current usually approximated
as being equal to the locked rotor current of the motor.
•As a close approximation with a margin of safety use:
Esecondary line-line
√3 (ZT)
Step 8. Determine the motor load. Add up the full load
motor currents. The full load motor current in the system is
generally a percentage of the transformer full load current,
depending upon the types of loads. The generally
accepted procedure assumes 50% motor load when both
motor and lighting loads are considered, such as supplied
by 4 wire, 208Y/120V and 480Y/277V volt 3-phase
systems.)
•Iasym motor contrib
= (5) x (Ifull load motor)
Step 15. The total asymmetrical short-circuit RMS current is
calculated as:
Itotal S.C. asym RMS = (IS.C. asym RMS) + (Iasym motor contrib)
*For simplicity of calculations all ohmic values are single phase distance one way, later compensated for in the three phase short-circuit formula by the factor,
(See Step 7.)
**UL Listed transformers 25 KVA and larger have a ±10% impedance tolerance. Short circuit amperes can be affected by this tolerance.
†Only X is considered in this procedure since utility X/R ratios are usually quite high. For more finite details obtain R of utility source.
•A more exact determination depends upon the sub-transient reactance of the motors in question and associated circuit impedances. A less conservative
method would involve the total motor circuit impedance to a common bus (sometimes referred to as a “zero reactance bus”).
††Arithmetical addition results in conservative values of fault current. More finite values involve vectorial addition of the currents.
Note: The ohms of the circuit components must be referred to the same voltage. If there is more than one voltage transformation in the system, the ohmic
method becomes more complicated. It is recommended that the per-unit method be used for ease in calculation when more than one voltage transformation
exists in the system.
6
√3.
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Ohmic Method – To Fault X1 – System A
One-Line Diagram
Impedance Diagram
Available Utility
S.C. MVA 100,000
1500 KVA Transformer,
480V, 3Ø,
3.5%Z, 3.45%X , 0.56%R
R
X
X=
1000(.48)2
= 0.0000023
100,000,000
—
0.0000023
X=
(10) (3.45) (.48)2
= 0.0053
1500
—
0.0053
R=
(10) (.56) (.48)2
= 0.00086
1500
0.00086
—
X=
25' 0.0379
x
= 0.000158
1000
6
—
0.000158
R=
25' 0.0244
x
= 0.000102
1000
6
0.000102
—
—
0.000050
0.000962
0.00551
(Table 1.2)
If.l. trans = 1804A
25’ - 500 kcmil
6 Per Phase
Service Entrance
Conductors in Steel Conduit
(Table 5)
2000A Switch
(Table 3) X = 0.000050
KRP-C-2000SP Fuse
Fault X1
Motor Contribution
1
1
M
Total R and X =
M
Ztotal per = √ (0.000962)2 + (0.00551) 2 = 0.0056Ω
phase
IS.C. sym RMS =
480
√3 (.0056)
= 49,489A
Isym motor contrib = 4 x 1804 = 7216A
(100% motor load)
Itotal S.C. sym RMS = 49,489 + 7216 = 56,705A
(fault X1)
X/Rratio = .00551 = 5.73
.000962
Asym Factor = 1.294 (Table 8)
IS.C. asym RMS = 1.294 x 49,489 = 64,039A
Iasym motor contrib = 5 x 1804 = 9,020A
(100% motor load)
Itotal S.C. asym RMS = 64,039 + 9,020 = 73,059A
(fault X1)
Note: See Ohmic Method Procedure for Formulas.
7
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Ohmic Method – To Fault X2 – System A
One-Line Diagram
Impedance Diagram
Adjusted Impedance
to Fault X1
Fault X1
1
R
X
X = 0.00551
—
0.00551
R = 0.000962
0.000962
—
—
0.00008
1
400A Switch
LPS-RK-400SP Fuse
(Table 3) X = .00008
50’ - 500 kcmil
Feeder Cable
in Steel Conduit
50’ x
.0379 = 0.00189
1000
—
0.00189
R=
50’ x
.0244 = 0.00122
1000
0.00122
—
0.002182
0.00748
(Table 5)
Fault X2
Motor Contribution
X=
2
M
2
Total R and X =
M
Ztotal per = √ (0.002182)2 + (0.00748)2 = 0.00778Ω
phase
480
= 35,621A
3
(.00778)
√
IS.C. sym RMS =
Isym motor contrib = 4 x 1804 = 7216A
(100% motor load)
Itotal S.C. sym RMS = 35,621 + 7,216 = 42,837A
(fault X2)
X/Rratio =
.00748 =
3.43
.002182
Asym Factor = 1.149 (Table 8)
IS.C. asym RMS = 1.149 x 35,621 = 40,929A
Iasym motor contrib = 5 x 1804 = 9,020A
(100% motor load)
Itotal S.C. asym RMS = 40,929 + 9,020 = 49,949A
(fault X2)
Note: See Ohmic Method Procedure for Formulas. Actual motor contribution
will be somewhat smaller than calculated due to the impedance of the
feeder cable.
8
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Ohmic Method – To Fault X1 – System B
To use the OHMIC Method through a second transformer,
the following steps apply:
Step 1b. Reflect X and R values of all components to
secondary side of transformer
Step 1a. Summarize X and R values of all components on
primary side of transformer.
Vs2
V2
(Xp)
Rs = s 2 (Rp)
Vp
Vp2
and proceed with steps 2 thru 15 from page 6.
One-Line Diagram
Xs =
Impedance Diagram
Available Utility
500,000 S.C. KVA
1000KVA Transformer,
480V, 3Ø,
3.45% X, .60% R
X
X=
1000 (.48)2
= .000461
500,000
—
.000461
X=
(10) (3.45) (.48)2
= .00795
1000
—
.00795
R=
(10) (.60) (.48)2
= .00138
1000
.00138
—
X=
30' .0303 =
x
.000227
1000
4
—
.000227
R=
30' .0220
x
= .000165
1000
4
.000165
—
—
.00005
.001545
.008688
(Table 1.2)
30' - 500 kcmil
4 Per Phase
Copper in PVC Conduit
R
(Table 5)
1600A Switch
KRP-C-1500SP Fuse
(Table 3) X = .000050
1
Total R and X =
1
Ztotal per = √ (.001545)2 + (.008688)2 = .008824Ω
phase
IS.C. sym RMS =
X/Rratio =
480
= 31,405A
√3 (.008824)
.008688 =
5.62
.001545
Asym Factor = 1.285 (Table 8)
IS.C. asym RMS = 31,405 x 1.285 = 40,355A
9
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Ohmic Method – To Fault X2 – System B
One-Line Diagram
Adjusted Impedance
to fault X1
400A Switch
R
X
X = .008688
R = .001545
—
.001545
.008688
—
X = .00008
—
.00008
Impedance Diagram
1
1
LPS-RK-350SP Fuse
X=
20' x .0327 =
.000327
1000
2
(Table 5)
20' - 2/0
2 Per Phase
Copper in PVC Conduit
R=
20' x .0812 =
.000812
1000
2
Total R and X (480V) =
To Reflect X and R to secondary:
(208)2 x (.009095)
= .001708
Xtotal =
(480)2
(208V)
Rtotal =
(208V)
225KVA Transformer,
208/120V,
.998%X, .666%R
(208)2 x (.002357)
= .000442
(480)2
—
.000327
.000812
—
.002357
.009095
—
.001708
.000442
—
X=
(10) (.998) (.208)2
= .00192
225
—
.00192
R=
(10) (.666) (.208)2
= .00128
225
.00128
—
.001722
.003628
(Table 1.2)
2
Total R and X (208V) =
2
Ztotal per = √(.001722)2 + (.003628)2 = .004015Ω
phase
IS.C. sym RMS =
X/Rratio =
208
√3 (.004015)
= 29,911A
.003628 =
2.10
.001722
Asym Factor = 1.0491 (Table 8)
IS.C. asym RMS = 29,911 x 1.0491 = 31,380A
10
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Per-Unit Method
3ø Short Circuit Calculation Per-Unit Method*
The per-unit method is generally used for calculating
short-circuit currents when the electrical system is more
complex.
Step 9. The symmetrical motor contribution can be
approximated by using an average multiplying factor
associated with the motors in the system. This factor varies
according to motor design and in this text may be chosen
as 4 times motor full load current for approximate
calculation purposes. To solve for the symmetrical motor
contribution:
After establishing a one-line diagram of the system,
proceed to the following calculations: **
Step 1.
† PUX utility
=
***
Isym motor contrib = (4) x (Ifull load motor)
KVAbase
S.C. KVA utility
Step 10. The total symmetrical short-circuit rms current is
calculated as:
Step 2.
Step 3.
Step 4.
PUX trans =
(%X•)(KVAbase )
(100)(KVAtrans)
PUR trans =
(%R•)(KVAbase)
(100)(KVAtrans)
••
switches, CT, bus)
sym RMS =
(IS.C. sym RMS) + (Isym motor contrib)
Step 11. Determine X/R ratio of the system to the point of
fault.
X/Rratio =
(XΩ)(KVAbase)
PUXcomponent (cable, =
(1000)(KV) 2
switches, CT, bus)
PURcomponent (cable, =
Itotal S.C.
PUX total
PURtotal
Step 12. From Table 8, Column Mm, obtain the asymmetrical
factor corresponding to the X/R ratio determined in Step
11. This multiplier will provide the worst case asymmetry
occurring in the first 1/2 cycle. When the average 3-phase
multiplier is desired use column Ma.
(RΩ)( KVAbase)
(1000)(KV) 2
Step 5. Next, total all per-unit X and all per-unit R in system
to point of fault.
Step 13. The asymmetrical RMS short-circuit current can
be calculated as:
Step 6. Determine the per-unit impedance of the system by:
IS.C. asym RMS = (IS.C. sym RMS) x (Asym Factor)
PUZ total = √(PURtotal)2 + (PUX total)2
Step 14. The short-circuit current that the motor load can
contribute is an asymmetrical current usually approximated
as being equal to the locked rotor current of the motor.***
As a close approximation with a margin of safety use:
Step 7. Calculate the symmetrical RMS short-circuit current
at the point of fault.
IS.C. sym RMS =
KVAbase
***I
asym motor contrib
√3 (KV)(PUZtotal)
= (5) x (Ifull load motor)
Step 15. The total asymmetrical short-circuit RMS current
is calculated as:
Step 8. Determine the motor load. Add up the full load
motor currents.(Whenever motor and lighting loads are
considered, such as supplied by 4 wire, 208Y/120 and
480Y/277 volt 3 phase systems, the generally accepted
procedure is to assume 50% motor load based on the full
load current rating of the transformer.)
••
ItotalS.C. asym RMS = (IS.C. asym RMS) + (Iasym motor contrib)
* The base KVA used throughout this text will be 10,000 KVA.
** As in the ohmic method procedure, all ohmic values are single-phase distance one way, later compensated for in the three phase short-circuit formula by the
factor, √3. (See Step 7.)
• UL Listed transformers 25KVA and larger have a ± 10% impedance tolerance. Short circuit amperes can be affected by this tolerance.
† Only per-unit X is considered in this procedure since utility X/R ratio is usually quite high. For more finite details obtain per-unit R of utility source.
*** A more exact determination depends upon the sub-transient reactance of the motors in question and associated circuit impedances. A less conservative
method would involve the total motor circuit impedance to a common bus (sometimes referred to as a “zero reactance bus”).
•• Arithmetical addition results in conservative values of fault current. More finite values involve vectorial addition of the currents.
11
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Per-Unit Method – To Fault X1 – System A
One-Line Diagram
10,000 KVA Base
PUR
PUX
Impedance Diagram
Available Utility
S.C. MVA 100,000
1500 KVA Transformer,
480V, 3Ø,
3.5%Z, 3.45%X, .56%R
If.l. trans = 1804A
PUX =
10,000
= 0.0001
100,000,000
—
0.0001
PUX =
(3.45) (10,000)
= 0.2300
(100) (1500)
—
0.2300
PUR =
(.56) (10,000)
= 0.0373
(100) (1500)
0.0373
—
(25') (.0379)
x
x (10,000)
(1000)
(6)
=
= 0.00685 —
PUX
(1000) (.480)2
25’ - 500kcmil
6 Per Phase
Service Entrance
Conductors in Steel Conduit
(25') (.0244)
x
x (10,000)
(1000)
(6)
= 0.0044
=
PUR
(1000) (.480)2
2000A Switch
PUX =
KRP-C-2000SP Fuse
1
M
(.00005) (10,000)
= 0.00217
(1000) (.480)2
Total PUR and PUX =
1
M
0.00685
0.0044
—
—
0.00217
0.0417
0.2391
PUZtotal = √(0.0417)2 + (0.2391)2 = .2430
IS.C. sym RMS =
10,000
= 49,489A
√3 (.480)(.2430)
Isym motor contrib = 4 x 1804 = 7,216A
Itotal S.C. sym RMS = 49,489 + 7,216 = 56,705A
(fault X1)
X/Rratio =
* Asym
.2391 =
5.73
.0417
Factor = 1.294 (Table 8)
IS.C. asym RMS = 49,489 x 1.294 = 64,039A
Iasym motor contrib = 5 x 1804 = 9,020A
(100% motor load)
Itotal S.C. asym RMS = 64,039 + 9,020 = 73,059A
(fault X1)
Note: See Per Unit Method Procedure for Formulas.
Actual motor contribution will be somewhat smaller than calculated
due to impedance of the feeder cable.
12
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations - Procedures and Methods
Per-Unit Method – To Fault X2 – System A
One-Line Diagram
10,000 KVA Base
PUR
PUX
Impedance Diagram
Adjusted Impedance
to Fault X1
Fault X1
PUX = .2391
PUR = .0417
1
—
.0417
.2391
—
—
.0034
—
.0822
.0529
—
.0946
.3247
1
400A Switch
LPS-RK400SP Fuse
PUX =
(.00008) (10,000)
= .0034
(1000) (.480)2
50’ x
(.0379) x (10,000)
1000
= .0822
PUX =
(1000) (.480)2
50’ - 500kcmil
Feeder Cable in
Steel Conduit
50’ x (.0244) x (10,000)
1000
= .0529
PUR =
(1000) (.480)2
2
Motor Contribution
M
2
Total PUR and PUX =
M
PUZtotal = √(.0946)2 + (.3247)2 = 0.3380
IS.C. sym RMS =
10,000
= 35,621A
√3 (.480)(.3380)
Isym motor contrib = 4 x 1804 = 7,216A
Itotal S.C. sym RMS = 35,621 + 7,216 = 42,837A
(fault X2)
X/Rratio =
.32477
= 3.43
.09465
Asym Factor = 1.149 (Table 8)
IS.C. asym RMS = 1.149 x 35,621 = 40,929A
Iasym motor contrib = 5 x 1804 = 9,020A
(100%motor load)
Itotal S.C. asym RMS = 40,929 + 9,020 = 49,949A
(fault X2)
13
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Per-Unit Method – To Fault X1 – System B
One-Line Diagram
10,000KVA Base
PUR
PUX
Impedance Diagram
Available Utility
S.C. KVA 500,000
1000 KVA Transformer,
480V, 3Ø
3.45%X, .60%R
PUX =
10,000 =
.02
500,000
—
.02
PUX =
(3.45) (10,000)
= .345
(100) (1000)
—
.345
PUR =
(.6) (10,000)
= .06
(100) (1000)
.06
—
—
.0099
.0072
—
—
.0022
.0672
.3771
(30') (.0303)
x (10,000)
x
(1000)
(4)
= .0099
PUX =
(1000) (.48)2
30' - 500kcmil
4 Per Phase
Copper in PVC Conduit
(30') (.0220)
x (10,000)
x
(1000)
(4)
= .0072
=
PUR
(1000) (.48)2
1600A Switch
PUX =
KRP-C-1500SP Fuse
1
(.00005) (10,000)
= .0022
(1000) (.48)2
Total PUR and PUX =
1
PUZtotal = √(.0672)2 + (.3771)2 = .383
IS.C. sym RMS =
10,000
= 31,405A
√ 3 (.48)(.383)
X/Rratio = .3771 = 5.62
.0672
Asym Factor = 1.285 (Table 8)
IS.C.asym RMS = 31,405 x 1.285 = 40,355A
14
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Per-Unit Method – To Fault X2 – System B
One-Line Diagram
10,000 KVA
PUR
PUX
Impedance Diagram
X1 = .3771
R1 = .0672
Adjusted Impedance to
Fault X1
1
.3771
—
—
.0035
—
.0142
.0352
—
1
400A Switch
PUX =
LPS-RK-350SP Fuse
(.00008) (10,000)
= .0035
(1000) (.48)2
(20') (.0327)
x
x (10,000)
(1000)
(2)
= .0142
PUX =
(1000) (.48)2
20’ - 2/0
2 Per Phase
Copper in PVC conduit
(20') (.0812)
x
x (10,000)
(1000)
(2)
= .0352
PUR =
(1000) (.48)2
225KVA Transformer,
208V, 3Ø
.998%X, .666%R
2
—
.0672
PUX =
(.998) (10,000)
= .4435
(100) (225)
—
.4435
PUR =
(.666) (10,000)
= .296
(100) (225)
.296
—
.3984
.8383
2
Total PUR and PUX
PUZtotal = √ (.3984)2 + (.8383)2 = .928
IS.C.sym RMS =
X/Rratio =
10,000
√(3)(.208)(.928)
= 29,911A
.8383 =
2.10
.3984
Asym Factor = 1.0491 (Table 8)
IS.C. asym RMS = 29,911 x 1.0491 = 31,380A
15
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
TRON Computer Software Method
®
BUSSPOWER® is a Computer Software Program which
calculates three phase fault currents. It is a part of the
TRON ® Software Package for Power Systems Analysis.
The user inputs data which includes:
- Cable and Busway Lengths and Types
- Transformer Rating and Impedence
- Fault sources such as Utility Available and Motor
Contribution.
Following the data input phase, the program is
executed and an output report reviewed.
The following is a partial output report of System A
being studied.
TRON ® Software Fault Calculation Program –
Three Phase Fault Report
SYSTEM A
Bus Record
Name
X1
X2
Fault Study Summary
Voltage
Available RMS Duties
L-L
3 Phase
Momentary
(Sym)
(Asym)
480
58414
77308
480
44847
53111
The following is a par tial output repor t of the
distribution System B.
SYSTEM B
Bus Record
Name
X1
X2
Fault Study Summary
Voltage
Available RMS Duties
L-L
3 Phase
Momentary
(Sym)
(Asym)
480
31,363
40,141
208
29,980
31,425
A fur ther description of this program and its
capabilities is on the back cover of this bulletin.
16
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3ø Short-Circuit Current Calculations – Procedures and Methods
Point-to-Point Method
The application of the point-to-point method permits the
determination of available short-circuit currents with a
reasonable degree of accuracy at various points for either
3ø or 1ø electrical distribution systems. This method can
assume unlimited primary short-circuit current (infinite bus).
At some distance from the terminals, depending upon wire size, the L-N fault
current is lower than the L-L fault current. The 1.5 multiplier is an
approximation and will theoretically vary from 1.33 to 1.67. These figures are
based on change in turns ratio between primary and secondary, infinite
source available, zero feet from terminals of transformer, and 1.2 x %X and
1.5 x %R for L-N vs. L-L resistance and reactance values. Begin L-N
calculations at transformer secondary terminals, then proceed point-to-point.
Basic Point-to-Point Calculation Procedure
Step 1. Determine the transformer full load amperes from
either the nameplate or the following formulas:
x
3Ø Transformer
If.l. = KVA 1000
EL-L x 1.732
Step 5. Calculate "M" (multiplier).
1Ø Transformer
M= 1
1+f
Step 6. Calculate the available short-circuit symmetrical
RMS current at the point of fault.
If.l. = KVA x 1000
EL-L
IS.C. sym RMS = IS.C. x M
Step 2. Find the transformer multiplier.
Multiplier = 100
*%Z trans
Calculation of Short-Circuit Currents
at Second Transformer in System
Use the following procedure to calculate the level of
fault current at the secondary of a second, downstream
transformer in a system when the level of fault current at the
transformer primary is known.
* Note. Transformer impedance (Z) helps to determine what the short circuit
current will be at the transformer secondary. Transformer impedance is
determined as follows: The transformer secondary is short circuited. Voltage
is applied to the primary which causes full load current to flow in the
secondary. This applied voltage divided by the rated primary voltage is the
impedance of the transformer.
Example: For a 480 volt rated primary, if 9.6 volts causes secondary full load
current to flow through the shorted secondary, the transformer impedance is
9.6/480 = .02 = 2%Z.
In addition, UL listed transformer 25KVA and larger have a ± 10%
impedance tolerance. Short circuit amperes can be affected by this
tolerance.
MAIN
TRANSFORMER
IS.C. primary
Step 3. Determine the transformer let-thru short-circuit
current**.
IS.C. secondary
H.V. UTILITY
CONNECTION
IS.C. = If.l. x Multiplier
IS.C. primary
** Note. Motor short-circuit contribution, if significant, may be added to the
transformer secondary short-circuit current value as determined in Step 3.
Proceed with this adjusted figure through Steps 4, 5 and 6. A practical
estimate of motor short-circuit contribution is to multiply the total motor
current in amperes by 4.
Procedure for Second Transformer in System
Step 1. Calculate the "f" factor (IS.C. primary known)
Step 4. Calculate the "f" factor.
3Ø Faults
3Ø Transformer
(IS.C. primary and
IS.C. secondary are
3Ø fault values)
f = 1.732 x L x I
C x EL-L
1Ø Line-to-Line (L-L)
Faults on 1Ø Center
Tapped Transformer
x x
f =2 L I
C x EL-L
1Ø Line-to-Neutral
(L-N) Faults on 1Ø
Center Tapped Transformer
x x †
f=2 L I
C x EL-N
IS.C. secondary
f=
IS.C. primary x Vprimary x 1.73 (%Z)
100,000 x KVA trans
1Ø Transformer
(IS.C. primary and
IS.C. primary x Vprimary x (%Z)
IS.C. secondary are
f=
100,000 x KVA trans
1Ø fault values:
IS.C. secondary is L-L)
Where:
L = length (feet) of circuit to the fault.
C = constant from Table 6, page 27. For parallel
runs, multiply C values by the number of
conductors per phase.
I = available short-circuit current in amperes at
beginning of circuit.
Step 2. Calculate "M" (multiplier).
M=
1
1+f
Step 3. Calculate the short-circuit current at the secondary
of the transformer. (See Note under Step 3 of "Basic Pointto-Point Calculation Procedure".)
† Note. The L-N fault current is higher than the L-L fault current at the
secondary terminals of a single-phase center-tapped transformer. The
short-circuit current available (I) for this case in Step 4 should be adjusted
at the transformer terminals as follows:
At L-N center tapped transformer terminals,
I = 1.5 x L-L Short-Circuit Amperes at Transformer Terminals
IS.C. secondary =
17
Vprimary
Vsecondary
x M x IS.C. primary
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3Ø Short-Circuit Current Calculations – Procedures and Methods
Point-to-Point Method – To Faults X1 & X2 – System A
One-Line Diagram
Fault X1
Available Utility
S.C. MVA 100,000
1500 x 1000 =
1804A
480 x 1.732
Step 1.
If.l. =
Step 2.
Multiplier = 100 = 28.57
3.5
Step 3.
IS.C.= 1804 x 28.57 = 51,540A
Step 4.
f=
1.732 x 25 x 51,540 =
0.0349
6 x 22,185 x 480
2000A Switch
Step 5.
M=
1
= .9663
1 + .0349
KRP-C-2000SP Fuse
Step 6.
IS.C.sym RMS = 51,540 x .9663 = 49,803A
1500 KVA Transformer,
480V, 3Ø, 3.5%Z,
3.45%X, 56%R
If.l. =1804A
25' - 500kcmil
6 Per Phase
Service Entrance
Conductors in Steel Conduit
Fault X1
1
IS.C.motor contrib = 4 x 1,804 = 7,216A
400A Switch
ItotalS.C. sym RMS = 49,803 + 7,216 = 57,019A
LPS-RK-400SP Fuse
( fault X1)
Fault X2
50' - 500 kcmil
Feeder Cable
in Steel Conduit
Step 4.
Use IS.C.sym RMS @ Fault X1 to calculate "f"
f=
Fault X2
Motor Contribution
1
= .7117
1 + .4050
Step 5.
M=
Step 6.
IS.C.sym RMS = 49,803 x .7117 = 35,445A
2
M
1.732 x 50 x 49,803 =
.4050
22,185 x 480
Isym motor contrib = 4 x 1,804 = 7,216A
Itotal S.C. sym RMS = 35,445 + 7,216 = 42,661A
(fault X2)
18
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
3Ø Short-Circuit Current Calculations – Procedures and Methods
Point-to-Point Method – To Faults X1 & X2 - System B
Fault X1
One-Line Diagram
Available Utility
500,000 S.C KVA
1000 KVA Transformer,
480V, 3Ø,
3.5%Z
If.l.= 1203A
30’ - 500 kcmil
4 Per Phase
Copper in PVC Conduit
Step 1.
If.l. = 1000 x 1000 = 1203A
480 x 1.732
Step 2.
Multiplier =
Step 3.
IS.C. = 1203 x 28.57 = 34,370A
Step 4.
f=
Step 5.
M=
Step 6.
IS.C.sym RMS = 34,370 x .9664 = 33,215A
100 =
28.57
3.5
1.732 x 30 x 34,370 =
.0348
4 x 26,706 x 480
1600A Switch
KRP-C-1500SP Fuse
Fault X1
1
= .9664
1 + .0348
1
400A Switch
Fault X2
LPS-RK-350SP Fuse
20’ - 2/0
2 Per Phase
Copper in PVC Conduit
225 KVA transformer,
208V, 3Ø
1.2%Z
Step 4.
f = 1.732 x 20 x 33,215 = .1049
2 x 11,423 x 480
Step 5.
M=
Step 6.
IS.C.sym RMS = 33,215 x .905 = 30,059A
1
= .905
1 + .1049
Fault X2
f=
2
30,059 x 480 x 1.732 x 1.2 =
1.333
100,000 x 225
M=
1
= .4286
1 + 1.333
IS.C. sym RMS =
480 x .4286 x 30,059 =
29,731A
208
3Ø Short-Circuit Current Calculations – RMS Amperes
Comparison of Results
System A
X1
W/O Motor
W/Motor
X2
W/O Motor
W/Motor
System B
Ohmic
Sym.
Asym.
Per-Unit
Sym.
Asym.
TRON®
Sym.
Asym.
PTP
Sym.
49,489 64,039
56,705 73,059
49,489 64,039
56,705 73,059
49,992 64,430
58,414 77,308
49,803
57,019
35,621 40,929
42,837 49,949
35,621 40,929
42,837 49,949
36,126 41,349
44,847 53,111
35,445
42,661
X1
X2
Notes:
1. OHMIC and PER UNIT methods assume 100% motor contribution at X1,
then at X2.
2. TRON modeled 100% motor contribution by assuming 1500 HP load,
located at Point X2.
3. PTP method added symmetrical motor contribution at X1, then at X2.
19
Ohmic
Sym.
Asym.
31,405 40,355
29,911 31,380
Per-Unit
Sym.
Asym.
31,405 40,355
29,911 31,380
TRON®
Sym.
Asym.
31,363 40,145
29,980 31,425
PTP
Sym.
33,215
29,731
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
1ø Short-Circuit Current Calculations – 1ø Transformer System
Procedures and Methods
Short-circuit calculations on a single-phase center
tapped transformer system require a slightly different
procedure than 3Ø faults on 3Ø systems.
1. It is necessary that the proper impedance be used to
represent the primary system. For 3Ø fault calculations, a
single primary conductor impedance is only considered
from the source to the transformer connection. This is
compensated for in the 3 Ø short-circuit formula by
multiplying the single conductor or single-phase
impedance by 1.73.
A
B
C
PRIMARY
SECONDARY
SHORT
CIRCUIT
However, for single-phase faults, a primary conductor
impedance is considered from the source to the
transformer and back to the source. This is compensated in
the calculations by multiplying the 3 Ø primary source
impedance by two.
2. The impedance of the center-tapped transformer must
be adjusted for the half-winding (generally line-to-neutral)
fault condition.
The diagram at the right illustrates that during line-toneutral faults, the full primary winding is involved but, only
the half-winding on the secondary is involved. Therefore,
the actual transformer reactance and resistance of the halfwinding condition is different than the actual transformer
reactance and resistance of the full winding condition.
Thus, adjustment to the %X and %R must be made when
considering line-to-neutral faults. The adjustment multipliers
generally used for this condition are as follows:
PRIMARY
SECONDARY
SHORT CIRCUIT
L2
N
L1
1.5 times full winding %R on full winding basis.
1.2 times full winding %X on full winding basis.
Note: %R and %X multipliers given in Table 1.3 may be used, however,
calculatios must be adjusted to indicate transformer KVA/2.
3. The impedance of the cable and two-pole switches on
the system must be considered "both-ways" since the
current flows to the fault and then returns to the source. For
instance, if a line-to-line fault occurs 50 feet from a
transformer, then 100 feet of cable impedance must be
included in the calculation.
L1
SHORT CIRCUIT
N
The calculations on the following pages illustrate 1 ø
fault calculations on a single-phase transformer system.
Both line-to-line and line-to-neutral faults are considered.
L2
50 feet
Note in these examples:
a. The multiplier of 2 for some electrical components to
account for the single-phase fault current flow,
b. The half-winding transformer %X and %R multipliers for
the line-to-neutral fault situation,and
c. The KVA and voltage bases used in the per-unit
calculations
20
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
1ø Short-Circuit Current Calculations –1ø Transformer System
Per-Unit Method – Line-to-Line Fault @ 240V – Fault X1
One-Line Diagram
10,000KVA Base
PUR
PUX
Impedance Diagram
100,000 KVA
3Ø Source
PUX(3Ø) =
10,000 = .1
100,000
PUX(1Ø) = 2 x .1 = .2000
—
.2000
PUX =
(1.22) (10,000)
= 1.6267
(100) (75)
—
1.6267
PUR =
(.68) (10,000)
= .9067
(100) (75)
.9067
—
PUX =
2(.00008) (10,000)
= .0278
(1000) (.240)2
—
.0278
—
.3289
.2118
—
1.1185
2.1834
75KVA, 1Ø Transformer,
1.22%X, .68%R
Negligible Distance
400A Switch
LPN-RK-400SP Fuse
2x
PUX =
2x
25' - 500kcmil
PUR =
Magnetic Conduit
1
25' x
.0379 x 10,000
1000
= .3289
(1000) (.240)2
25'
x .0244 x 10,000
1000
= .2118
(1000) (.240)2
Total PUR and PUX =
1
PUZtotal = √(1.1185)2 + (2.1834)2 = 2.4532
IS.C. sym RMS =
L-L @ 240V
10,000
= 16,984A
(.240) (2.4532)
Note: See "Data Section" for impedance data for the electrical components.
21
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
1ø Short-Circuit Current Calculations – 1ø Transformer System
Per-Unit Method – Line-to-Neutral Fault @ 120V – Fault X1
One-Line Diagram
10,000KVA Base
PUR
PUX
Impedance Diagram
100,000 KVA
3Ø Source
PUX(3Ø) =
10,000 = .1
100,000
PUX(1Ø) = 2 x .1 = .2000
—
.2000
PUX =
(1.2) (1.22) (10,000)
= 1.952
(100) (75)
—
1.952
PUR =
(1.5) (.68) (10,000)
= 1.3600
(100) (75)
1.3600
—
—
.0556
25' x
.0379 x 10,000
1000
= 1.316
(1000) (.120)2
—
1.316
25' x
.0244 x 10,000
1000
= .8472
(1000) (.120)2
.8472
—
2.2072
3.5236
75KVA, 1Ø Transformer,
1.22%X, .68%R
Negligible Distance
400A Switch
PUX* =
LPN-RK-400SP Fuse
(.00008) (10,000)
= .0556
(1000) (.120)2
2x
PUX** =
2x
25' - 500kcmil
PUR** =
Magnetic Conduit
1
Total PUR and PUX =
1
PUZtotal = √(2.2072)2 + (3.5236)2 = 4.158
IS.C. sym RMS =
L-N @ 120V
10,000
= 20,041A
(.120) (4.158)
Note: See "Data Section" for impedance data for the electrical components.
* The multiplier of two (2) is not applicable since on a line to neutral fault, only
one switch pole is involved.
** Assumes the neutral conductor and the line conductor are the same size.
22
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
1ø Short-Circuit Current Calculations – 1ø Transformer System
Point-to-Point Method – Line-to-Line Fault @ 240V – Fault X1
Fault X1
One-Line Diagram
Available Utility
S.C.KVA 100,000
3Ø Source
75KVA, 1Ø Transformer,
1.22%X, .68%R
1.40%Z
120/240V
Negligible Distance
400A Switch
LPN-RK-400SP Fuse
25' - 500kcmil
Magnetic Conduit
1
23
75 x 1000 =
312.5A
240
Step 1.
If.l. =
Step 2.
Multiplier =
Step 3.
IS.C. = 312.5 x 71.43 = 22,322A
Step 4.
x
x
f = 2 25 22,322 = .2096
22,185 x 240
Step 5.
M=
Step 6.
IS.C. L-L (X1) = 22,322 x .8267 = 18,453A
100 =
71.43
1.40
1
= .8267
1 + .2096
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
1ø Short-Circuit Current Calculations – 1ø Transformer System
Point-to-Point Method – Line-to-Neutral Fault @ 120V – Fault X1
Fault X1
One-Line Diagram
Available Utility
S.C.KVA 100,000
3Ø Source
Step 1.
x
If.l. = 75 1000 = 312.5A
240
Step 2.
Multiplier =
Step 3.
IS.C. (L-L) = 312.5 x 71.43 = 22,322A
100 =
71.43
1.40
IS.C. (L-N) = 22,322 x 1.5 = 33,483A
75KVA, 1Ø Transformer,
1.22% X, .68%R,
1.40%Z
120/240V
f=
Step 5.
M=
Step 6.
IS.C. L-N (X1) = 33,483 x .6139 = 20,555A
Negligible Distance
400A Switch
2* x 25 x 22,322 x 1.5
= .6288
22,185 x 120
Step 4.
1
= .6139
1 + .6288
LPN-RK-400SP Fuse
* Assumes the Neutral conductor and the line conductor are the same size.
25' - 500kcmil
Magnetic Conduit
1
1Ø Short Circuit Calculations – RMS Amperes
Comparison of Results
Per-Unit Method vs. Point-to-Point Method
X1
Line-Line
Line-Neutral
Per-Unit
Method
PTP
Method
16,984A
20,041A
18,453A
20,555A
24
Impedance and Reactance Data–Transformers and Switches
Table 1.1. Transformer Impedance Data
(X/R Ratio of Transformers – Based on ANSI/IEEE C37.010-1979)
Table 1.4. Impedance Data for Single Phase and Three Phase
Transformers-Supplement†
KVA
1Ø
10
15
Suggested
%Z
X/R Ratio for Calculation
1.2
1.1
1.3
1.1
75
1.11
1.5
150
1.07
1.5
225
1.12
1.5
300
1.11
1.5
333
1.9
4.7
500
2.1
5.5
† These represent actual transformer nameplate ratings taken from field
installations.
Note: UL Listed transformers 25KVA and greater have a ±10% tolerance on
their impedance nameplate.
50
40
Typical X/R
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
30
20
10
0
0.5 1
2
5
10 20
50
100 200
Self-Cooled Transformer Rating in MVA
500
3Ø
Table 2. Current Transformer Reactance Data
Approximate Reactance of Current Transformers*
1000
This table has been reprinted from IEEE Std 141-1986, IEEE Recommended
Practice for Electric Power Distribution for Industrial Plants, Copyright© 1986
by the Institute of Electrical and Electronics Engineers, Inc with the
permission of the IEEE Standards Department.
Primary Current
Ratings - Amperes
100 - 200
250 - 400
500 - 800
1000 - 4000
Note: Values given are
facturers' data.
Table 1.2. Impedance Data for Three Phase Transformers
KVA
%R
%X
%Z
X/R
3.0
3.7600
1.0000
3.8907
0.265
6.0
2.7200
1.7200
3.2182
0.632
9.0
2.3100
1.1600
2.5849
0.502
15.0
2.1000
1.8200
2.7789
0.867
30.0
0.8876
1.3312
1.6000
1.5
45.0
0.9429
1.4145
1.7000
1.5
75.0
0.8876
1.3312
1.6000
1.5
112.5
0.5547
0.8321
1.0000
1.5
150.0
0.6657
0.9985
1.2000
1.5
225.0
0.6657
0.9985
1.2000
1.5
300.0
0.6657
0.9985
1.2000
1.5
500.0
0.7211
1.0816
1.3000
1.5
750.0
0.6317
3.4425
3.5000
5.45
1000.0
0.6048
3.4474
3.5000
5.70
1500.0
0.5617
3.4546
3.5000
6.15
2000.0
0.7457
4.9441
5.0000
6.63
2500.0
0.7457
4.9441
5.0000
6.63
Note: UL Listed transformers 25KVA and greater have a ±10% tolerance on
their nameplate impedance.
Reactance in Ohms for
Various Voltage Ratings
600-5000V
7500V
15,000V
0.0022
0.0040
—
0.0005
0.0008
0.0002
0.00019
0.00031
0.00007
0.00007
0.00007
0.00007
in ohms per phase. For actual values, refer to manu-
This table has been reprinted from IEEE Std 241-1990, IEEE Recommended
Practice for Commercial Building Power Systems, Copyright© 1990 by the
Institute of Electrical and Electronics Engineers, Inc. with the permission of
the IEEE Standards Department.
Table 3. Disconnecting Switch Reactance Data
(Disconnecting-Switch Approximate Reactance Data, in Ohms*)
Table 1.3. Impedance Data for Single Phase Transformers
Suggested
Normal Range
Impedance Multipliers**
X/R Ratio
of Percent
For Line-to-Neutral
kVA
for
Impedance (%Z)*
Faults
1Ø
Calculation
for %X
for%R
25.0
1.1
1.2–6.0
0.6
0.75
37.5
1.4
1.2–6.5
0.6
0.75
50.0
1.6
1.2–6.4
0.6
0.75
75.0
1.8
1.2–6.6
0.6
0.75
100.0
2.0
1.3–5.7
0.6
0.75
167.0
2.5
1.4–6.1
1.0
0.75
250.0
3.6
1.9–6.8
1.0
0.75
333.0
4.7
2.4–6.0
1.0
0.75
500.0
5.5
2.2–5.4
1.0
0.75
* National standards do not speciify %Z for single-phase transformers. Consult
manufacturer for values to use in calculation.
** Based on rated current of the winding (one–half nameplate kVA divided by
secondary line-to-neutral voltage).
Switch Size
Reactance
(Amperes)
(Ohms)
200
400
600
800
1200
1600
2000
3000
4000
0.0001
0.00008
0.00008
0.00007
0.00007
0.00005
0.00005
0.00004
0.00004
1 Pole
Note: The reactance of disconnecting switches for low-voltage circuits
(600V and below) is in the order of magnitude of 0.00008 - 0.00005
ohm/pole at 60 Hz for switches rated 400 - 4000 A, respectively.
*For actual values, refer to manufacturers’ data.
This table has been reprinted from IEEE Std 241-1990, IEEE Recommended
Practice for Commercial Building Power Systems, Copyright© 1990 by the
Institute of Electrical and Electronics Engineers, Inc. with the permission of
the IEEE Standards Department.
Note: UL Listed transformers 25 KVA and greater have a ± 10% tolerance on
their impedance nameplate.
This table has been reprinted from IEEEStd 242-1986 (R1991), IEEE
Recommended Practice for Protection and Coordination of Industrial and
Commercial Power Systems, Copyright© 1986 by the Institute of Electrical
and Electronics Engineers, Inc. with the permission of the IEEE Standards
Department.
25
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
Impedance & Reactance Data-Circuit Breakers and Conductors
Table 4. Circuit Breaker Reactance Data
(a) Reactance of Low-Voltage Power Circuit Breakers
Circuit-Breaker
Interrupting
Circuit-Breaker
Rating
Rating
Reactance
(amperes)
(amperes)
(ohms)
15,000
15 - 35
0.04
and
50 - 100
0.004
25,000
125 - 225
0.001
250 - 600
0.0002
50,000
200 - 800
0.0002
1000 - 1600
0.00007
75,000
2000 - 3000
0.00008
100,000
4000
0.00008
(b)Typical Molded Case Circuit Breaker Impedances
Molded-Case
Circuit-Breaker
Rating
Resistance
Reactance
(amperes)
(ohms)
(ohms)
20
0.00700
Negligible
40
0.00240
Negligible
100
0.00200
0.00070
225
0.00035
0.00020
400
0.00031
0.00039
600
0.00007
0.00017
Notes:
(1) Due to the method of rating low-voltage power
circuit breakers, the reactance of the circuit breaker
which is to interrupt the fault is not included in
calculating fault current.
(2) Above 600 amperes the reactance of molded case
circuit breakers are similar to those given in (a)
* For actual values, refer to manufacturers’ data.
This table has been reprinted from IEEE Std 241-1990,
IEEE Recommended Practice for Commercial Building
Power Systems, copyright © 1990 by the Institute of
Electrical and Electronics Engineers, Inc. with the
permission of the IEEE Standards Department.
Table 5. Impedance Data - Insulated Conductors
(Ohms/1000 ft. each conductor - 60Hz)
Size
AWG or
kcM
14
12
10
8
6
4
2
1
1/0
2/0
3/0
4/0
250
300
350
400
500
600
750
1000
Resistance (25C)
Copper
Metal
NonMet
2.5700
2.5700
1.6200
1.6200
1.0180
1.0180
.6404
.6404
.4100
.4100
.2590
.2590
.1640
.1620
.1303
.1290
.1040
.1020
.0835
.0812
.0668
.0643
.0534
.0511
.0457
.0433
.0385
.0362
.0333
.0311
.0297
.0273
.0244
.0220
.0209
.0185
.0174
.0185
.0140
.0115
Aluminum
Metal
Nonmet
4.2200
4.2200
2.6600
2.6600
1.6700
1.6700
1.0500
1.0500
.6740
.6740
.4240
.4240
.2660
.2660
.2110
.2110
.1680
.1680
.1330
.1330
.1060
.1050
.0844
.0838
.0722
.0709
.0602
.0592
.0520
.0507
.0460
.0444
.0375
.0356
.0319
.0298
.0264
.0240
.0211
.0182
Reactance - 600V - THHN
Single Conductors
1 Multiconductor
Mag.
Nonmag. Mag
Nonmag.
.0493
.0394
.0351
.0305
.0468
.0374
.0333
.0290
.0463
.0371
.0337
.0293
.0475
.0380
.0351
.0305
.0437
.0349
.0324
.0282
.0441
.0353
.0328
.0235
.0420
.0336
.0313
.0273
.0427
.0342
.0319
.0277
.0417
.0334
.0312
.0272
.0409
.0327
.0306
.0266
.0400
.0320
.0300
.0261
.0393
.0314
.0295
.0257
.0399
.0319
.0299
.0261
.0393
.0314
.0295
.0257
.0383
.0311
.0290
.0254
.0385
.0308
.0286
.0252
.0379
.0303
.0279
.0249
.0382
.0305
.0278
.0250
.0376
.0301
.0271
.0247
.0370
.0296
.0260
.0243
Note: Increased resistance of conductors in magnetic raceway is due to the effect of hysteresis
losses. The increased resistance of conductors in metal non-magnetic raceway is due to the effect
of eddy current losses. The effect is essentially equal for steel and aluminum raceway. Resistance
values are acceptable for 600 volt, 5KV and 15 KV insulated Conductors.
Size
AWG or
kcM
8
6
4
2
1
1/0
2/0
3/0
4/0
250
300
350
400
500
600
750
1000
Reactance - 5KV
Single Conductors
Mag.
Nonmag.
.0733
.0586
.0681
.0545
.0633
.0507
.0591
.0472
.0571
.0457
.0537
.0430
.0539
.0431
.0521
.0417
.0505
.0404
.0490
.0392
.0478
.0383
.0469
.0375
.0461
.0369
.0461
.0369
.0439
.0351
.0434
.0347
.0421
.0337
1 Multiconductor
Mag.
Nonmag.
.0479
.0417
.0447
.0389
.0418
.0364
.0393
.0364
.0382
.0332
.0360
.0313
.0350
.0305
.0341
.0297
.0333
.0290
.0323
.0282
.0317
.0277
.0312
.0274
.0308
.0270
.0308
.0270
.0296
.0261
.0284
.0260
.0272
.0255
Reactance - 15KV
Single Conductors
Mag.
Nonmag.
–
–
.0842
.0674
.0783
.0626
.0727
.0582
.0701
.0561
.0701
.0561
.0661
.0561
.0614
.0529
.0592
.0491
.0573
.0474
.0557
.0458
.0544
.0446
.0534
.0436
.0517
.0414
.0516
.0414
.0500
.0413
.0487
.0385
1 Multiconductor
Mag.
Nonmag.
–
–
.0584
.0508
.0543
.0472
.0505
.0439
.0487
.0424
.0487
.0424
.0458
.0399
.0427
.0372
.0413
.0359
.0400
.0348
.0387
.0339
.0379
.0332
.0371
.0326
.0357
.0317
.0343
.0309
.0328
.0301
.0311
.0291
These are only representative figures. Reactance is affected by cable insulation type, shielding,
conductor outside diameter, conductor spacing in 3 conductor cable, etc. In commercial buildings
meduim voltage impedances normally do not affect the short circuit calculations significantly.
This table has been reprinted from IEEE Std 241-1990, IEEE Recommended Practice for Commercial
Building Power Systems, copyright © 1990 by the Institute of Electrical and Electronics Engineers,
Inc. with the permission of the IEEE Standards Department.
26
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
"C" Values for Conductors and Busway
Table 6. “ C” Values for Conductors and Busway
Copper
AWG Three Single Conductors
Three-Conductor Cable
or
Conduit
Conduit
kcmil Steel
Nonmagnetic
Steel
600V
5KV
15KV
600V
5KV
15KV
600V
5KV
15KV
14
389
389
389
389
389
389
389
389
389
12
617
617
617
617
617
617
617
617
617
10
981
981
981
981
981
981
981
981
981
8
1557
1551
1557
1558
1555
1558
1559
1557
1559
6
2425
2406
2389
2430
2417
2406
2431
2424
2414
4
3806
3750
3695
3825
3789
3752
3830
3811
3778
3
4760
4760
4760
4802
4802
4802
4760
4790
4760
2
5906
5736
5574
6044
5926
5809
5989
5929
5827
1
7292
7029
6758
7493
7306
7108
7454
7364
7188
1/0
8924
8543
7973
9317
9033
8590
9209
9086
8707
2/0
10755
10061
9389
11423
10877
10318
11244
11045
10500
3/0
12843
11804
11021
13923
13048
12360
13656
13333
12613
4/0
15082
13605
12542
16673
15351
14347
16391
15890
14813
250
16483
14924
13643
18593
17120
15865
18310
17850
16465
300
18176
16292
14768
20867
18975
17408
20617
20051
18318
350
19703
17385
15678
22736
20526
18672
19557
21914
19821
400
20565
18235
16365
24296
21786
19731
24253
23371
21042
500
22185
19172
17492
26706
23277
21329
26980
25449
23125
600
22965
20567
47962
28033
25203
22097
28752
27974
24896
750
24136
21386
18888
28303
25430
22690
31050
30024
26932
1000 25278
22539
19923
31490
28083
24887
33864
32688
29320
Aluminum
14
236
236
236
236
236
236
236
236
236
12
375
375
375
375
375
375
375
375
375
10
598
598
598
598
598
598
598
598
598
8
951
950
951
951
950
951
951
951
951
6
1480
1476
1472
1481
1478
1476
1481
1480
1478
4
2345
2332
2319
2350
2341
2333
2351
2347
2339
3
2948
2948
2948
2958
2958
2958
2948
2956
2948
2
3713
3669
3626
3729
3701
3672
3733
3719
3693
1
4645
4574
4497
4678
4631
4580
4686
4663
4617
1/0
5777
5669
5493
5838
5766
5645
5852
5820
5717
2/0
7186
6968
6733
7301
7152
6986
7327
7271
7109
3/0
8826
8466
8163
9110
8851
8627
9077
8980
8750
4/0
10740
10167
9700
11174
10749
10386
11184
11021
10642
250
12122
11460
10848
12862
12343
11847
12796
12636
12115
300
13909
13009
12192
14922
14182
13491
14916
14698
13973
350
15484
14280
13288
16812
15857
14954
15413
16490
15540
400
16670
15355
14188
18505
17321
16233
18461
18063
16921
500
18755
16827
15657
21390
19503
18314
21394
20606
19314
600
20093
18427
16484
23451
21718
19635
23633
23195
21348
750
21766
19685
17686
23491
21769
19976
26431
25789
23750
1000 23477
21235
19005
28778
26109
23482
29864
29049
26608
Note: These values are equal to one over the impedance per foot for impedances found in Table 5, Page 26.
Ampacity
225
400
600
800
1000
1200
1350
1600
2000
2500
3000
4000
Busway
Plug-In
Copper
28700
38900
41000
46100
69400
94300
119000
129900
142900
143800
144900
—
Feeder
Aluminum
23000
34700
38300
57500
89300
97100
104200
120500
135100
156300
175400
—
Copper
18700
23900
36500
49300
62900
76900
90100
101000
134200
180500
204100
277800
High Impedance
Aluminum Copper
12000
—
21300
—
31300
—
44100
—
56200
15600
69900
16100
84000
17500
90900
19200
125000
20400
166700
21700
188700
23800
256400
—
Note: These values are equal to one over the impedance per foot for
impedances in Table 7, Page 28.
27
Nonmagnetic
600V
5KV
389
389
617
617
981
981
1559
1558
2433
2428
3837
3823
4802
4802
6087
6022
7579
7507
9472
9372
11703
11528
14410
14118
17482
17019
19779
19352
22524
21938
22736
24126
26915
26044
30028
28712
32236
31258
32404
31338
37197
35748
15KV
389
617
981
1559
2420
3798
4802
5957
7364
9052
11052
13461
16012
18001
20163
21982
23517
25916
27766
28303
31959
236
375
598
951
1482
2353
2958
3739
4699
5875
7372
9242
11408
13236
15494
16812
19587
22987
25750
25682
32938
236
375
598
951
1479
2344
2958
3709
4646
5771
7201
8977
10968
12661
14658
16500
18154
20978
23294
23491
29135
236
375
598
951
1481
2349
2958
3724
4681
5851
7328
9164
11277
13105
15299
17351
19243
22381
25243
25141
31919
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
Busway Impedance Data
Table 7. Busway Impedance Data (Ohms per 1000 Feet – Line-to-Neutral, 60 Cycles)
Plug-In Busway
Copper Bus Bars
Ampere Rating
Resistance
225
0.0262
400
0.0136
600
0.0113
800
0.0105
1000
0.0071
1200
0.0055
1350
0.0040
1600
0.0036
2000
0.0033
2500
0.0032
3000
0.0031
4000
0.0030
5000
0.0020
Low-Impedance Feeder Busway
225
0.0425
400
0.0291
600
0.0215
800
0.0178
1000
0.0136
1200
0.0110
1350
0.0090
1600
0.0083
2000
0.0067
2500
0.0045
3000
0.0041
4000
0.0030
5000
0.0023
Reactance
0.0229
0.0218
0.0216
0.0190
0.0126
0.0091
0.0072
0.0068
0.0062
0.0062
0.0062
0.0062
0.0039
Impedance
0.0348
0.0257
0.0244
0.0217
0.0144
0.0106
0.0084
0.0077
0.0070
0.0070
0.0069
0.0069
0.0044
Aluminum Bus Bars
Resistance
Reactance
0.0398
0.0173
0.0189
0.0216
0.0179
0.0190
0.0120
0.0126
0.0080
0.0080
0.0072
0.0074
0.0065
0.0070
0.0055
0.0062
0.0054
0.0049
0.0054
0.0034
0.0054
0.0018
—
—
—
—
Impedance
0.0434
0.0288
0.0261
0.0174
0.0112
0.0103
0.0096
0.0083
0.0074
0.0064
0.0057
—
—
0.0323
0.0301
0.0170
0.0099
0.0082
0.0070
0.0065
0.0053
0.0032
0.0032
0.0027
0.0020
0.0015
0.0534
0.0419
0.0274
0.0203
0.0159
0.0130
0.0111
0.0099
0.0074
0.0055
0.0049
0.0036
0.0027
0.0767
0.0378
0.0305
0.0212
0.0166
0.0133
0.0110
0.0105
0.0075
0.0055
0.0049
0.0036
—
0.0832
0.0470
0.0320
0.0227
0.0178
0.0143
0.0119
0.0110
0.0080
0.0060
0.0053
0.0039
—
0.0323
0.0280
0.0099
0.0081
0.0065
0.0053
0.0045
0.0034
0.0031
0.0023
0.0020
0.0015
—
The above data represents values which are a composite of those obtained by a survey of industry; values tend to be on the low side.
28
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
Asymmetrical Factors
Table 8. Asymmetrical Factors
Ratio to Symmetrical RMS Amperes
Short Circuit Short
Maximum 1 phase
Maximum 1 phase
Average 3 phase
Power Factor, Circuit
Instantaneous
RMS Amperes at
RMS Amperes at
1/2 Cycle Mm
1/2 Cycle Ma*
Percent*
X/R Ratio Peak Amperes Mp
(Asym.Factor)*
0
∞
2.828
1.732
1.394
1
100.00
2.785
1.697
1.374
2
49.993
2.743
1.662
1.354
3
33.322
2.702
1.630
1.336
4
24.979
2.663
1.599
1.318
5
19.974
2.625
1.569
1.302
6
16.623
2.589
1.540
1.286
7
14.251
2.554
1.512
1.271
8
13.460
2.520
1.486
1.256
9
11.066
2.487
1.461
1.242
10
9.9301
2.455
1.437
1.229
11
9.0354
2.424
1.413
1.216
12
8.2733
2.394
1.391
1.204
13
7.6271
2.364
1.370
1.193
14
7.0721
2.336
1.350
1.182
15
6.5912
2.309
1.331
1.172
16
6.1695
2.282
1.312
1.162
17
5.7947
2.256
1.295
1.152
18
5.4649
2.231
1.278
1.144
19
5.16672
2.207
1.278
1.135
20
4.8990
2.183
1.247
1.127
21
4.6557
2.160
1.232
1.119
22
4.4341
2.138
1.219
1.112
23
4.2313
2.110
1.205
1.105
24
4.0450
2.095
1.193
1.099
25
3.8730
2.074
1.181
1.092
26
3.7138
2.054
1.170
1.087
27
3.5661
2.034
1.159
1.081
28
3.4286
2.015
1.149
1.076
29
3.3001
1.996
1.139
1.071
30
3.1798
1.978
1.130
1.064
31
3.0669
1.960
1.122
1.062
32
2.9608
1.943
1.113
1.057
33
2.8606
1.926
1.106
1.057
34
2.7660
1.910
1.098
1.050
35
2.6764
1.894
1.091
1.046
36
2.5916
1.878
1.085
1.043
37
2.5109
1.863
1.079
1.040
38
2.4341
1.848
1.073
1.037
39
2.3611
1.833
1.068
1.034
40
2.2913
1.819
1.062
1.031
41
2.2246
1.805
1.058
1.029
42
2.1608
1.791
1.053
1.027
43
2.0996
1.778
1.049
1.024
44
2.0409
1.765
1.045
1.023
45
1.9845
1.753
1.041
1.021
46
1.9303
1.740
1.038
1.019
47
1.8780
1.728
1.035
1.017
48
1.8277
1.716
1.032
1.016
49
1.7791
1.705
1.029
1.014
50
1.7321
1.694
1.026
1.013
55
1.5185
1.641
1.016
1.008
60
1.3333
1.594
1.009
1.004
65
1.1691
1.517
1.005
1.001
70
1.0202
1.517
1.002
1.001
75
0.8819
1.486
1.0008
1.0004
80
0.7500
1.460
1.0002
1.0001
85
0.6198
1.439
1.00004
1.00002
100
0.0000
1.414
1.00000
1.00000
*Reprinted by permission of National Electrical Manufacturer's Association from
NEMA Publication AB-1, 1986, copyright 1986 by NEMA.
29
Selective Coordination (Blackout Prevention)
Having determined the faults that must be
interrupted, the next step is to specify Protective
Devices that will provide a Selectively Coordinated
System with proper Interrupting Ratings.
Such a system assures safety and reliability
under all service conditions and prevents needless
interruption of service on circuits other than the one
on which a fault occurs.
The topic of Selectivity will be Discussed in the
next Handbook, EDP II.
Component Protection (Equipment Damage Prevention)
Proper protection of electrical equipment
requires that fault current levels be known. The
characteristics and let-through values of the
overcurrent device must be known, and compared
to the equipment withstand ratings. This topic of
Component Protection is discussed in the third
Handbook, EDP III.
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Engineering
Dependable
Protection
For An
Electrical
Distribution
System
Bulletin EDP-2
(2004-2)
Part 2
Selective Coordination
Of Overcurrent
Protective Devices
For
Low Voltage Systems
Bussmann
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
Basic Considerations of Selective Coordination
Engineering Dependable Protection
Part I has provided a simple method to calculate shortcircuit currents that occur in electrical systems. With this
information, selective coordination studies of the systems
can be performed in order to prevent blackouts.
remainder of the system undisturbed and preserving
continuity of service.
We may then define selective coordination as "THE
ACT OF ISOLATING A FAULTED CIRCUIT FROM THE
REMAINDER OF THE ELECTRICAL SYSTEM, THEREBY
ELIMINATING UNNECESSARY POWER OUTAGES. THE
FAULTED CIRCUIT IS ISOLATED BY THE SELECTIVE
OPERATION OF ONLY THAT OVERCURRENT PROTECTIVE
DEVICE CLOSEST TO THE OVERCURRENT CONDITION."
Figures 1 and 2 illustrate a non-selective system and a
selectively coordinated system, respectively.
What Is Selective Coordination?
Today, more than ever, one of the most important parts
of any installation - whether it is an office building, an
industrial plant, a theater, a high-rise apartment or a
hospital - is the electrical distribution system. Nothing will
stop all activity, paralyze production, inconvenience and
disconcert people and possibly cause a panic more
effectively than a major power failure.
ISOLATION of a faulted circuit from the remainder of
the installation is MANDATORY in today's modern electrical
systems. Power BLACKOUTS CANNOT be tolerated.
It is not enough to select protective devices based
solely on their ability to carry the system load current and
interrupt the maximum fault current at their respective
levels. A properly engineered system will allow ONLY the
protective device nearest the fault to open, leaving the
Popular Methods of Performing a Selective Coordination Study
Currently two methods are most often used to perform
a coordination study:
1. Overlays of Time-Current Curves, which utilize a
light table and manufacturers' published data, then hand
plot on log-log paper.
2. Computer programs that utilize a PC and allow the
designer to select time current curves published by
manufacturers and transfer to a plotter or printer, following
proper selections.
This text will apply to both methods.
It is also possible that non-selective OPENING could
be due to overload conditions on the branch circuit.
Non-Selective Coordination Resulting in a Blackout
A fault on a branch circuit opens protective devices
"D", "C" and "B". The entire power supply to the building is
completely shut down. This non-selective operation is
normally due to a medium to high level short circuit. This
fault may be L-L, L-G, or 3 phase bolted in nature.
Selective Coordination
A fault on a branch circuit opens protective device " D"
only. Since A, B and C are not disturbed, the remainder of
the electrical system is still energized.
Not Affected
Not Affected
A
A
De-energized
Portion of System
Not Affected
Also Opens
B
B
Not
Affected
Also Opens
C
C
Opens
Opens
D
D
Branch
Circuit
Fault
De-energized
Portion of System.
(This is the only part of
the system affected).
Figure 1
Figure 2
3
Fault
100A
Overloads and Low Level Fault Currents
400A
Reading Time-Current Curves
600
This infor mation is presented as an aid to
understanding time-current characteristic curves of fuses
and circuit breakers, and will discuss the major
considerations in properly applying electrical protective
devices. A thorough understanding of time-current
characteristic curves of overcurrent protective devices is
essential to provide a Selectively Coordinated System.
It should be noted that the study of time-current curves
indicates performance during overload and low level fault
conditions. The performance of overcurrent devices that
operate under medium to high level fault conditions are not
reflected on standard curves. Other engineering methods
must be utilized.
400
300
200
400A
100
Point E
80
Point C
60
40
100A
30
Available
Fault
Current
Level
1000A
20
Fuse Curves
Figure 3 illustrates the time-current characteristic
curves for two sizes of time-delay, dual-element fuses in
series, as depicted in the one-line diagram in Figure 3a.
The horizontal axis of the graph represents the RMS
symmetrical current in amperes. The ver tical axis
represents the time, in seconds, until the fault occurs .
For example: Assume an available fault current level
of 1000 amperes RMS symmetrical on the load side of the
100 ampere fuse. To determine the time it would take this
fault current to open the two fuses, first find 1000 amperes
on the horizontal axis (Point A), follow the dotted line
vertically to the intersection of the total clear curve of the
100 ampere time-delay dual-element fuse (Point B) and the
minimum melt curve of the 400 ampere time-delay dualelement fuse (Point C). Then, horizontally from both
intersection points, follow the dotted lines to Points D and
E. At 1.75 seconds, Point D represents the maximum time
the 100 ampere time-delay dual-element fuse will take to
open the 1000 ampere fault. At 88 seconds, Point E
represents the minimum time at which the 400 ampere
time-delay dual-element fuse could open this available fault
current. Thus, selective operation is assured.
The two fuse curves can be examined by the same
procedure at various current levels along the horizontal axis
(for example, see Points F and G at the 2000 ampere fault
level). It can be deter mined that the two fuses are
selectively coordinated, since the 100 ampere time-delay
dual-element fuse will open before the 400 ampere timedelay dual-element fuse can melt.
TIME IN SECONDS
Point G
10
8
Figure 3a.
6
4
3
2
Point B
Point D
1
.8
.6
.4
.3
.2
Point F
.1
.08
Minimum Melt
Total Clearing
.06
.04
.03
Figure 3
4
6000
4000
3000
2000
800
1000
400
600
CURRENT IN AMPERES
20,000
Point A 1000A
300
200
.01
8000
10,000
.02
100
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Reading Time-Current Curves
Circuit Breaker Curves
The dashed portion of Figure 4 represents the same
400 ampere breaker with an I.T. = 10x, or 10 times 400
amperes = 4000 amperes. At this setting the overload trip
will operate up to approximately 4000 amperes (±10%).
Overcurrents greater than 4000 amperes (±10%) would be
cleared by the instantaneous trip.
3. Unlatching Times - As explained above, the unlatching
time indicates the point at which the breaker senses an
overcurrent in the instantaneous region and releases the
latch holding the contacts. However, the fault current
continues to flow through the breaker and the circuit to the
point of fault until the contacts can physically separate and
extinguish the arc. Once the unlatching mechanism has
sensed an overcurrent and unlatched, the circuit breaker
will open. The final interruption of the current represented
on the breaker curve in the instantaneous region occurs
after unlatching, but within the maximum interruption time.
The relatively long delay between unlatching and the
actual interruption of the overcurrent in the instantaneous
region is the primary reason that molded case breakers
are very difficult to coordinate. This is an inherent problem
since the breaking of current is accomplished by
mechanical means.
4. Interrupting Rating - The interrupting rating of a circuit
breaker is a critical factor concerning protection and
safety. The interrupting rating of a circuit breaker is the
maximum fault current the breaker has been tested to
interrupt in accordance with testing laboratory standards.
Fault currents in excess of the interrupting rating can result
in destruction of the breaker and equipment and possible
injury to personnel. In other words, when the fault level
exceeds the circuit breaker interrupting rating, the circuit
breaker is no longer a protective device.
Looking at Figure 10, the interrupting ratings at 480
volts are 14,000 amperes for the 90 ampere breaker and
30,000 amperes for the 400 ampere breaker. The
interrupting ratings on circuit breakers vary according to
breaker type and voltage level.
When drawing circuit breaker time-current curves,
deter mine the proper interrupting rating from the
manufacturer's literature and represent this interrupting
rating on the drawing by a vertical line at the right end of
the curve.
Figure 4 illustrates a typical thermal magnetic molded
case circuit breaker curve with an overload region and an
instantaneous trip region (two instantaneous trip settings
are shown). Circuit breaker time-current characteristic
curves are read similar to fuse curves. The horizontal axis
represents the current, and the vertical axis represents the
time at which the breaker interrupts the circuit.
When using molded case circuit breakers of this type,
there are four basic curve considerations that must be
understood. These are:
1.Overload Region
2.Instantaneous Region
3.Unlatching Time
4.Interrupting Rating
1. Overload Region - The opening of a molded case circuit
breaker in the overload region (see Figure 4) is generally
accomplished by a thermal element, while a magnetic coil
is generally used on power breakers. Electronic sensing
breakers will utilize CT's. As can be seen, the overload
region has a wide tolerance band, which means the
breaker should open within that area for a particular
overload current.
2. Instantaneous Region - The instantaneous trip setting
indicates the multiple of the full load rating at which the
circuit breaker will open as quickly as possible. The
instantaneous region is represented in Figure 4 and is
shown to be adjustable from 5x to 10x the breaker rating.
When the breaker coil senses an overcurrent in the
instantaneous region, it releases the latch which holds the
contacts closed.
In Figure 4, the unlatching time is represented by the
curve labeled "average unlatching time for instantaneous
tripping". After unlatching, the overcurrent is not halted until
the breaker contacts are mechanically separated and the
arc is extinguished. Consequently, the final overcurrent
termination can vary over a wide range of time, as is
indicated by the wide band between the unlatching time
curve and the maximum interrupting time curve in Figure 4.
The instantaneous trip setting for larger molded case
and power breakers can usually be adjusted by an external
dial. Figure 4 shows two instantaneous trip settings for a
400 amp breaker. The instantaneous trip region, drawn with
the solid line, represents an I.T. = 5x, or five times 400
amperes = 2000 amperes. At this setting, the circuit
breaker will trip instantaneously on currents of
approximately 2000 amperes or more. The ± 25% band
represents the area in which it is uncertain whether the
overload trip or the instantaneous trip will operate to clear
the overcurrent.
5
Reading Time-Current Curves
1000
800
600
400 Ampere Circuit Breaker
400
d
rloa
Ove
300
200
ion
Reg
Minimum
Unlatching
Time
Maximum
Interrrupting Time
100
80
60
40
30
Average Unlatching Times
Breaker Tripping Magnetically
20
Current in
RMS Amps
5,000
10,000
15,000
20,000
25,000
10
8
6
Adjustable Magnetic
Instantaneous Trip
Set at 10 Times
I.T. = 10X
(± 10% Band)
4
TIME IN SECONDS
3
Time in
Seconds
.0045
.0029
.0024
.0020
.0017
Interrupting Rating
2
RMS Sym.
240V
480V
600V
1
.8
Adjustable
Instantaneous Trip
Set at 5 Times
I.T. = 5X
(± 25% Band)
.6
.4
.3
.2
.1
.08
.06
Maximum
Interrupting
Time
.04
.03
.02
Instantanous Region
.01
.008
.006
.004
Interrupting
Rating
at 480 Volt
.003
CURRENT IN AMPERES
Figure 4. Typical Circuit Breaker Time-Current Characteristic Curve
6
80,000
100,000
60,000
40,000
30,000
20,000
8000
10,000
6000
4000
3000
800
1000
600
400
300
200
.001
2000
Average Unlatching
Times for
Instantaneous Tripping
.002
100
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Amps
42,000
30,000
22,000
Reading Time-Current Curves
5. Short Time Delay And Instantaneous Override - Circuit
breaker short-time-delay (STD) mechanisms allow an
intentional delay to be installed on Low Voltage Power
Circuit Breakers (Figure 5). Short-time-delays allow the fault
current to flow for several cycles, which subjects the
electrical equipment being protected to unnecessarily high
mechanical and thermal stress. Most equipment ratings,
such as short-circuit ratings for bus duct and switchboard
bus, do not apply when short-time-delay settings are
employed. The use of short-time-delay settings on circuit
breakers requires the system equipment to be reinforced to
withstand the available fault current for the duration of the
short-time-delay. Ignoring equipment ratings in relation to
the protective device opening time and let-thru
characteristics can be disastrous.
An Insulated Case Circuit Breaker (ICCB) may also be
equipped with short-time-delay. However, ICCB's will have
a built-in override mechanism (Figure 6). This is called the
instantaneous override function, and will override the STD
for medium to high level faults. This override may "kick in"
for faults as low as 12x the breaker's ampere rating. This
can result in non-selective tripping of the breaker and load
side breakers where overlaps occur. This can be seen in
the example given in Figure 7. As the overlap suggests, for
any fault condition greater than 21,000 amperes, both
devices will open, causing a blackout.
1000
800
600
400
300
200
LVPCB
100
80
60
40
30
TIME IN SECONDS
20
10
8
6
4
3
2
Note: Choosing overcurrent protective devices strictly on
the basis of voltage, current, and interrupting rating will not
assure component protection from short-circuit currents.
The interrupting rating of a protective device pertains only
to that device and has absolutely no bearing on its ability to
protect connected downstream components. High
interrupting rated electro-mechanical overcurrent protective
devices, such as circuit breakers, especially those that are
not current-limiting, may not be capable of protecting wire,
cable or other components within the higher short-circuit
ranges. Quite often, the component is completely
destroyed under shor t-circuit conditions while the
protective device is opening the faulted circuit.
1
.8
.6
.4
.3
STD = 21 Cycles
CURRENT IN AMPERES
Figure 5
7
80,000
100,000
60,000
40,000
30,000
20,000
8,000
10,000
6,000
4,000
3,000
.1
2,000
.2
1,000
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
.01
Figure 6
8
80,000
60,000
40,000
100,000
CURRENT IN AMPERES
30,000
20,000
10,000
8000
6000
4000
3000
60
2000
1000
800
600
400
300
200
100
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Reading Time-Current Curves
1000
800
600
400
300
200
100
80
ICCB
40
30
20
10
8
6
4
3
2
.8
1
.6
.4
.3
.2
Instantaneous
Override
= 12X
.08
.1
.06
.04
.03
.02
.01
Figure 7
CURRENT IN AMPERES
9
.04
80,000
200
100,000
400
60,000
40,000
30,000
20,000
10,000
8000
6000
4000
3000
2000
60
1000
800
600
400
300
200
100
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Reading Time-Current Curves
1000
800
600
2000A
300
100A
100
80
40
100A CB
2000A ICCB
30
20
10
8
6
4
3
2
.8
1
.6
.4
.3
.2
.08
.1
.06
BLACKOUT!
.03
.02
Current Limiting Fuses
Medium to High Level Fault Currents
Figure 8 shows that the available short-circuit current
will reach a peak value of I p during the first half cycle
unless a protective device limits the peak fault current to a
value less than Ip. A current-limiting fuse will reduce the
available peak current to less than Ip, namely I'p, and will
clear the fault in approximately one-half cycle or less. Note
that tc is the total clearing time of the fuse, tm the melting
time and ta the arcing time of the fuse. Where high values
of fault current are available, the sub-cycle region becomes
the most critical region for selective operation of currentlimiting fuses.
The area under the current curves indicates the energy
let-thru. If no protective device were present, or if
mechanical type overcurrent devices with opening times of
one-half cycle or longer were present, the full available
short-circuit energy would be delivered to the system. The
amount of energy delivered is directly proportionate to the
square of the current. So we can see how important it is to
have fuses which can limit the current being delivered to
the system to a value less than the available current. The
amount of energy being produced in the circuit while the
fuse is clearing is called the total clearing energy and is
equal to the melting energy plus the arcing energy.
Selectivity between two fuses operating under shortcircuit conditions exists when the total clearing energy of
the load side fuse is less than the melting energy of the line
side fuse (See Figure 9).
Available
Short-Circuit Current
Current
Ip
Limited Current Results
When Fuse Clears
I'p
ta
tm
Time
tc
Fault is Initiated Here
Figure 8
An engineering tool has been developed to aid in the
proper selection of fuses for selective coordination. This
Selectivity Ratio Guide (SRG) is shown below.
* Selectivity Ratio Guide (Line-Side to Load-Side) for Blackout Prevention
Circuit
Load-Side Fuse
Current Rating
Type
601-6000A
TimeDelay
LOW-PEAK
(L)
KRP-CSP
601-4000A
TimeDelay
LIMITRON
(L)
KLU
0-600A
Dual-Element
Time-Delay
LOW-PEAK
(RK1)
(J)**
LPN-RKSP LPJSP
LPS-RKSP
2:1
2:1
601-6000A
Fast-Acting
0-600A
0-1200A
Fast-Acting
Trade Name &
FUSETRON LIMITRON LIMITRON T-TRON
Class
(RK5)
(L)
(RK1)
(T)
Buss
FRN-R
KTU
KTN-R
JJN
Symbol
FRS-R
KTS-R
JJS
601 to Time- LOW-PEAK KRP-CSP 2:1
2.5:1
4:1
2:1
2:1
2:1
6000A Delay (L)
601 to Time- LIMITRON KLU
2:1
2:1
2:1
2:1
4:1
2:1
2:1
2:1
4000A Delay (L)
LOW-PEAK LPN-RKSP –
–
2:1
2:1
8:1
–
3:1
3:1
0
Dual (RK1)
LPS-RKSP
to
Ele(J)
LPJSP** –
–
2:1
2:1
8:1
–
3:1
3:1
600A ment FUSETRON FRN-R
–
–
1.5:1
1.5:1
2:1
–
1.5:1
1.5:1
(RK5)
FRS-R
601 to
LIMITRON KTU
2:1
2.5:1
2:1
2:1
6:1
2:1
2:1
2:1
6000A
(L)
0 to
Fast- LIMITRON KTN-R
–
–
3:1
3:1
8:1
–
3:1
3:1
600A Acting (RK1)
KTS-R
0 to
T-TRON
JJN
–
–
3:1
3:1
8:1
–
3:1
3:1
1200A
(T)
JJS
0 to
LIMITRON JKS
–
–
2:1
2:1
8:1
–
3:1
3:1
600A
(J)
0 to
Time- SC
SC
–
–
3:1
3:1
4:1
–
2:1
2:1
60A
Delay (G)
* Note: At some values of fault current, specified ratios may be lowered to permit closer fuse sizing. Plot fuse curves or consult with Bussmann.
General Notes: Ratios given in this Table apply only to Buss fuses. When fuses are within the same case size, consult Bussmann.
** Consult Bussmann for latest LPJSP ratios.
Line-Side Fuse
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
0-600A
0-60A
TimeDelay
LIMITRON SC
(J)
(G)
JKS
SC
2:1
N/A
2:1
N/A
3:1
4:1
3:1
1.5:1
4:1
1.5:1
2:1
N/A
3:1
4:1
3:1
4:1
3:1
4:1
2:1
2:1
ampere ratings is 5:1 (1000:200) which indicates
coordination between these fuses. Continuing further into
the system the LPS-RK-200SP feeds a LPJ60SP. This ratio
of ampere ratings is 3.33:1 (200:60), which also indicates a
selectively coordinated system.
As an example, refer to Figure 9 and the SRG for Low
Peak fuses. The SRG suggests that the minimum ratio
between line side and load side fuse should be at least 2:1.
The one-line illustrated in Figure 9 shows Low Peak fuses
KRP-C1000SP feeding a LPS-RK200SP. The ratio of
10
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Current Limiting Fuses
Available
Short-Circuit
Current
KRP-C-1000SP Amp Fuse
Let-Thru Energy*
480/277
Volts
Line Side
LOW-PEAK®
Time-Delay Fuse
KRP-C-1000SP
tm
tc
Load Side
LOW-PEAK®
LPS-RK-200SP
Dual-Element Fuse
LPS-RK-200SP
Amp Fuse
Let-Thru Energy*
tm
tc
Line Side
Load Side
LOW-PEAK®
LPJ-60SP
Dual-Element Fuse
LPJ-60SP
Amp Fuse
Let-Thru Energy*
Fault
tc
Figure 9
Requirements for selectivity—Total clearing energy of load side fuse is less than melting energy of line side fuse.
*Area under the curves indicates let-thru energy.
11
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Circuit Breakers
Medium to High Level Fault Currents
Figure 10 illustrates a 400 ampere circuit breaker
ahead of a 90 ampere breaker. Any fault above 1500
amperes on the load side of the 90 ampere breaker will
open both breakers. The 90 ampere breaker will generally
unlatch before the 400 ampere breaker. However, before
the 90 ampere breaker can separate its contacts and clear
the fault current, the 400 ampere breaker has unlatched
and also will open.
Assume a 4000 ampere short circuit exists on the load
side of the 90 ampere circuit breaker. The sequence of
events would be as follows:
1. The 90 ampere breaker will unlatch (Point A) and
free the breaker mechanism to start the actual opening
process.
2. The 400 ampere breaker will unlatch (Point B) and it,
too, would begin the opening process. Once a breaker
unlatches, it will open. At the unlatching point, the process
is irreversible.
3. At Point C, the 90 ampere breaker will have
completely interrupted the fault current.
4. At Point D, the 400 ampere breaker also will have
completely opened the circuit.
Consequently, this is a non-selective system, causing
a complete blackout to the load protected by the 400
ampere breaker.
As printed by one circuit breaker manufacturer, "One
should not overlook the fact that when a high fault current
occurs on a circuit having several circuit breakers in series,
the instantaneous trip on all breakers may operate.
Therefore, in cases where several breakers are in series,
the larger upstream breaker may start to unlatch before the
smaller downstream breaker has cleared the fault. This
means that for faults in this range, a main breaker may
open when it would be desirable for only the feeder breaker
to open."
12
Figure 10
CURRENT IN AMPERES
13
1,500A
4,000A
14,000A
I.R.
30,000A
I.R.
80,000
100,000
.004
•
A•
60,000
300
40,000
80
30,000
.006
20,000
.02
8000
.03
10,000
90Amp
Circuit Breaker
6000
3000
2000
1000
800
600
400
300
200
100
80
60
40
30
20
10
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination –
Circuit Breakers
1000
800
600
400
400A
200
100
90A
4000A
60
40
30
20
400Amp Circuit Breaker
I.T. = 5X
10
8
6
4
3
2
.8
1
.6
.4
.3
.2
.08
.1
.06
.04
•D
•C
.008
.01
B
.003
.002
.001
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Selective Coordination Study –
Recommended Procedures
2. Short Circuit Study
Perform a short circuit analysis, calculating maximum
available short circuit currents at critical points in the
distribution system (such as transformers, main switchgear,
panelboards, motor control centers, load centers, and large
motors and generators.) (Reference: Bussmann Bulletin,
Engineering Dependable Protection - EDPI.)
The following steps are recommended when
conducting a selective coordination study.
1. One-Line Diagram
Obtain the electrical system one-line diagram that identifies
important system components, as given below.
a. Transformers
Obtain the following data for protection and coordination information of transformers:
- KVA rating
- Inrush points
- Primary and secondary connections
- Impedance
- Damage curves
- Primary and secondary voltages
- Liquid or dry type
b. Conductors - Check phase, neutral, and equipment
grounding. The one-line diagram should include information such as:
- Conductor size
- Number of conductors per phase
- Material (copper or aluminum)
- Insulation
- Conduit (magnetic or non-magnetic)
From this information, short circuit withstand curves can be
developed. This provides information on how overcurrent
devices will protect conductors from overload and short
circuit damage.
c. Motors
The system one-line diagram should include motor
information such as:
- Full load currents
- Horsepower
- Voltage
- Type of starting characteristic
(across the line, etc.)
- Type of overload relay
(Class 10, 20, 30)
Overload protection of the motor and motor circuit can be
determined from this data.
d. Fuse Characteristics
Fuse Types/Classes should be identified on the one-line
diagram.
e. Circuit Breaker Characteristics
Circuit Breaker Types should be identified on the one-line
diagram.
f. Relay Characteristics
Relay Types should be identified on the one-line diagram.
3. Helpful Hints
a. Determine the Ampere Scale Selection. It is most
convenient to place the time current curves in the center of
the log-log paper. This is accomplished by multiplying or
dividing the ampere scale by a factor of 10.
b. Determine the Reference (Base) Voltage. The best
reference voltage is the voltage level at which most of the
devices being studied fall. (On most low voltage industrial
and commercial studies, the reference voltage will be 208,
240, or 480 volts). Devices at other voltage levels will be
shifted by a multiplier based on the transformer turn ratio.
The best reference voltage will require the least amount of
manipulation. Modern computer programs will automatically make these adjustments when the voltage levels of
devices are identified by the input data.
c. Commencing the Analysis. The starting point can be
determined by the designer. Typically, studies begin with
the main circuit devices and work down through the
feeders and branches. (Right to left on your log-log paper.)
d. Multiple Branches. If many branches are taken off one
feeder, and the branch loads are similar, the largest rated
branch circuit should be checked for coordination with
upstream devices. If the largest branch will coordinate, and
the branch devices are similar, they generally will
coordinate as well. (The designer may wish to verify other
areas of protection on those branches, conductors, etc.)
e. Don't Overcrowd the Study. Many computer generated
studies will allow a maximum of ten device characteristics
per page.
f. One-Line Diagram. A one-line diagram of the study
should be drawn for future reference.
14
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Examples of Selective Coordination Studies
The following pages will analyze in detail the system
shown in Figure 11. It is understood that a short circuit
study has been completed, and all devices have adequate
interrupting ratings. A Selective Coordination Analysis is the
next step.
This simple radial system will involve three separate
time current curve studies, applicable to the three feeder/
branches shown.
13.8KV
Overcurrent Relay
IFLA=42A
1000KVA
∆-Y
480/277V
JCN80E
#6 XLP
5.75% Z
1600A Main Bus
Fault X1 20,000A RMS Sym
LOW-PEAK®
KRP-C-1600SP
Main Switchboard
1
LOW-PEAK®
LPS-RK-225SP
LOW-PEAK®
LPS-RK-400SP
LOW PEAK®
LPS-RK-200SP
400A Feeder
200A Feeder
PDP
150KVA
∆-Y
208/120V
2% Z
#3/0 THW
LOW-PEAK®
LPN-RK-500SP
LOW-PEAK®
LPS-RK-100SP
LP1
20A Branch
20A CB
20A CB
250 kcmil
2/Ø THW
100A Motor Branch
#12 THW
#1 THW
60HP 3Ø
Figure 11
M 77A FLA
15
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Example –
Time Current Curve #1 (TCC1)
Notes:
1. TCC1 includes the primary fuse, secondary main fuse,
200 ampere feeder fuse, and 20 ampere branch circuit
breaker from LP1.
2. Analysis will begin at the main devices and proceed
down through the system.
3. Reference (base) voltage will be 480 volts, arbitrarily
chosen since most of the devices are at this level.
4. Selective coordination between the feeder and branch
circuit is not attainable for faults above 2500 amperes that
occur on the 20 amp branch circuit, from LP1. Notice the
overlap of the 200 ampere fuse and 20 ampere circuit
breaker.
5. The required minimum ratio of 2:1 is easily met between
the KRP-C-1600SP and the LPS-RK-200SP.
Device ID
Description
Comments
1
1000KVA XFMR
Inrush Point
12 x FLA
@ .1 Seconds
2
1000KVA XFMR
Damage Curves
5.75%Z, liquid
filled
(Footnote 1)
(Footnote 2)
3
JCN 80E
E-Rated Fuse
4
#6 Conductor
Damage Curve
Copper, XLP
Insulation
5
Medium Voltage
Relay
Needed for XFMR
Primary Overload
Protection
6
KRP-C-1600SP
Class L Fuse
11
LPS-RK-200SP
Class RK1 Fuse
12
3/0 Conductor
Damage Curve
Copper THW
Insulation
13
20A CB
Thermal Magnetic
Circuit Breaker
14
#12 Conductor
Damage Curve
Copper THW
Insulation
Footnote 1: Transformer damage curves indicate when it will be damaged,
thermally and/or mechanically, under overcurrent conditions.
Transformer impedance, as well as primary and secondary
connections, and type, all will determine their damage
characteristics.
Footnote 2: A ∆-Y transformer connection requires a 15% shift, to the right,
of the L-L thermal damage curve. This is due to a L-L
secondary fault condition, which will cause 1.0 p.u. to flow
through one primary phase, and .866 p.u. through the two
faulted secondary phases. (These currents are p.u. of 3-phase
fault current.)
16
Time Current Curve #1 (TCC1)
1000
800
2
600
400
3
FLA
2
300
XFMR
DAMAGE
200
11
100
80
60
JCN 80E
20A MCCB
LPS-RK-200SP
40
5
30
KRP-C-1600SP
20
MV OLR
TIME IN SECONDS
10
8
6
4
#6 DAMAGE
3
3/0 DAMAGE
2
#12 DAMAGE
12
13.8KV
14
4
1
.8
.6
Overcurrent
Relay
13
.4
.3
JCN80E
#6 XLP
.2
1000KVA
5.75%Z
∆-Y
480/277V
1
TX
INRUSH
.1
.08
.06
KRP-C-1600SP
6
.04
.03
.02
CURRENT IN AMPERES X 10 @ 480V
20A CB
20A CB
#12 THW
17
8000
10,000
6000
4000
3000
2000
800
1000
600
400
300
200
80
100
60
40
30
20
8
6
4
3
200A .01
Feeder
10
#3/0 THW
2
LPS-RK-200SP
1
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Example –
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Example –
Time Current Curve #2 (TCC2)
Device ID
Notes:
1. TCC2 includes the primary fuse, secondary main fuse,
400 ampere feeder fuse, 100 ampere motor branch fuse,
77 ampere motor and overload relaying.
2. Analysis will begin at the main devices and proceed
down through the system.
3. Reference (base) voltage will be 480 volts, arbitrarily
chosen since most of the devices are at this level.
Description
Comment
1
1000KVA XFMR
Inrush Point
12 x FLA
@ .1 seconds
2
1000KVA XFMR
Damage Curves
5.75%Z, liquid
filled
(Footnote 1)
(Footnote 2)
3
JCN 80E
E-Rated Fuse
4
#6 Conductor
Damage Curve
Copper, XLP
Insulation
5
Medium Voltage
Relay
Needed for XFMR
Primary Overload
Protection
6
KRP-C-1600SP
Class L Fuse
21
LPS-RK-100SP
Class RK1 Fuse
22
Motor Starting Curve
Across the Line
Start
23
Motor Overload Relay
Class 10
24
Motor Stall Point
Part of a Motor
Damage Curve
25
#1 Conductor
Damage Curve
Copper THW
Insulation
Footnote 1: Transformer damage curves indicate when it will be damaged,
thermally and/or mechanically, under overcurrent conditions.
Transformer impedance, as well as primary and secondary
connections, and type, all will determine their damage
characteristics.
Footnote 2: A ∆-Y transformer connection requires a 15% shift, to the right,
of the L-L thermal damage curve. This is due to a L-L
secondary fault condition, which will cause 1.0 p.u. to flow
through one primary phase, and .866 p.u. through the two
faulted secondary phases. (These currents are p.u. of 3-phase
fault current.)
18
Time Current Curve #2 (TCC2)
1000
800
2
600
400
200
3
FLA
2
300
XFMR DAMAGE
MTR OLR
MS
24
100
23
80
13.8KV
60
40
Overcurrent
Relay
JCN 80E
TIME IN SECONDS
#6 XLP
1000KVA
5.75%Z
∆-Y
480/277V
JCN80E
MTR START
30
LPS-RK-100SP
20
MV OLR
KRP-C-1600SP
5
10
8
6
4
#6 DAMAGE
3
2
22
KRP-C-1600SP
1
25
#1 DAMAGE
4
.8
.6
.4
21
LPS-RK-400SP
.3
400A Feeder
.2
TX
INRUSH
.1
LPS-RK-100SP
1
.08
#1 THW
.06
6
.04
.03
.02
CURRENT IN AMPERES X 10 @ 480V
19
8000
10,000
6000
4000
3000
2000
800
1000
600
400
300
200
80
100
60
40
30
20
8
10
6
4
.01
3
M
2
60HP
1
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Example –
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Example –
Time Current Curve #3 (TCC3)
Notes:
1. TCC3 includes the primary fuse, secondary main fuse,
225 ampere feeder/transformer primary and secondary
fuses.
2. Analysis will begin at the main devices and proceed
down through the system.
3. Reference (base) voltage will be 480 volts, arbitrarily
chosen since most of the devices are at this level.
4. Relative to the 225 ampere feeder, coordination between
primary and secondary fuses is not attainable, noted by
overlap of curves.
5. Overload and short circuit protection for the 150 KVA
transformer is afforded by the LPS-RK-225SP fuse.
Device ID
Description
Comment
1
1000KVA XFMR
Inrush Point
12 x FLA
@ .1 seconds
2
1000KVA XFMR
Damage Curves
5.75%Z, liquid
filled
(Footnote 1)
(Footnote 2)
3
JCN 80E
E-Rated Fuse
4
#6 Conductor
Damage Curve
Copper, XLP
Insulation
5
Medium Voltage
Relay
Needed for XFMR
Primary Overload
Protection
6
KRP-C-1600SP
Class L Fuse
31
LPS-RK-225SP
Class RK1 Fuse
32
150 KVA XFMR
Inrush Point
12 x FLA
@.1 Seconds
33
150 KVA XFMR
Damage Curves
2.00% Dry Type
(Footnote 3)
34
LPN-RK-500SP
Class RK1 Fuse
35
2-250kcmil Conductors Copper THW
Damage Curve
Insulation
Footnote 1: Transformer damage curves indicate when it will be damaged,
thermally and/or mechanically, under overcurrent conditions.
Transformer impedance, as well as primary and secondary
connections, and type, all will determine their damage
characteristics.
Footnote 2: A ∆-Y transformer connection requires a 15% shift, to the right,
of the L-L thermal damage curve. This is due to a L-L
secondary fault condition, which will cause 1.0 p.u. to flow
through one primary phase, and .866 p.u. through the two
faulted secondary phases. (These currents are p.u. of 3-phase
fault current.)
Footnote 3: Damage curves for a small KVA (<500KVA) transformer,
illustrate thermal damage characteristics for ∆-Y connected.
From right to left, these reflect damage characteristics, for a
line-line fault, 3Ø fault, and L-G fault condition.
20
Time Current Curve #3 (TCC3)
1000
800
2
600
3
FLA
FLA
400
2
300
XFMR DAMAGE
200
100
80
60
JCN80E
40
13.8KV
5
LPS-RK-225SP
30
LPN-RK-500SP
MV OLR
20
KRP-C1600SP
Overcurrent
Relay
TIME IN SECONDS
JCN 80E
31
34
10
8
6
2-250 DAMAGE
35
4
#6 DAMAGE
3
#6 XLP
33
XFMR DAMAGE
2
1000KVA
5.75%Z
∆-Y
480/277V
4
1
.8
.6
.4
KRP-C-1600SP
.3
.2
TX
INRUSH
TX
INRUSH
LPS-RK-225SP
1
32
.1
.08
150KVA
2.0%Z
∆-Y
208/120V
.06
.04
6
.03
CURRENT IN AMPERES X 10 @ 480V
21
8000
10,000
6000
4000
3000
2000
800
1000
600
400
300
200
80
100
60
40
30
20
8
10
6
4
.01
3
250 kcmil
2/Ø THW
.02
2
LPN-RK-500SP
1
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Example –
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Conclusions
Unnecessar y power OUTAGES, such as the
BLACKOUTS we so often experience, can be stopped by
isolating a faulted circuit from the remainder of the system
through the proper selection of MODERN CURRENTLIMITING FUSES.
Time-Delay type current-limiting fuses can be sized
close to the load current and still hold motor-starting
currents or other har mless transients, thereby
ELIMINATING nuisance OUTAGES.
The SELECTIVITY GUIDE on page 10 may be used for
an easy check on fuse selectivity regardless of the shortcircuit current levels involved. Where medium and high
voltage primary fuses are involved, the time-current
characteristic curves of the fuses in question should be
plotted on standard NEMA log-log graph paper for proper
study.
The time saved by using the SELECTIVITY GUIDE will
allow the electrical systems designer to pursue other areas
for improved systems design.
22
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
Engineering Dependable Protection
Engineering Dependable Protection - Part III
"Component Protection for Electrical Systems"
Table of Contents
Page
Basic Considerations of Component Protection …………………………………………………………3
- The National Electrical Code and Component Protection …………………………………………3
- Calculating Short-Circuit Currents ……………………………………………………………………3
Current-Limitation ……………………………………………………………………………………………4
- Analysis of Current-Limiting Fuse Let-Thru Charts …………………………………………………5
Let-Thru Data Pertinent to Equipment Withstand …………………………………………………………6
- How to Use the Let-Thru Charts ………………………………………………………………………6
A Practical Approach - Protecting System Components ………………………………………………7
- A. Wire and Cable Protection …………………………………………………………………………7
- B. Bus Short-Circuit Rating and Bracing Requirements …………………………………………15
- C. Low Voltage Motor Controllers ……………………………………………………………………18
- D. Molded Case Circuit Breakers ……………………………………………………………………19
- E. Transformers…………………………………………………………………………………………21
- F. Ballasts ………………………………………………………………………………………………22
- G.Transfer Switches …………………………………………………………………………………23
- H. HVAC Equipment……………………………………………………………………………………24
Data Section - BUSS® Fuse Let-Thru Charts
1. LOW-PEAK YELLOW™ Class L Time-Delay Fuses KRP-C_SP …………………………………25
2 LOW-PEAK YELLOW™ Class J Dual-Element T-D Fuses LPJ_SP ………………………………26
3. LOW-PEAK YELLOW™ Class RK1 Dual-Element T-D Fuses LPN-RK_SP, LPS-RK_SP ………27
4. FUSETRON® Class RK5 Dual-Element T-D Fuses FRN-R, FRS-R ………………………………28
5. TRON® Class T Fast-Acting Fuses JJN, JJS ………………………………………………………29
6. LIMITRON® Class RK1 Fast-Acting Fuses KTN-R, KTS-R ………………………………………30
7. LIMITRON® Class J Fast-Acting Fuses JKS ………………………………………………………31
Buss Fuse Selection Chart …………………………………………………………..……………………32
2
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Engineering
Dependable
Protection
For An
Electrical
Distribution
System
Bulletin EDP-3
(2004-3)
Part 3
Component
Protection for
Electrical
Systems
Bussmann
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
Basic Considerations of Component Protection
This requires that overcurrent protective devices, such
as fuses and circuit breakers be selected in such a manner
that the short-circuit withstand ratings of the system
components will not be exceeded should a short-circuit
occur.
The “short-circuit withstand rating” is the maximum
short-circuit current that a component can safely withstand.
Failure to provide adequate protection may result in
component destruction under short-circuit conditions.
Engineering Dependable Protection
Part I provided the tools necessary to examine
electrical distribution systems from the standpoint of
reliability, to insure proper interrupting ratings of protective
devices.
Part II dealt with selective coordination in order to
prevent blackouts.
This handbook is Part III, "Component Short Circuit
Protection". It will help the engineer understand the
withstand rating of various system components, thus
enabling him or her to explore the protection of the
components that make up the system.
Calculating Short-Circuit Currents
Before proceeding with a systems analysis of wire,
cable and other component protection requirements, it will
be necessary to establish the short-circuit current levels
available at various points in the electrical system. This can
be accomplished by using Engineering Dependable
Protection - Part I (BUSS Bulletin EDP-I). After calculating
the fault levels throughout the electrical system, the next
step is to check the withstand rating of wire and cable, bus,
circuit breakers, transfer switches, starters, etc., not only
under overload conditions but also under short-circuit
conditions.
Introduction
This issue analyzes the protection of electrical system
components from fault currents. It gives the specifier the
necessary information regarding the withstand rating of
electrical circuit components, such as wire, bus, motor
starters, etc. Proper protection of circuits will improve
reliability and reduce the possibility of injury. Electrical
systems can be destroyed if the overcurrent devices do not
limit the short-circuit current to within the withstand rating of
the system’s components. Merely matching the ampere
rating of a component with the ampere rating of a
protective device will not assure component protection
under short-circuit conditions.
In the past several years, there have been numerous
reports in newspapers, magazines and insurance company
files about destroyed electrical systems. Recognizing this
as a serious problem to safety of life and property, much
more emphasis has been placed on COMPLIANCE with
THE NATIONAL ELECTRICAL CODE.
The National Electrical Code covers COMPONENT
PROTECTION in several sections. The first section to note
is Section 110-10.
Note: The let-thru energy of the protective device must be
equal to or less than the short-circuit withstand rating of the
component being protected.
CAUTION: Choosing overcurrent protective devices
strictly on the basis of voltage, current, and interrupting
rating alone will not assure component protection from
short-circuit currents. High interrupting capacity
electro-mechanical overcurrent protective devices,
especially those that are not current-limiting, may not
be capable of protecting wire, cable or other
components within high short-circuit ranges. The
interrupting rating of a protective device pertains only
to that device and has absolutely no bearing on its
ability to protect connected downstream components.
Quite often, an improperly protected component is
completely destroyed under short-circuit conditions
while the protective device is opening the faulted
circuit.
Component Protection and the National Electrical Code
Section 110-10. Circuit Impedance and Other
Characteristics. The overcurrent protective devices, the
total impedance, the component short-circuit withstand
ratings, and other characteristics of the circuit to be
protected shall be so selected and coordinated as to
permit the circuit protective devices used to clear a fault
without the occurrence of extensive damage to the
electrical components of the circuit. This fault shall be
assumed to be either between two or more of the circuit
conductors, or between any circuit conductor and the
grounding conductor or enclosing metal raceway.
Before proceeding with the study of component
withstandability, the technology concerning “CurrentLimitation” will be reviewed.
3
Current-Limitation
A Definition of Current-Limitation
Today, most electrical distribution systems are capable
of delivering very high short-circuit currents, some in
excess of 200,000 amperes. If the components are not
capable of handling these short-circuit currents, they could
easily be damaged or destroyed. The current-limiting ability
of today’s modern fuses allows components with low shortcircuit withstand ratings to be specified in spite of high
available fault currents.
Section 240-11 of the NEC offers the following
definition of a current limiting device:
“A current-limiting overcurrent protective device is a
device which, when interrupting currents in its currentlimiting range, will reduce the current flowing in the faulted
circuit to a magnitude substantially less than that
obtainable in the same circuit if the device were replaced
with a solid conductor having comparable impedance.”
The concept of current-limitation is pointed out in
Figure 1, where the prospective available fault current is
shown in conjunction with the limited current resulting when
a current-limiting fuse clears. The area under the current
curve indicates the amount of short-circuit energy being
dissipated in the circuit. Since both magnetic forces and
thermal energy are directly proportional to the square of the
current, it is important to limit the short-circuit current to as
small a value as possible. Magnetic forces vary as the
square of the “PEAK” current and thermal energy varies as
the square of the “RMS” current.
Prospective available short-circuit
current that would flow when a
Fuse is not used.
100,000
Current
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
Peak Let-Thru
Current of Fuse
10,000
0
Time
tc
Total Clearing Time of Fuse
Figure 1. Current Limiting Effect of Fuses
Thus, the current-limiting fuse in this example would
limit the let-thru energy to a fraction of the value which is
available from the system. In the first major loop of fault
current, standard non-current limiting, electro-mechanical
protective devices would let-through approximately 100
times* as much destructive energy as the fuse would letthrough.
*
4
2
(100,000
10,000 )
Current-Limitation
Analysis of Current-Limiting Fuse Let-Thru Charts
The degree of current-limitation of a given size and
type of fuse depends, in general, upon the available shortcircuit current which can be delivered by the electrical
system. Current-limitation of fuses is best described in the
form of a let-thru chart which, when applied from a
practical point of view, is useful to determine the let-thru
currents when a fuse opens.
Fuse let-thru charts are similar to the one shown in
Figure 2 and are plotted from actual test data. The test
circuit that establishes line A-B corresponds to a shortcircuit power factor of 15%, which is associated with an X/R
ratio of 6.6. The fuse curves represent the cutoff value of
the prospective available short-circuit current under the
given circuit conditions. Each type or class of fuse has its
own family of let-thru curves.
The let-thru data has been generated by actual short circuit
tests of current-limiting fuses. It is important to understand
how the curves are generated, and what circuit parameters
affect the let-thru curve data. Typically, there are three
circuit parameters that can affect fuse let-thru performance
for a given available short circuit current. These are:
1. Short circuit power factor
2. Short circuit closing angle
3. Applied voltage
Current-limiting fuse let-thru curves are generated
under worst case conditions, based on these three
variable parameters. The benefit to the user is a
conservative resultant let-thru current (both Ip and IRMS).
Under actual field conditions, changing any one or a
combination of these will result in lower let-thru currents.
This provides for an additional degree of reliability when
applying fuses for equipment protection.
See charts and tables on pages 25 thru 31 for
Bussmann fuse let-thru current data.
B
400,000
300,000
Available Peak ShortCircuit Current = 198,000A
I
200,000
Available RMS ShortCircuit Current = 86,000A
100,000
80,000
60,000
Peak Let-Thru Current
of Fuse= 49,000A
800A
RMS Let-Thru Current
of Fuse = 21,000A
30,000
20,000
tm
10,000
8000
6000
AMPERE
RATING
A
200,000
100,000
60,000
80,000
30,000
40,000
20,000
6000
4000
3000
2000
1000
8000
10,000
2000
PROSPECTIVE SHORT CIRCUIT CURRENT – SYMMETRICAL RMS AMPS
Figure 2. Analysis of a Current-LImiting Fuse
5
TIME
ta
tc
4000
3000
1000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
tm = Fuse Melt Time
ta = Fuse Arc Time
tc = Fuse Clearing Time
Let-Thru Data Pertinent to Equipment Withstand
B. Determine the APPARENT PROSPECTIVE RMS SYMMETRICAL
LET-THRU CURRENT.
Prior to using the Fuse Let-Thru Charts, it must be
determined what let-thru data is pertinent to equipment
withstand ratings.
Equipment withstand ratings can be described as:
How Much Fault Current can the equipment handle, and
for How Long? Based on standards presently available,
the most important data which can be obtained from the
Fuse Let-Thru Charts and their physical effects are the
following:
A. Peak let-thru current - mechanical forces
B. Apparent prospective RMS symmetrical let-thru
current - heating effect
Step 1. Enter the chart on the Prospective Short-Circuit
current scale at 86,000 amperes and proceed vertically
until the 800 ampere fuse curve is intersected.
Step 2. Follow horizontally until line A-B is intersected.
Step 3. Proceed vertically down to the Prospective ShortCircuit Current.
Step 4. Read the APPARENT PROSPECTIVE RMS
SYMMETRICAL LET-THRU CURRENT as 21,000 amperes.
(The RMS SYMMETRICAL LET-THRU CURRENT would be
86,000 amperes if there were no fuse in the circuit.)
30,000
20,000
10,000
8000
6000
A
2000
200,000
60,000
80,000
100,000
A
B
1000
30,000
40,000
Step 1. Enter the chart on the Prospective Short-Circuit
current scale at 86,000 amperes and proceed vertically
until the 800 ampere fuse curve is intersected.
AMPERE
RATING
4000
3000
1000
A. Determine the PEAK LET-THRU CURRENT.
800A
D
20,000
How to Use the Let-Thru Charts
Using the example given in Figure 3, one can
determine the pertinent let-thru data for the KRP-C800SP
ampere LOW-PEAK fuse. The Let-Thru Chart pertaining to
the 800 ampere LOW-PEAK fuse is illustrated in Figure 4.
C
100,000
80,000
60,000
8000
10,000
Figure 3. 800 Ampere LOW-PEAK® Current-Limiting Time-Delay
Fuse and Associated Let-Thru Data
200,000
6000
A. Peak Let-Thru Current
B. Apparent Prospective RMS Sym
Let-Thru Current
300,000
4000
Short-Circuit
B
400,000
3000
86,000 Amps
RMS Sym
Available
KRP-C800SP
Ampere Fuse
2000
Figure 3 is a typical example showing the short-circuit
current available to an 800 ampere circuit, an 800 ampere
LOW-PEAK® current-limiting time-delay fuse, and the letthru data of interest.
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
PROSPECTIVE SHORT CIRCUIT CURRENT – SYMMETRICAL RMS AMPS
A IRMS Available = 86,000 Amps
B IRMS Let-Thru = 21,000 Amps
Step 2. Follow horizontally until the Instantaneous Peak LetThru Current scale is intersected.
C Ip Available = 198,000 Amps
D Ip Let-Thru = 49,000 Amps
Step 3. Read the PEAK LET-THRU CURRENT as 49,000
amperes. (If a fuse had not been used, the peak current
would have been 198,000 amperes.)
Figure 4. Current-Limitation Curves – Bussmann LOW-PEAK®
Time-Delay Fuse KRP-C800SP
6
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Electrical Distribution System
A Practical Approach – Protecting System Components
The following components will be analyzed by
establishing the short-circuit withstand data of each
component and then selecting the proper current limiting
fuses for protection:
A. Wire and Cable
B. Bus (Busway, Switchboards, Motor Control Centers
and Panelboards)
C. Low-Voltage Motor Controllers
D. Circuit Breakers
E. Low-Voltage Transformers
F. Ballasts
G. Transfer Switches
H. HVAC Equipment
Most electrical equipment has a withstand rating that is
defined in terms of an RMS Symmetrical-Short Circuit
Current, and in some cases, Peak Let-Thru Current. These
values have been established through short-circuit testing
of that equipment according to an accepted industry
standard. Or, as is the case with conductors, the withstand
rating is based on a mathematical calculation and is also
expressed in an RMS short circuit current.
If both the let-thru currents (IRMS and I p ) of the
current limiting fuse and the time it takes to clear the
fault are less than the withstand rating of the electrical
component, then that component will be protected from
short-circuit damage.
Protecting System Components
A. Wire and Cable
The circuit shown in Figure 5 originates at a distribution
panel where 40,000 amperes RMS symmetrical is available.
To determine the proper fuse, first establish the short-circuit
withstand data for the #10 THW copper cable shown in the
diagram.
40,000 Amps
RMS Sym
Available
Distribution
Panel
LOW-PEAK® Dual-Element
Fuse LPS-RK30SP
been established for various insulation as follows:
Paper, rubber and varnished cloth
200°C.
Thermoplastic
150°C.
The following charts show the currents which, after
flowing for the times indicated, will produce these
maximum temperatures for each conductor size. Figures 6,
7 and 8 give data for copper conductors, and Figures 9, 10
and 11 for aluminum conductors. The system short-circuit
capacity, the conductor cross-sectional area and the
overcurrent protective device opening time should be such
that these maximum allowable short-circuit currents are not
exceeded.
Thus, if the protective device requires one cycle to
open (such as a circuit breaker) then 1/0 THW copper
cables must be specified for the 30 ampere circuit in Figure
5 in order to prevent damaging temperature rise to the
insulation. (Refer to Figure 6 for 1/0 withstand data.)
Using the formula shown on each ICEA protection
table will allow the engineer to calculate withstand ratings
of cable not shown on these pages. It may be
advantageous to calculate withstand ratings below one
cycle, when the opening time of the current-limiting device
is known.
An example of additional withstand ratings for 75°C
copper wire is shown in Table 1.
Short-Circuit
To Load
#10 THW Copper
Figure 5. Example Showing Short-Circuit Protection of Wire and
Cable.
Figures 6 thru 11 show the short-circuit withstand of
copper and aluminum wire and cable based on Insulated
Cable Engineers Association (ICEA) formulae.
The short-circuit withstand of the #10 THW copper
conductor, from Figure 7 is 4,300 amperes for one cycle
(.0167 seconds). Short-circuit protection of this conductor
requires the selection of an overcurrent device which will
limit the 40,000 amperes RMS symmetrical available to a
value less than 4,300 amperes, and clear the fault in one
cycle or less.
The LOW-PEAK® dual-element fuse let-thru chart (page
27) shows that the LPS-RK30SP LOW-PEAK dual-element
fuse will let-through an apparent prospective RMS current
of less than 1,800 amperes, when 40,000 amperes is
available (and would clear the fault in less than 1/2 cycle).
Table 1. Copper, 75° Thermoplastic Insulated Cable Damage
Table (Based on 60 HZ)
Copper
Wire Size
75°C
Thermoplastic
#14
#12
#10
*#8
#6
#4
Short-Circuit Currents for Insulated Cables*
The recent increase in KVA capacity of power distribution systems has resulted in possible short-circuit currents
of extremely high magnitude. Conductor insulation may be
seriously damaged by fault induced, high conductor
temperatures. As a guide in preventing such serious
damage, maximum allowable short-circuit temperatures,
which damage the insulation to a slight extent only, have
Maximum Short-Circuit Withstand Current
in Amperes
For
For
For
1/2 Cycle
1 Cycle
2 Cycles
2,400
1,700
1,200
3,800
2,700
1,900
6,020
4,300
3,000
9,600
6,800
4,800
15,200
10,800
7,600
24,200
17,100
12,100
For
3 Cycles
1,000
1,550
2,450
3,900
6,200
9,900
Permission has been given by ICEA to reprint these charts. These charts
have been reproduced on pages 8 thru 13.
7
A. Wire and Cable
Allowable Short-Circuit Currents for Insulated Copper Conductors*
100
80
60
50
30
–
S
LE
YC
YC
30
16
8
8
C
C
4
C
2
10
S
LE
C
1
YC
C
YC
LE
–
0.
20
01
0.
67
YC LE
03
–
SE
C
0
3
S
L
.0
60
YC
3
C
ES –
6
S
O
67
C
0.
LE
10
E
N
–
Y
13
C
0
D
S
S
C
0
O
3
E
C
.
L
–
2
N
3
C
ES
YC
66 S
0.
D
O
50
E
7
LE
N
–
D
00 SE CO
1.
S
00
–
SE CO ND
00
1.
N
C
66
67 SEC ON D
D
SE
O
N
C
D
O
N
D
40
SHORT CIRCUIT CURRENT - THOUSANDS OF AMPERES
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
6
5
4
3
CONDUCTOR: COPPER
INSULATION: PAPER, RUBBER, VARNISHED
CLOTH
2
CURVES BASED ON FORMULA:
1
.8
 I 2t = .0297 log T2 + 234
A 
T1 + 234
.6
WHERE:
.5
I
.4
A = CONDUCTOR AREA - CIRCULAR MILS
t
= SHORT-CIRCUIT CURRENT - AMPERES
= TIME OF SHORT-CIRCUIT - SECONDS
.3
T1 = MAXIMUM OPERATING TEMPERATURE 75°C
.2
T2 = MAXIMUM SHORT-CIRCUIT TEMPERATURE 200°C
.1
10
8
6
4
2
1 1/0
2/0 3/0 4/0 AWG
250 MCM
CONDUCTOR SIZE
500
1000
Figure 6. Short-Circuit Current Withstand Chart for Copper Cables with Paper, Rubber, or Varnished Cloth Insulation
*Copyright 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers Association (ICEA). Permission has been given by ICEA to reprint this chart.
8
A. Wire and Cable
Allowable Short-Circuit Currents for Insulated Copper Conductors*
100
80
60
50
40
20
C
2
1
C
10
YC
YC LE
C
LE –
8 YC
0
S
16 CY LE
– .01
S
C
6
C
0.
30
YC LE –
03 7 S
C
0
S
3
L
.
60
YC
3 EC
ES – 06
SE O
6
0
LE
10 CY
.1 7
–
C ND
0
S
S
C
3
0.
O
33 EC
C
L
–
2
N
YC ES
66
0.
D
S
O
50
EC N
7
LE –
SE
D
00
1.
S
O
0
N
C
–
SE
D
O
1. 000
N
C
66
D
S
O
67 EC
N
D
SE O
N
C
O D
N
D
30
8
4
SHORT CIRCUIT CURRENT - THOUSANDS OF AMPERES
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
6
5
4
3
CONDUCTOR: COPPER
INSULATION: THERMOPLASTIC
2
CURVES BASED ON FORMULA:
 I 2t = .0297 log T2 + 234 
A 
T1 + 234 
1
.8
WHERE:
.6
I
.5
A = CONDUCTOR AREA - CIRCULAR MILS
.4
t
.3
T1 = MAXIMUM OPERATING TEMPERATURE 75°C
= TIME OF SHORT-CIRCUIT - SECONDS
T2 = MAXIMUM SHORT-CIRCUIT TEMPERATURE 150°C
.2
.1
10
= SHORT-CIRCUIT CURRENT - AMPERES
8
6
4
2
1 1/0
2/0 3/0 4/0 AWG
250 MCM
CONDUCTOR SIZE
500
1000
Figure 7. Short-Circuit Current Withstand Chart for Copper Cables with Thermoplastic Insulation
*Copyright 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers Association (ICEA). Permission has been given by ICEA to reprint this chart.
9
A. Wire and Cable
Allowable Short-Circuit Currents for Insulated Copper Conductors*
100
80
60
50
30
–
LE
C
30
16
8
C
8
C
4
YC
YC
2
10
S
LE
C
1
YC
C
YC
LE
–
0.
20
01
S
0.
67
YC LE
03
–
SE
C
0
3
S
.0
60
YC LE
3
C
–
6
S
O
6 7 SE
C
10
N
YC LES – 0.13
C
0
D
S
0
O
33
E
C
.
L
–
2
N
C
YC ES
6
0.
D
O
50 67 SE
LE
N
–
C
S
D
0
1
S
E O
0
.0
–
SE CO ND
1. 000
N
C
66
67 SEC ON D
D
SE O
C ND
O
N
D
40
SHORT CIRCUIT CURRENT - THOUSANDS OF AMPERES
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
6
5
4
CONDUCTOR: COPPER
3
INSULATION: CROSSLINKED POLYETHYLENE &
ETHYLENE PROPYLENE RUBBER
2
CURVES BASED ON FORMULA:
 I 2t = .0297 log T2 + 234 
A 
T1 + 234 
1
WHERE:
.8
I
.6
A = CONDUCTOR AREA - CIRCULAR MILS
.5
t
.4
T1 = MAXIMUM OPERATING TEMPERATURE - 90°C
= SHORT-CIRCUIT CURRENT - AMPERES
= TIME OF SHORT-CIRCUIT - SECONDS
T2 = MAXIMUM SHORT-CIRCUIT TEMPERATURE 250°C
.3
.2
.1
10
8
6
4
2
1 1/0
2/0 3/0 4/0 AWG
250 MCM
CONDUCTOR SIZE
500
1000
Figure 8. Short-Circuit Current Withstand Chart for Copper Cables with Crosslinked Polyethylene & Ethylene Propylene Rubber Insulation
*Copyright 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers Association (ICEA). Permission has been given by ICEA to reprint this chart.
10
A. Wire and Cable
Allowable Short-Circuit Currents for Insulated Aluminum Conductors*
100
80
60
50
40
20
4
2
8
C
1
C
10
YC
YC LE
C
LE –
8 YC
0
S
16 CY LE
– .01
S
C
6
C
0.
30
YC LE –
03 7 S
C
0
S
3
L
.
60
YC
3 EC
ES – 06
SE O
6
0
LE
10 CY
.1 7
–
C ND
0
S
S
C
3
0.
O
33 EC
C
L
–
2
N
YC ES
66
0.
D
S
O
50
EC N
7
LE –
SE
D
00
1.
S
O
0
N
C
–
SE
D
O
1. 000
N
C
66
D
S
O
67 EC
N
D
SE O
N
C
O D
N
D
30
SHORT CIRCUIT CURRENT - THOUSANDS OF AMPERES
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
6
5
4
3
CONDUCTOR: ALUMINUM
2
INSULATION: PAPER, RUBBER, VARNISHED
CLOTH
1
CURVES BASED ON FORMULA:
.8
 I 2t = .0125 log T2 + 228 
A 
T1 + 228 
.6
WHERE:
.5
I
.4
A = CONDUCTOR AREA - CIRCULAR MILS
t
.3
= TIME OF SHORT-CIRCUIT - SECONDS
T1 = MAXIMUM OPERATING TEMPERATURE 75°C
.2
.1
10
= SHORT-CIRCUIT CURRENT - AMPERES
T2 = MAXIMUM SHORT-CIRCUIT TEMPERATURE 200°C
8
6
4
2
1 1/0
2/0 3/0 4/0 AWG
250 MCM
CONDUCTOR SIZE
500
1000
Figure 9. Short-Circuit Current Withstand Chart for Aluminum Cables with Paper, Rubber, or Varnished Cloth Insulation
*Copyright 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers Association (ICEA). Permission has been given by ICEA to reprint this chart.
11
A. Wire and Cable
Allowable Short-Circuit Currents for Insulated Aluminum Conductors*
100
80
60
50
40
30
C
SE
67
–
–
S
S
10
0
C
C
YC
C
YC
LE
YC
LE
S
LE
60
3
0.
0.
0.
–
S
YC
C
30
4
13
06
–
S
C
16
8
4
C
6
5
YC
2
LE
C
1
YC
C
8
LE
YC
LE
0.
–
03
0.
10
33
01
SE
67
SE
C
O
N
D
O
–
26 33
N
C
YC LES
0.
D
67 SE ON
50
LE –
SE CO D
00
1.
S
00
N
C
–
S
D
O
00
EC
1.
N
66
D
S
O
67 EC
N
D
SE O
N
C
O D
N
D
20
SHORT CIRCUIT CURRENT - THOUSANDS OF AMPERES
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
CONDUCTOR: ALUMINUM
2
INSULATION: THERMOPLASTIC
1
CURVES BASED ON FORMULA:
.8
 I 2t = .0125 log T2 + 228 
A 
T1 + 228 
.6
WHERE:
.5
I
.4
A = CONDUCTOR AREA - CIRCULAR MILS
t
= SHORT-CIRCUIT CURRENT - AMPERES
= TIME OF SHORT-CIRCUIT - SECONDS
.3
T1 = MAXIMUM OPERATING TEMPERATURE 75°C
.2
T2 = MAXIMUM SHORT-CIRCUIT TEMPERATURE 150°C
.1
10
8
6
4
2
1 1/0
2/0 3/0 4/0 AWG
250 MCM
CONDUCTOR SIZE
500
1000
Figure 10. Short-Circuit Current Withstand Chart for Aluminum Cable with Thermoplastic Insulation
*Copyright 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers Association (ICEA). Permission has been given by ICEA to reprint this chart.
12
A. Wire and Cable
Allowable Short-Circuit Currents for Insulated Aluminum Conductors*
100
80
60
50
40
33
0.
–
LE
S
–
S
LE
YC
60
C
C
YC
C
30
16
6
5
YC
LE
C
8
4
C
YC
2
8
06
S
LE
C
1
YC
C
YC
10
–
LE
0.
–
03
0.
01
67
SE
20
SE CO
S
0. 67
LE
N
–
Y
1
C
0
D
0. 33 SE
O
C CLE S –
26 3
N
C
YC
0.
D
S
S
O
67
5
E
LE –
SE CO ND
1. 000
S
0
N
C
–
SE
D
O
1. 000
N
C
66
D
67 SEC ON
SE O D
C ND
O
N
D
30
4
10
SHORT CIRCUIT CURRENT - THOUSANDS OF AMPERES
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
3
CONDUCTOR: ALUMINUM
2
INSULATION: CROSSLINKED POLYETHYLENE &
ETHYLENE PROPYLENE RUBBER
1
CURVES BASED ON FORMULA:
.8
 I 2t = .0125 log T2 + 228 
A 
T1 + 228 
.6
WHERE:
.5
I
.4
A = CONDUCTOR AREA - CIRCULAR MILS
t
= SHORT-CIRCUIT CURRENT - AMPERES
= TIME OF SHORT-CIRCUIT - SECONDS
.3
T1 = MAXIMUM OPERATING TEMPERATURE - 90°C
.2
T2 = MAXIMUM SHORT-CIRCUIT TEMPERATURE 250°C
.1
10
8
6
4
2
1 1/0
2/0 3/0 4/0 AWG
250 MCM
CONDUCTOR SIZE
500
1000
Figure 11. Short-Circuit Current Withstand Chart for Aluminum Cables with Crosslinked Polyethylene & Ethylene Propylene Rubber
Insulation
*Copyright 1969 (reaffirmed March, 1992) by the Insulated Cable Engineers Association (ICEA). Permission has been given by ICEA to reprint this chart.
13
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
A. Wire and Cable
It becomes obvious that the word “Minimum” in the
heading of table 250-95 means just that - the values in the
table are a minimum - they may have to be increased due
to the available short-circuit current and the current-limiting,
or non-current-limiting ability of the overcurrent protective
device.
Good engineering practice requires the calculation of
the available short-circuit currents (3-phase and phase-toground values) wherever equipment grounding conductors
are used. Overcurrent protective device (fuse or circuit
breaker) manufacturers’ literature must be consulted. Letthru energies for these devices should be compared with
the short-circuit ratings of the equipment grounding
conductors. Wherever let-thru energies exceed the
“minimum” equipment grounding conductor withstand
ratings, the equipment grounding conductor size must be
increased until the withstand ratings are not exceeded.
Protecting Equipment Grounding Conductors
Safety issues arise when the analysis of equipment
grounding conductors is discussed. Table 250-95 of the
NEC offers minimum sizing for equipment grounding
conductors.
The problem of protecting equipment grounding
conductors was recognized more than 30 years ago when
Eustace Soares, wrote his famous grounding book
“Grounding Electrical Distribution Systems for Safety". In his
book he states that the “validity” rating corresponds to the
amount of energy required to cause the copper to become
loose under a lug after the conductor has had a chance to
cool back down. This validity rating is based upon raising
the copper temperature from 75°C to 250°C.
In addition to this and the ICEA charts, a third method
promoted by Onderdonk allows the calculation of the
energy necessary to cause the conductor to melt (75°C to
1,083°C).
Table 2 offers a summary of these values associated
with various size copper conductors.
Table 2. Comparison of Equipment Grounding Conductor Short-Circuit Withstand Ratings
Conductor
Size
14
12
10
8
6
4
3
2
1
1/0
2/0
3/0
4/0
250
300
350
400
500
600
700
750
800
900
1,000
5 Sec. Rating (Amps)
ICEA
Soares
P32-382
1 Amp/30 cm
Insulation
Validity
Damage
150°C
250°C
97
137
155
218
246
346
391
550
621
875
988
1,391
1,246
1,754
1,571
2,212
1,981
2,790
2,500
3,520
3,150
4,437
3,972
5,593
5,009
7,053
5,918
8,333
7,101
10,000
8,285
11,667
9,468
13,333
11,835
16,667
14,202
20,000
16,569
23,333
17,753
25,000
18,936
26,667
21,303
30,000
23,670
33,333
Onderdonk
Melting
Point
1,083°C
253
401
638
1,015
1,613
2,565
3,234
4,078
5,144
6,490
8,180
10,313
13,005
15,365
18,438
21,511
24,584
30,730
36,876
43,022
46,095
49,168
55,314
61,460
14
I2 t Rating x106 (Ampere Squared Seconds)
ICEA
Soares
Onderdonk
P32-382
1 Amp/30 cm
Melting
Insulation
Validity
Point
Damage
150°C
250°C
1,083°C
.047
.094
.320
.120
.238
.804
.303
.599
2.03
.764
1.51
5.15
1.93
3.83
13.0
4.88
9.67
32.9
7.76
15.4
52.3
12.3
24.5
83.1
19.6
38.9
132.0
31.2
61.9
210.0
49.6
98.4
331.0
78.9
156.0
532.0
125.0
248.0
845.0
175.0
347.0
1,180.0
252.0
500.0
1,700.0
343.0
680.0
2,314.0
448.0
889.0
3,022.0
700.0
1,389.0
4,721.0
1,008.0
2,000.0
6,799.0
1,372.0
2,722.0
9,254.0
1,576.0
3,125.0
10,623.0
1,793.0
3,556.0
12,087.0
2,269.0
4,500.0
15,298.0
2,801.0
5,555.0
18,867.0
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
B. Bus Short-Circuit Rating and Bracing Requirements
Bus Short-Circuit Rating Requirements When Protected by
Current-Limiting Fuses
NEMA Standards require that busways have a
symmetrical short-circuit withstand rating at least as great
as the average available symmetrical short-circuit current.*
Since the shor t-circuit ratings of busways are
established on the basis of minimum three-cycle duration
tests, these ratings will not apply unless the protective
device used will remove the fault within three cycles or
less.*
BUSWAYS MAY BE USED ON CIRCUITS HAVING
AVAILABLE SHORT-CIRCUIT CURRENTS GREATER THAN
THE BUSWAY RATING WHEN PROPERLY COORDINATED,
AND RATED WITH CURRENT-LIMITING DEVICES.*
If a busway has been listed or labeled for a maximum
short-circuit current with a specific overcurrent device, it
cannot be used where greater fault currents are available
without violating the listing or labeling. If a busway has
been listed or labeled for a maximum short-circuit current
without a specific overcurrent device (i.e., for three cycles),
current-limiting fuses can be used to reduce the available
short-circuit current to within the withstand rating of the
busway.
Refer to Figure 12 for an analysis of the short-circuit
rating requirements for the 800 ampere plug-in bus.
65,000 Amps
RMS Sym
Available
KRP-C800SP Amp LOW-PEAK®
Time-Delay Fuses
800 Amp Switch
Bracing Required?
The busway short-circuit short time rating has a
mechanical limit. Exceeding this limit invites mechanical
damage due to the high magnetic forces associated with
the peak current of the fault. The mechanical limit typically
applies for high faults near and below the busway shortcircuit rating. Allowable durations of short-circuit current,
longer than the 3-cycles at 60 Hz (0.05 seconds) required
at the maximum short-circuit rating, are obtained from a
constant I2t “mechanical damage limit” curve.
Typically, for currents below one-half of the short-circuit
current rating, where mechanical stresses are reduced to
one-quar ter of those at the maximum rating, the
mechanical capabilities become less important than the
thermal capability. The lower limit duration at one-half the
busway rating is determined by the busway thermal (I2t)
capabilities.
The following examples compare busway short-circuit
overcurrent protection by low voltage circuit breakers and
current-limiting fuses. This study looks at the development
of the busway mechanical withstand curves and the timecurrent curves of the breakers and fuses.
In this example, the 800 ampere plug-in busway has a
65 kA short-circuit rating.
A plot of the busway mechanical limit characteristic on
log-log paper (Figure 13) passes through the short-circuit
rating at (65 kA, 0.05 seconds) and is a constant I2t down
to 32.5 kA (one-half the short-circuit rating of 65 kA).
Assume the available short-circuit current at the
busways is equal to the 65 kA rating. The overcurrent
devices are assumed to have the proper interrupting
capacity.
In order to coordinate selectively with circuit breakers
that are instantaneously tripped, the power circuit breaker
protecting the busway does not have an instantaneous trip.
There is a problem with the protection of this busway.
The short time-delay needed to achieve coordination
results in a lack of protection of the 800 ampere busway. A
short-circuit on this busway can result in damage. As noted
on the curve, a 65,000 ampere fault will intersect the
mechanical damage curve before the breaker trips.
This busway would have to be braced to withstand
65,000 amperes of short-circuit current for a minimum of 12
cycles.
A plot of the same system utilizing LOW-PEAK Class L
and Class RK1 fuses is given in Figure 14. Current
limitation by the KRP-C800SP will offer short-circuit
protection for the busway, as it lets through 19,000
amperes.
Short-Circuit
800 Amp Plug-in Bus
Figure 12. Determining the Short-Circuit Ratings of Busway
The 800 ampere plug-in bus in Figure 12 could be
subjected to 65,000 amperes at its line side; however, the
KRP-C800SP ampere LOW-PEAK® time-delay fuse would
limit this available current. Upon checking the Data Section,
page 25, when protected by KRP-C800SP ampere LOWPEAK time-delay fuses, the 800 ampere bus need only be
braced for 19,000 amperes RMS symmetrical. This would
allow a standard 22,000 ampere RMS symmetrical (3cycle) rated bus to be specified, whereas, if a non-currentlimiting type protective device were specified, the bracing
requirements would have been 65,000 amperes for three
cycles.
CURRENT-LIMITING FUSES GENERALLY REDUCE
BUS BRACING REQUIREMENTS TO ALLOW A STANDARD
SHORT-CIRCUIT RATED BUSWAY TO BE SPECIFIED.
When applying air frame circuit breakers with short
time-delay (STD), the engineer must specify additional
short-circuit bracing based on the STD time setting. For
example, an 800 ampere air frame circuit breaker may
have an intentional 18 cycle STD to selectively coordinate
with downstream breakers. It is imperative that the 800
ampere busway also be braced for this 18 cycles to avoid
damage or destruction.
Note: The Busway is protected by the fast speed of
response in the high short-circuit region. Protection is
achieved, as is selective coordination, with the downstream
LPS-RK400SP fuse.
*NEMA Pub. No. BU1-1988.
15
B. Bus Short-Circuit Rating and Bracing Requirements
1,000
Table 3.
NEMA (Standard Short-Circuit Ratings of Busway*)
600
Short-Circuit Current Ratings
(Symmetrical Amperes)
Plug-In Duct
Feeder Duct
10,000
–
14,000
–
22,000
–
22,000
42,000
22,000
42,000
42,000
75,000
42,000
75,000
42,000
75,000
65,000
100,000
65,000
100,000
65,000
150,000
85,000
150,000
85,000
200,000
–
200,000
200
800A AFCB
400A MCCB
100
80
60
40
30
20
10
8
800A
AFCB
6
4
3
2
400A CB
1
.8
.6
800A Plug-in
Busway
.4
.3
.2
.1
.08
Short Time
Delay - 6 Cycles
.06
.04
Busway
Mechanical
Capability
.03
Figure 13.
16
80,000
100,000
60,000
40,000
30,000
20,000
8,000
CURRENT IN AMPERES
10,000
6,000
4,000
3,000
2,000
800
600
400
.01
1,000
.02
300
*Reprinted with permission of
NEMA, Pub. No. BU1-1988.
300
200
Table 3 pertains to feeder and plug-in
busway. For switchboard and panelboard
standard ratings refer to manufacturer.
U.L. Standard 891 details short-circuit
durations for busway within switchboards
for a minimum of three cycles, unless the
main overcurrent device clears the short in
less than three cycles.
400
100
Continuous Current
Rating of Busway
(Amperes)
100
225
400
600
800
1000
1200
1350
1600
2000
2500
3000
4000
5000
800
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
65,000A
Short-Circuit
.01
CURRENT IN AMPERES
Figure 14.
17
80,000
100,000
60,000
.04
40,000
.06
30,000
20,000
10,000
8,000
6,000
4,000
3,000
2,000
1,000
800
600
400
2
300
200
100
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
B. Bus Short-Circuit Rating and Bracing Requirements
1,000
800
600
400
300
200
LPS-RK400SP
100
KRP-C800SP
80
60
40
30
20
10
8
6
4
KRP-C800SP
3
LPS-RK400SP
.8
1
.6
.4
.3
.2
.08
.1
Busway
Mechanical
Capability
.03
.02
65,000A
Short-Circuit
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
C. Low Voltage Motor Controllers
The diagram in Figure 15 shows a Size 2, combination
motor controller supplying a 460 volt, 3Ø, 20HP motor. The
short-circuit withstand of this and other motor controllers
are established so that they may be properly protected
from short-circuit damage.
40,000 RMS
Symmetrical
Available
3Ø, 460V
In order to properly select a branch circuit protective
device that not only provides motor branch circuit
protection, but also protects the circuit components from
damage, the designer must look beyond mere safety
standards. Coordination of the branch circuit protective
device and the motor starter is necessary to insure that
there will be no damage or danger to either the starter or
the surrounding equipment. There is an IEC (International
Electrotechnical Commission) Standard that offers
guidance in evaluating the level of damage likely to occur
during a short-circuit with various branch circuit protective
devices. IEC Publication 947, “Low Voltage Switchgear and
Control, Par t 4-1: Contactors and Motor Star ters",
addresses the coordination between the branch circuit
protective device and the motor starter. It also provides a
method to measure the performance of these devices
should a short-circuit occur. IEC defines two levels of
protection (coordination) for the motor starter:
Type 1. Considerable damage to the contactor and
overload relay is acceptable. Replacement of components
or a completely new starter may be needed. There must be
no discharge of parts beyond the enclosure.
Type 2. No damage is allowed to either the contactor
or overload relay. Light contact welding is allowed, but
must be easily separable.
Where Type 2 protection is desired, the controller
manufacturer must verify that Type 2 protection can be
achieved by using a specified protective device. U.S.
manufacturers have recently begun having both their
NEMA and IEC motor controllers verified to meet the Type 2
requirements outlined in IEC 947-4. As of this writing only
current-limiting fuses have been able to provide the
current-limitation necessary to provide verified Type 2
protection. In many cases, Class J, Class RK1, or Class CC
fuses are required, because Class RK5 fuses and circuit
breakers aren’t fast enough under short-circuit conditions
to provide Type 2 protection.
Section 430-52 of the National Electrical Code allows
Dual Element Time-Delay fuses and other overcurrent
protective devices to be sized for branch circuit protection
(short-circuit protection only). Controller manufacturers
often affix labels to the inside of the motor starter cover
which recommend the maximum size fuse for each
overload relay size.
A paragraph in Section 430-52 states:
“Where maximum branch circuit protective device
ratings are shown in the manufacturer’s overload relay table
for use with a motor controller or are otherwise marked on
the equipment, they shall not be exceeded even if higher
values are allowed as shown above.”**
This paragraph means that the branch circuit
overcurrent protection for overload relays in motor
controllers must be no greater than the maximum size as
shown in the manufacturer’s overload relay table. These
maximum branch circuit sizes must be observed even
though other portions of Section 430-52 allow larger sizing
of branch circuit overcurrent protection.
The reason for this maximum overcurrent device size is
to provide short-circuit protection for the overload relays
and motor controller.
20HP
M 3Ø, 460V
27 F.L.A.
Low-Peak®
Dual Element
Time Delay Fuse
Typical Size 2 Controller
Figure 15. Short-Circuit Protection of Motor Controller
There are several independent organizations engaged
in regular testing of motor controllers under short-circuit
conditions. One of these, Underwriter’s Laboratories, tests
controllers rated one horsepower or less and 300 volts or
less with 1000 amperes short-circuit current available to the
controller test circuit. Controllers rated 50HP or less are
tested with 5000 amperes available and controllers rated
above 50HP to 200HP are tested with 10,000 amperes
available. See Table 4 for these values.*
Table 4.
Motor Controller
HP Rating
1HP or less and 300V or less
50HP or less
Greater than 50HP to 200HP
201HP to 400HP
401HP to 600HP
601HP to 900HP
901HP to 1600HP
Test Short Circuit
Current Available
1,000A
5,000A
10,000A
18,000A
30,000A
42,000A
85,000A
It should be noted that these are basic short-circuit
requirements. Higher, combination ratings are attainable if
tested to an applicable standard. However, damage is
usually allowed.
Type 1 vs. Type 2 Protection
UL has developed a short-circuit test procedure
designed to verify that motor controllers will not be a safety
hazard and will not cause a fire.
Compliance to the standard allows deformation of the
enclosure, but the door must not be blown open and it must
be possible to open the door after the test. In the standard
short-circuit tests, the contacts must not disintegrate, but
welding of the contacts is considered acceptable. When
testing with fuses, damage to the overload relay is not
allowed, and it must perform in accordance with the
calibration requirements. Tests with circuit breakers allow
the overload relay to be damaged with burnout of the
current element completely acceptable.
For short-circuit ratings in excess of the standard
levels listed in UL508, the damage allowed is even more
severe. Welding or complete disintegration of contacts is
acceptable and complete burnout of the overload relay is
allowed. Therefore, a user cannot be certain that the motor
starter will not be damaged just because it has been U.L.
Listed for use with a specific branch circuit protective
device. U.L. tests are for safety, but do allow a significant
amount of damage as long as it is contained within the
enclosure.
**“Above” refers to other portions of Section 430-52 not shown here.
*From Industrial Control Equipment, U.L. #508.
18
D. Molded Case Circuit Breakers
Ip = 48,026A
Until recently, molded case circuit breakers were
protected the same way as other electrical equipment.
Quicker acting circuit breakers, as well as test circuits that
cause short-circuit test parameters to change, have
required additional considerations in recommended
protection procedures.
As has been discussed previously, the two parameters
IRMS and Ip, must be compared to the equipment withstand
rating. The rule is simple: if the RMS and peak let-thru value
of the fuse are less than the equipment withstand rating,
the equipment will be protected. This philosophy holds true
for various static components, such as wire and cable,
busway, and motor star ters. This basic protection
requirement is mandated in NEC Section 110-10. It will also
be true for non-current-limiting circuit breakers when their
opening time is greater than one-half cycle.
In the past, as long as the fuse let-thru values were
less than the breaker’s interrupting rating, the system was
considered sound. THIS METHOD HAS A SOLID HISTORY
OF SUCCESSFUL APPLICATIONS. However, due to
changes in circuit breaker design, the method may not
always work with today’s circuit breakers. Selecting a
current-limiting fuse to protect a downstream molded case
circuit breaker has now become an increasingly more
complex problem.
IRMS = 22,000A
Amps
P.F. = 20%
IRMS = 22,000 Amp
Time
Figure 16
S.C.P.F. = 20%
S.C. Avail. = 22,000A
RLINE
20A
XLINE
RCB
XCB
RLOAD
XLOAD
RS
XS
4' Rated Wire (#12 Cu)
SOURCE:
10" Rated Wire (#12 Cu)
Note: For calculations, R CB and X CB are assumed negligible.
Figure 17
Quicker Operating Circuit Breakers
Simply put, if the total clearing energy of a quicker
acting molded case circuit breaker is less than the melting
energy of a larger upstream fuse, the molded case circuit
breaker will interrupt the full value of the system fault
without the benefit of the fuse’s current-limiting effect. This
situation can have catastrophic effects on the circuit
breaker as it tries to interrupt faults beyond its interrupting
capacity. Currently, there is no available engineering
method to predict protection of these faster breakers.
Standard interrupting rating tests will allow for a
maximum 4-foot rated wire on the line side, and a 10-inch
rated wire on the load side of the circuit breaker.
Performing a short-circuit analysis of this test circuit results
in the following short-circuit parameters, as seen by the
circuit breaker.
Actual short-circuit RMS current = 9,900 amperes RMS
symmetrical
Actual short-circuit power factor = 88%
Actual short-circuit peak current = 14,001 amperes
Molded Case Circuit Breakers - Agency Test Procedures
Some agency standards allow a unique test set-up for
testing circuit breaker interrupting ratings. Figure 16
illustrates a typical calibrated test circuit waveform for a
20A, 240-volt, two-pole molded case circuit breaker, with a
marked interrupting rating of 22,000 amperes RMS
symmetrical. Figure 17 also illustrates the calibration
required by the standard, and the maximum peak current
available at a 20% power factor. However, agency
standards allow for a random close during the short-circuit
test, so the peak available current may be as low as 1.414
times the RMS current for one- and two-pole circuit
breakers. For three-pole circuit breakers, one pole may see
a peak of only 1.414 x RMS. The conservative approach
would therefore assume a 1.414 multiplier also for threepole breakers.
A graphic analysis of this actual short-circuit follows
(Figure 18).
Ip = 14,001A
P.F. = 88%
IRMS = 9,900 Amp
Amps
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
IRMS = 9,900A
Time
Figure 18
Agency standards allow for a random close during
the short-circuit test, so the peak available current may
be as low as 1.414 times the RMS symmetrical current.
Thus, the circuit breaker is actually tested to interrupt
9,900 amperes at 88% power factor, not 22,000 amperes at
20% power factor.
19
D. Molded Case Circuit Breakers
Figure 19 shows the waveforms superimposed for
comparison. Henceforth, this RMS test value will be
identified as the circuit breaker interrupting capacity. (Don’t
confuse this with the circuit breaker marked interrupting
rating.)
Following is a partial table showing the actual Ip and
IRMS values to which a circuit breaker may be tested.
Table 5. 240V - 2-Pole CB Interrupting Capacities (KA)
CB
Rating
15
20
25
30
40
50
60
70
80
90
100
Ip = 48,026A
IRMS = 22,000A
Ip = 14,001A
P.F. = 88%
IRMS = 9,900 Amp
Amps
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
IRMS = 9,900A
Time
10kA
Ip
7.2
8.9
10.7
10.7
11.7
11.7
12.5
13.0
13.0
13.2
13.2
IRMS
5.1
6.3
7.5
7.5
8.3
8.3
8.8
9.2
9.2
9.3
9.3
14kA
Ip
8.7
11.4
14.2
14.2
16.0
16.0
17.3
18.1
18.1
18.3
18.3
IRMS
6.1
8.1
10.1
10.1
11.3
11.3
12.2
12.8
12.8
12.9
12.9
18kA
Ip
9.3
12.6
16.5
16.5
19.2
19.2
21.3
22.6
22.6
23.0
23.0
IRMS
6.6
8.9
11.7
11.7
13.6
13.6
15.1
16.0
16.0
16.3
16.3
After reviewing the values to which the circuit breaker
can be tested (its interrupting capacity) it becomes obvious
that a circuit breaker’s interrupting rating cannot be
considered its short-circuit withstand rating (especially for
breakers with higher interrupting ratings).
Figure 19
“Fully Rated System”: A fully rated system is a
combination of overcurrent devices that have an
interrupting rating equal to or greater than the available
short-circuit current.
The following definitions should be noted:
Interrupting Rating (CB): The marked rating shown
on the Circuit Breaker. It has been established by testing.*
“Series Rated System”: Although there is no official
definition, a series rated system can be described as a
combination of circuit breakers or fuses and breakers that
can be applied at available fault levels above the
interrupting rating of the load side circuit breakers, but not
above that of the main or line side device (formerly known
as a Cascaded System).
Interrupting Capacity (CB): Actual test Ip and IRMS
the circuit breaker sees during the tests for standard circuit
breaker applications.*
Equally important, the short-circuit power factor is
greatly affected due to the high R values of the small, rated
wire. This results in a lower peak value that the circuit
breaker must tolerate during the first one-half cycle.
Bussmann’s recommendation is to use fully rated
overcurrent devices. But, when recently produced lighting
and receptacle circuit breakers are utilized at values
beyond their interrupting rating, the recommended
alternative is to use listed systems which utilize tested and
recognized combinations of main fuses and load side
circuit breakers.
* These definitions paraphrase those given in the IEEE Standard Dictionary of
Electrical and Electronic Terms, page 462, 1984 edition.
20
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
E. Transformers
1. Overload Protection
The National Electrical Code has developed separate
sections and sizing recommendations for fuses with
primary voltages above and below 600 volts, nominal. The
following three paragraphs cover the basic requirements.
See NEC Sections 450-3 and 430-72 for the most common
exceptions.
Section 450-3a covers transformer protection when the
primar y voltage is greater than 600 volts. For low
impedance transformers, fuse protection on the primary
can be sized as high as 300% of primary current.
Secondary protection must be offered at 250% or 125% for
secondary voltages greater than 600 volts, or 600 volts or
less, respectively. See Figures 20 and 21.
Section 450-3b covers transformer protection when the
primary voltage is 600 volts or less. Primary fusing at 125%
of primary current will not require secondary protection.
Note: Secondary conductor and panelboard protection are
most often required by Articles 240 and 384 respectively.
Primary and secondary protection are required when
the primary fuse is greater than 125%. The primary fuse
may be sized no larger than 250% of primary current. The
secondary fuse should then be sized no larger than 125%
of the secondary current.
2. Magnetizing Inrush Currents
Primary fuses must be capable of handling the inrush
currents associated with the transformer during start-up. A
rule of thumb is that the fuse handle 12x full load current for
0.1 seconds, and 25x full load current for 0.01 seconds.
Dual-element time-delay fuses are best suited to meet the
sizing criteria of Article 450 and pass these initial surge
characteristics.
Refer to Bussmann Bulletin EDP II for a discussion of
these inrush points.
Table 6. 450-3(a)(1) Transformers Over 600 Volts
Maximum Rating or Setting for Overcurrent Device
Primary
Secondary
Over 600 Volts
Over 600 Volts
Transformer
Rated
Impedance
Not more
than 6%
More than 6%
and not more
than 10%
Circuit
Breaker
Setting
Fuse
Rating
Circuit
Breaker
Setting
Fuse
Rating
600 Volts
or Below
Circuit Breaker
Setting or
Fuse Rating
600%
300%
300%
250%
125%
400%
300%
250%
225%
125%
Z = 6% (or less)
PRI. SEC.
over 600V
600V or less
3. Short-Circuit Protection - Thermal and Magnetic
Withstand curves for distribution transformers define
how much current a transformer can withstand, and for how
long. As with any electrical component, if these curves are
exceeded the transformer may be damaged or destroyed.
These curves relate to both thermal and mechanical
damage, and are defined by different fault conditions.
Typically, three curves exist for a 3-phase transformer,
defined by phase-phase, phase-phase-phase, and phaseground fault conditions.
It is the designer’s goal to find a fuse time-current
curve that falls to the left of the damage curves and to the
right of the transformer inrush points.
Refer to Bussmann Bulletin EDP II for a discussion of
how to analyze these curves and protection levels.
Unsupervised
Location
Fuse at 125% of
F.L.A. of secondary
Fuse at 300% of
F.L.A. of primary
Figure 20
PRIMARY PROTECTION ONLY
PRI. & SEC.
600V or less
Fuse must not be
larger than 125%
of F.L.A. of primary
No secondary
protection
PRIMARY AND SECONDARY PROTECTION ONLY
PRI. & SEC.
600V or less
Fuse no larger than
250% of F.L.A.
of primary when
secondary fuses
are provided at 125%
125% of F.L.A.
of secondary
(except as noted above)
Figure 21
21
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
F. Ballasts
The National Electrical Code requires integral thermal
protection for ballasts in Section 410-73(e).
Testing agencies list ballasts for general use in lighting
fixtures which pass specific thermal and short-circuit tests.
The ballast must incorporate a thermal protector to sense
certain over-temperature conditions and must also be able
to withstand 200 amperes of short-circuit current when
tested with a 20 ampere fuse. See Figure 22 for a typical
test for ballasts.
Most systems today will deliver more than 200
amperes of short-circuit current to a row of fixtures. (See
Figure 23.) The fixtures should, therefore, be specified to
incorporate individual ballast fusing within the fixture and
external to the ballast.
Fusing each fixture, as shown in Figure 23, will also
provide isolation of the faulted ballast and reduce costly
and dangerous blackouts. When a ballast does fail, only
the fuse protecting that individual fixture opens - the
remaining fixtures continue in normal operation. Without this
individual ballast protection, a faulted ballast could cause
the branch circuit protective device to open, thereby
shutting off all the lights. With individual fusing, the
maintenance electrician can trouble shoot the problem
much more quickly because only one fixture is “out”. And
this trouble shooting can be performed as part of a
scheduled maintenance procedure. It doesn’t have to
become an “emergency” because employees are left in the
dark.
Short
Thermal Protector
Ballast
200A
0.9-1.0 P.F.
20 Amp Fuse
Ballast Winding
Figure 22. Underwriters’ Laboratories Short-Circuit Test for
Ballast Protectors.
20' #10 THW Wire
277 Volt
Lighting
Panel
2,000 Amperes Available
Row of Lighting
Fixtures
Fuse
Opens
Fixture
Faulted Ballast
Ballasts
Figure 23. Fusing Fixture Ballasts to Provide Short-Circuit
Protection and Isolation of Faulted Ballast. Good
Ballasts Remain on the Line.
Note: Refer to fixture manufacturer for recommended fuse
size.
22
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
G. Transfer Switches
Table 7. U.L. 1008 Minimum Withstand Test Requirement
Transfer switches are designed to transfer power
sources under load in order to feed a system, typically an
emergency system, on critical loads. These devices are
tested to meet basic short-circuit testing requirements.
Transfer switches are often tested per U.L. Standard 1008.
Transfer switches should always be evaluated on the
basis of the maximum available short-circuit currents. The
automatic transfer switch must withstand: a) the magnetic
stresses imposed by the instantaneous peak current
available at the point of application, and b) the thermal
stresses imposed by the available RMS short-circuit
current. The short-circuit current withstand rating of the
transfer switch must be equal to or greater than the
available short-circuit current at the point of application.
When properly coordinated with current-limiting
devices, automatic transfer switches can be used on
circuits having available short-circuit currents greater than
their unprotected withstand short-circuit current rating.
Modern current-limiting fuses, when properly sized, limit the
short-circuit current to within the withstand rating of a
transfer switch.
Transfer switches must withstand minimum short-circuit
currents at specified power factors, as listed in U.L.
Standard 1008, until the overcurrent protective devices
open. See Table 7.
Automatic Transfer
Switch Rating
100 Amps or less
101-400 Amps
401 Amps and greater
U.L. Minimum
Current Amps
5,000
10,000
20 times rating
but not less
than 10,000 Amps
U.L. Test Current
Power Factor
40% to 50%
40% to 50%
40% to 50% for
current of 10,000
Amps.
OR
25% to 30% for
currents of 20,000
Amps or less.
OR
20% or less for
current greater
than 20,000 Amps.
Transfer switch manufacturers generally publish the
withstand rating data for their products. When the available
short-circuit current exceeds the withstand rating of the
transfer switch, current-limitation is required. Properly sized
modern current-limiting fuses ahead of the transfer switch
limit the available short-circuit current to within the
withstand rating of a transfer switch, thereby protecting the
transfer switch. The transfer switch manufacturer will mark
the equipment with the fuse class and rating required to
achieve these higher short-circuit ratings.
23
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Protecting System Components
H. HVAC Equipment
Table 8. Short-Circuit Test Currents*
Heating and cooling equipment must meet short-circuit
test requirements in U.L. Standard 1995 and CSA-C22.2
No. 236-M90. Short-circuit tests are conducted at various
levels, up to a maximum of only 5000 amperes, depending
on the rated current and voltage of the equipment.
Where available fault currents exceed the values given
in Table 55.1 of U.L. 1995 (Table 8 at right) it is necessary
to specify a current limiting device to reduce the available
current down to within the withstand capabilities of the
equipment.
Class J and Class RK1 dual-element current-limiting
fuses will offer the best component short-circuit protection
and current-limiting characteristics for this type of
equipment.
Product Ratings, A
Single-Phase
110-120V
200-208V
220-240V
9.8 or less
5.4 or less
4.9 or less
9.9-16.0
5.5-8.8
5.0-8.0
16.1-34.0
8.9-18.6
8.1-17.0
34.1-80.0
18.7-44.0
17.1-40.0
Over 80.0
Over 44.0
Over 40.0
3-Phase
200-208V
220-240V
440-480V
2.12 or less
2.0 or less
–
2.13-3.7
2.1-3.5
1.8 or less
3.8-9.5
3.6-9.0
–
9.6-23.3
9.1-22.0
–
Over 23.3
Over 22.0
Over 1.8
*Table 55.1 of U.L. Standard 1995.
24
254-277V
–
6.65 or less
–
–
Over 6.65
550-600V
–
1.4 or less
–
–
Over 1.4
Circuit Capacity,
A
200
1000
2000
3500
5000
Circuit Capacity,
A
200
1000
2000
3500
5000
Low-Peak Yellow T M KRP-C_SP Fuses
Data Section Index
Page
1. LOW-PEAK YELLOW™ Class L Time-Delay Fuses KRP-C_SP …………………………………………………25
2. LOW-PEAK YELLOW™ Class J Dual-Element, Time-Delay Fuses LPJ_SP ……………………………………26
3. LOW-PEAK YELLOW™ Class RK1 Dual-Element Time-Delay Fuses LPN-RK_SP, LPS-RK_SP ………………27
4. FUSETRON® Class RK5 Dual-Element Time-Delay Fuses FRN-R, FRS-R ……………………………………28
5. TRON® Class T Fast-Acting Fuses JJN, JJS ………………………………………………………………………29
6. LIMITRON® Class RK1 Fast-Acting Fuses KTN-R, KTS-R ………………………………………………………30
7. LIMITRON® Class J Fast-Acting Fuses JKS ………………………………………………………………………31
LOW-PEAK YELLOW™ Class L Time-Delay Fuses KRP-C_SP
B
400,000
6,000
5,000
4,000
3,000
2,500
2,000
1,600
1,200
800
601
300,000
200,000
100,000
80,000
AMPERE
RATING
60,000
50,000
40,000
30,000
20,000
10,000
8,000
6,000
5,000
4,000
3,000
A
200,000
80,000
100,000
40,000
50,000
60,000
30,000
20,000
8,000
10,000
4,000
5,000
6,000
3,000
1,000
2,000
2,000
1,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Buss® Fuse Current- Limiting Let-Thru Charts
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
KRP-C_SP Fuse – RMS & Peak Let-Thru Currents (kA)
Prosp.
Short
C.C.
Fuse Size
601
IRMS
Ip
800
IRMS
Ip
1200
IRMS
Ip
1600
IRMS
Ip
2000
IRMS
Ip
2500
IRMS
Ip
3000
IRMS
Ip
4000
IRMS
Ip
5000
IRMS
Ip
6000
IRMS Ip
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
5
7
9
10
11
11
12
13
14
15
16
16
17
18
21
23
5
10
11
12
13
14
15
16
17
18
19
20
21
22
25
28
12
22
25
28
30
33
35
36
39
41
44
46
49
50
58
64
5
10
14
15
17
18
18
19
22
24
25
27
28
29
34
37
12
23
32
35
38
41
42
43
50
55
58
61
64
66
78
86
5
10
15
20
22
24
25
26
28
30
31
32
34
35
39
43
12
23
35
46
51
55
58
60
64
69
71
74
78
80
90
100
5
10
15
20
25
26
28
29
31
33
35
37
38
39
46
50
12
23
35
46
57
60
64
66
72
76
80
84
88
90
105
115
5
10
15
20
25
29
30
32
35
38
41
43
46
48
57
65
12
23
35
46
57
66
70
74
81
88
94
100
105
110
130
150
5
10
15
20
25
30
35
38
43
46
48
52
54
57
70
78
12
23
35
46
57
69
81
88
98
105
110
120
125
130
160
180
5
10
15
20
25
30
35
40
48
52
56
59
63
65
83
96
12
23
35
46
57
69
81
92
110
120
128
135
145
150
190
220
5
10
15
20
25
30
35
40
50
60
65
70
74
78
96
109
12
23
35
46
57
69
81
92
115
138
150
160
170
180
220
250
5
10
15
20
25
30
35
40
50
60
70
80
85
89
113
130
12
17
20
23
24
26
28
29
32
34
36
37
39
40
48
52
Note: For Ip and IRMS values at 300,000 amperes, consult Factory.
25
12
23
35
46
57
69
81
92
115
138
161
184
195
205
260
300
Low-Peak Yellow T M LPJ_SP Fuses
LOW-PEAK YELLOW™ Class J, Dual-Element Time-Delay Fuses
LPJ_SP
400
300
AMPERE
RATING
400
300
A
200
100
100
200,000
60,000
80,000
100,000
30,000
40,000
20,000
6,000
8,000
10,000
3,000
4,000
2,000
600
800
1,000
300
400
100
200
200
1,000
800
600
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
LPJ_SP – RMS & Peak Let-Thru Currents (kA)
Prosp.
Short
C.C.
Fuse Size
15
IRMS
Ip
30
IRMS
Ip
60
IRMS
Ip
100
IRMS
Ip
200
IRMS
Ip
400
IRMS
Ip
600
IRMS
Ip
1,000
3,000
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
80,000
100,000
150,000
200,000
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
2
0
1
1
1
1
1
1
2
2
2
2
2
2
2
2
3
1
2
2
2
3
3
3
4
4
4
4
4
5
5
6
6
1
1
1
2
2
2
2
2
3
3
3
3
3
4
4
4
2
3
3
4
4
5
5
5
6
6
6
7
7
8
9
10
1
2
2
2
3
3
3
4
4
4
4
5
5
5
6
7
2
4
4
6
7
7
8
8
9
9
10
11
12
12
14
16
1
2
3
4
4
4
5
5
5
6
6
6
7
8
9
10
2
5
6
8
9
10
12
12
12
13
14
15
17
18
21
23
1
3
4
6
6
7
8
8
8
9
9
10
11
12
14
16
2
7
10
13
15
16
17
18
19
21
22
23
26
28
33
36
1
3
5
8
9
10
11
12
13
13
14
15
16
17
19
21
2
7
12
18
21
23
26
27
29
31
32
35
37
40
44
47
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
4
200,000
1,000
800
600
2,000
60,000
80,000
100,000
2,000
4,000
3,000
20,000
A
6,000
8,000
10,000
4,000
3,000
3,000
4,000
AMPERE
RATING
1
100
60
50
40
30
20
15
10,000
8,000
6,000
200
10,000
8,000
6,000
200
20,000
2,000
7
6
3
20,000
600
400
40,000
30,000
600
800
1,000
10
300
400
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
40,000
30,000
100
B
100,000
80,000
60,000
30,000
40,000
B
100,000
80,000
60,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Buss® Fuse Current- Limiting Let-Thru Charts
Note: For Ip and IRMS values at 300,000 amperes, consult Factory.
26
Low-Peak YellowT M LPN-RK_SP, LPS-RK_SP Fuses
LOW-PEAK YELLOW™ Class RK1 Dual-Element Time-Delay Fuses
LPN-RK_SP
LOW-PEAK YELLOW™ Class RK1 Dual-Element Time-Delay Fuses
LPS-RK_SP
B
400,000
300,000
80,000
100,000
40,000
50,000
60,000
30,000
20,000
8,000
10,000
4,000
5,000
6,000
3,000
2,000
1,000
200,000
60
IRMS
1
1
1
2
2
2
2
3
3
3
3
3
3
3
4
4
4
4
5
Ip
2
3
3
4
4
5
6
6
6
7
7
7
8
8
8
9
9
10
11
100
IRMS
1
2
2
2
2
3
3
3
3
4
4
4
4
4
5
5
5
5
6
Ip
2
4
4
5
6
6
7
7
8
8
9
9
10
10
11
11
11
13
14
200
IRMS
1
2
3
3
4
4
5
5
5
6
6
6
7
7
7
7
8
8
9
Ip
2
5
6
7
9
10
11
12
12
13
13
14
15
16
16
17
18
19
21
400
IRMS
1
2
3
5
7
7
8
9
9
10
10
10
11
12
12
13
13
16
18
AMPERE
RATING
PROSPECTIVE SHORT CIRCUIT-CURRENT–SYMMETRICAL RMS AMPS
LPN-RK_SP – RMS & Peak Let-Thru Currents (kA)
Fuse Size
30
IRMS Ip
1
1
1
2
1
2
1
2
1
3
1
3
1
3
1
3
2
3
2
4
2
4
2
4
2
4
2
4
2
5
2
5
2
5
2
6
3
6
A
2,000
1,000
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
Prosp.
Short
C.C.
1,000
2,000
3,000
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
3,000
200,000
A
2,000
30
6,000
5,000
4,000
80,000
100,000
3,000
8,000
40,000
50,000
60,000
30
6,000
5,000
4,000
100
60
10,000
30,000
8,000
200
20,000
20,000
100
60
10,000
400
30,000
8,000
10,000
200
20,000
600
4,000
5,000
6,000
30,000
60,000
50,000
40,000
3,000
600
400
100,000
80,000
2,000
60,000
50,000
40,000
200,000
1,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
100,000
80,000
AMPERE
RATING
200,000
1,000
B
400,000
300,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Buss® Fuse Current- Limiting Let-Thru Charts
LPS-RK_SP – RMS & Peak Let-Thru Currents (kA)
Ip
2
5
7
12
15
17
19
20
21
22
23
24
26
27
28
29
30
36
42
600
IRMS
1
2
3
5
9
10
11
12
13
13
13
14
15
16
17
17
17
20
22
Prosp.
Short
C.C.
1,000
2,000
3,000
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
Ip
2
5
7
12
21
23
25
27
29
30
31
33
35
36
38
39
40
46
50
Fuse Size
30
IRMS Ip
1
1
1
2
1
2
1
2
1
3
1
3
1
3
2
4
2
4
2
4
2
4
2
5
2
5
2
5
2
5
2
5
2
6
3
6
3
7
60
IRMS
1
1
1
2
2
2
3
3
3
3
3
3
3
4
4
4
4
5
5
Ip
2
3
3
4
5
5
6
6
6
7
7
8
8
8
9
9
9
11
12
100
IRMS
1
2
2
2
3
3
3
3
4
4
4
4
4
5
5
5
5
6
7
Ip
2
4
4
5
6
7
7
8
8
9
9
10
10
11
11
12
12
14
15
200
IRMS
1
2
3
3
4
5
5
5
6
6
6
7
7
7
8
8
8
9
10
Ip
2
4
6
7
9
11
12
12
13
14
14
15
16
17
18
18
19
21
23
400
IRMS
1
2
3
5
7
8
8
9
10
10
10
11
12
13
13
13
14
16
17
Ip
2
4
7
12
16
18
19
21
22
23
24
26
28
29
30
31
32
36
40
Note: For Ip and IRMS values at 300,000 amperes, consult Factory.
Note: For Ip and IRMS values at 300,000 amperes, consult Factory.
27
600
IRMS
1
2
3
5
9
10
11
12
13
13
14
15
16
17
17
18
19
22
23
Ip
2
4
7
12
21
24
26
28
30
31
32
35
37
39
40
42
44
50
54
Buss Fuse Let-Thru Charts –
FUSETRON® Class RK5 Dual-Element Time-Delay Fuses
FRN-R
300,000
300,000
30
8,000
6,000
5,000
4,000
3,000
A
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
Fuse Size
30
IRMS Ip
1
2
1
3
1
3
2
4
2
4
2
4
2
4
2
5
2
5
2
6
3
6
3
6
3
6
3
7
3
8
4
8
60
IRMS
2
3
3
4
4
4
5
5
5
6
6
6
7
7
8
8
Ip
5
6
7
8
9
10
11
11
12
13
14
15
15
16
18
20
100
IRMS
3
5
6
7
7
7
8
8
9
9
10
10
10
10
11
12
Ip
8
11
13
15
16
17
18
19
20
21
22
23
23
24
26
27
200
IRMS
5
7
8
8
9
10
11
11
12
12
13
13
14
14
16
17
Ip
11
15
18
20
21
23
25
25
27
28
30
31
32
33
37
40
6,000
5,000
4,000
3,000
A
2,000
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
FRN-R – RMS & Peak Let-Thru Currents (kA)
Prosp.
Short
C.C.
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
8,000
1,000
200,000
80,000
100,000
40,000
50,000
60,000
30,000
20,000
8,000
10,000
4,000
5,000
6,000
3,000
2,000
1,000
2,000
30
10,000
200,000
10,000
100
60
20,000
80,000
100,000
60
40,000
50,000
60,000
20,000
30,000
30,000
100
20,000
30,000
200
8,000
10,000
200
600
400
60,000
50,000
40,000
4,000
5,000
6,000
60,000
50,000
40,000
100,000
80,000
3,000
600
400
2,000
100,000
80,000
200,000
1,000
AMPERE
RATING
200,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
400,000
AMPERE
RATING
B
400,000
1,000
FRN-R, FRS-R
FUSETRON® Class RK5 Dual-Element Time-Delay Fuses
FRS-R
B
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
FRS-R – RMS & Peak Let-Thru Currents (kA)
400
IRMS
5
9
11
12
13
14
15
15
16
17
18
19
19
20
23
24
Ip
12
21
25
28
30
32
34
35
37
40
41
43
44
46
52
56
600
IRMS
5
10
14
16
17
19
20
20
22
23
24
25
26
27
30
32
Prosp.
Short
C.C.
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
Ip
12
23
33
37
40
43
45
47
50
54
56
58
60
62
70
74
28
Fuse Size
30
IRMS Ip
1
3
2
4
2
5
2
6
3
6
3
7
3
7
3
7
3
8
4
9
4
9
4
9
4
10
4
10
5
12
6
13
60
IRMS
2
3
3
3
4
4
4
5
5
5
6
6
6
7
7
8
Ip
4
6
7
8
9
9
10
11
12
12
13
14
14
15
17
19
100
IRMS
3
5
5
6
7
7
7
8
8
9
9
9
10
10
11
11
Ip
8
11
13
14
16
17
17
18
19
20
21
22
22
23
25
26
200
IRMS
5
7
8
10
10
11
12
12
13
14
15
15
16
16
18
19
Ip
12
16
19
22
24
25
27
28
30
32
34
35
36
37
42
44
400
IRMS
5
10
13
14
16
17
18
19
20
21
22
23
23
24
26
27
Ip
12
23
30
33
37
39
41
43
46
49
50
52
54
56
60
63
600
IRMS
5
10
15
17
19
20
22
23
24
26
27
28
29
30
33
35
Ip
12
23
35
40
44
47
50
52
56
60
62
64
66
68
75
80
Buss Fuse Let-Thru Charts –
TRON® Class T Fast-Acting Fuses JJS
B
B
4,000
3,000
2,000
1,000
800
600
400
300
A
15
4,000
3,000
2,000
1,000
800
600
400
300
A
300
400
200
100
200,000
60,000
80,000
100,000
30,000
40,000
20,000
6,000
8,000
10,000
3,000
4,000
2,000
600
800
1,000
300
400
200
100
200
60,000
80,000
100,000
60
30
15
100
60
30
10,000
8,000
6,000
30,000
40,000
10,000
8,000
6,000
200
20,000
20,000
400
200
100
20,000
40,000
30,000
6,000
8,000
10,000
40,000
30,000
800
600
400
3,000
4,000
1200
800
600
100,000
80,000
60,000
2,000
100,000
80,000
60,000
200,000
600
800
1,000
AMPERE
RATING
200,000
AMPERE
RATING
400,000
300,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
400,000
300,000
200
JJN, JJS
200,000
TRON® Class T Fast-Acting Fuses JJN
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
JJN – RMS & Peak Let-Thru Current (kA)
Prosp.
Short
C.C.
500
1,000
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
Fuse Size
15
IRMS
Ip
0
0
0
1
0
1
1
1
1
1
1
2
1
2
1
2
1
2
1
2
1
2
1
2
1
3
1
3
1
3
1
3
1
3
2
4
30
IRMS
0
0
1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
60
IRMS
0
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
Ip
1
1
1
2
2
2
2
3
3
3
3
3
3
4
4
4
4
5
100
IRMS
1
1
1
2
2
2
2
2
3
3
3
3
3
3
3
4
4
4
Ip
1
1
2
3
3
3
4
4
4
4
4
5
5
5
5
6
6
7
Ip
1
2
3
4
4
5
5
5
6
6
6
7
7
8
8
8
9
10
200
IRMS
1
1
2
2
3
3
3
3
4
4
4
4
5
5
6
6
6
7
Ip
1
2
4
6
6
7
8
8
9
9
10
10
11
13
13
14
14
15
400
IRMS
1
1
3
4
4
5
5
5
5
6
6
7
7
7
8
8
9
9
600
IRMS
1
1
5
6
6
7
7
8
8
9
9
10
10
11
11
12
13
15
Ip
1
2
7
9
10
11
12
12
13
14
14
15
16
17
18
19
20
21
800
IRMS
1
1
5
7
9
10
10
11
11
11
12
13
14
15
15
16
17
19
Ip
1
2
11
13
15
16
17
18
19
20
22
23
24
25
26
27
30
34
Ip
1
2
12
17
20
22
23
25
25
26
28
30
32
34
35
36
40
44
1200
IRMS
1
1
5
9
10
11
12
13
13
13
15
16
17
17
18
19
22
23
Ip
1
2
12
20
23
25
27
29
30
31
34
36
38
40
42
44
50
54
JJS – RMS & Peak Let Thru Current (kA)
Prosp.
Short
C.C.
500
1,000
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
Fuse Size
15
IRMS
Ip
0
1
0
1
1
1
1
2
1
2
1
2
1
2
1
2
1
2
1
3
1
3
1
3
1
3
1
3
1
3
2
4
2
4
2
4
30
IRMS
0
0
1
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
Ip
1
1
2
2
3
3
3
3
3
4
4
4
5
5
5
5
6
7
60
IRMS
0
1
1
1
2
2
2
2
2
2
2
3
3
3
3
3
4
4
Ip
1
1
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
9
100
IRMS
1
1
2
2
3
3
3
3
3
3
4
4
4
4
4
5
6
6
Ip
1
2
4
5
6
6
7
7
8
8
9
9
9
10
10
11
13
14
29
200
IRMS
1
1
3
3
4
4
5
5
5
5
6
6
7
7
7
7
8
9
Ip
1
2
7
8
9
10
11
12
12
13
13
14
15
16
17
17
18
20
400
IRMS
1
1
4
6
7
7
7
8
9
9
10
10
11
11
12
12
14
16
Ip
1
2
10
13
15
17
17
19
20
21
22
24
25
26
27
28
32
36
600
IRMS
1
1
5
8
10
10
11
12
13
13
14
16
17
17
18
19
22
24
Ip
1
2
12
19
22
24
26
28
30
31
33
36
39
40
42
44
50
56
800
IRMS
1
1
5
9
11
12
13
14
15
15
17
18
19
20
21
22
25
28
Ip
1
2
12
21
25
27
30
32
34
35
38
41
44
46
48
50
58
64
Buss Fuse Let-Thru Charts –
LIMITRON® Class RK1 Fast-Acting Fuses KTN-R
B
3,000
A
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
60
IRMS
1
1
2
2
2
2
2
2
2
2
3
3
3
3
3
3
Ip
3
3
4
4
5
5
5
5
5
6
6
6
6
7
7
8
100
IRMS
2
2
2
3
3
3
3
3
4
4
4
4
5
5
5
6
Ip
4
5
6
6
7
7
8
8
9
9
9
10
10
11
13
14
200
IRMS
3
3
4
4
4
5
5
5
6
6
6
7
7
7
8
9
Ip
6
8
9
10
10
11
12
12
13
14
15
15
16
17
19
21
AMPERE
RATING
A
2,000
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
KTN-R – RMS & Peak Let-Thru Currents (kA)
Fuse Size
30
IRMS Ip
1
2
1
2
1
3
1
3
1
3
1
3
1
3
1
3
2
4
2
4
2
4
2
4
2
4
2
4
2
5
2
5
3,000
1,000
200,000
80,000
100,000
40,000
50,000
60,000
30,000
20,000
8,000
10,000
4,000
5,000
6,000
3,000
2,000
1,000
2,000
30
6,000
5,000
4,000
200,000
30
60
8,000
80,000
100,000
6,000
5,000
4,000
100
10,000
40,000
50,000
60,000
60
8,000
200
20,000
30,000
100
10,000
30,000
20,000
200
20,000
8,000
10,000
30,000
400
4,000
5,000
6,000
400
600
60,000
50,000
40,000
3,000
600
60,000
50,000
40,000
100,000
80,000
2,000
AMPERE
RATING
100,000
80,000
200,000
1,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
300,000
200,000
Prosp.
Short
C.C.
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
B
400,000
300,000
1,000
KTN-R, KTS-R
LIMITRON® Class RK1 Fast-Acting Fuses KTS-R
400,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
KTS-R – RMS & Peak Let-Thru Currents (kA)
400
IRMS
5
6
7
8
9
10
10
10
11
12
13
13
13
14
16
18
Ip
10
14
17
19
20
22
23
24
26
28
29
30
31
32
37
41
600
IRMS
5
8
10
11
12
13
13
14
15
17
17
18
19
20
23
26
Prosp.
Short
C.C.
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
Ip
12
19
22
25
27
29
31
32
36
38
40
42
44
46
53
59
30
Fuse Size
30
IRMS Ip
1
2
1
2
1
3
1
3
1
3
1
3
2
4
2
4
2
4
2
4
2
4
2
4
2
5
2
5
2
5
3
6
60
IRMS
1
2
2
2
2
2
2
2
3
3
3
3
3
3
4
4
Ip
3
4
4
5
5
5
5
6
6
6
7
7
7
7
8
9
100
IRMS
2
2
3
3
3
3
4
4
4
4
5
5
5
5
6
7
Ip
4
5
6
7
7
8
8
9
9
10
10
11
12
12
14
15
200
IRMS
3
4
4
5
5
5
6
6
6
7
7
7
8
8
9
10
Ip
6
8
10
11
12
13
13
14
14
15
16
17
18
19
21
23
400
IRMS
5
7
8
9
10
10
11
11
12
13
14
14
15
16
18
20
Ip
12
15
18
20
22
24
25
26
28
30
32
33
35
36
41
46
600
IRMS
5
9
11
12
13
14
15
16
17
19
20
21
22
23
26
29
Ip
12
20
24
28
31
33
35
37
40
43
45
48
50
52
60
66
Buss Fuse Let-Thru Charts – JKS
LIMITRON® Class J Fast Acting Fuses JKS
B
400,000
300,000
AMPERE
RATING
200,000
100,000
80,000
60,000
50,000
40,000
600
400
30,000
200
20,000
100
60
10,000
8,000
30
6,000
5,000
4,000
3,000
A
200,000
80,000
100,000
40,000
50,000
60,000
30,000
20,000
8,000
10,000
4,000
5,000
6,000
3,000
1,000
2,000
2,000
1,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Data Section
PROSPECTIVE SHORT-CIRCUIT CURRENT–SYMMETRICAL RMS AMPS
JKS – RMS & Peak Let-Thru Currents (kA)
Prosp.
Short
C.C.
5,000
10,000
15,000
20,000
25,000
30,000
35,000
40,000
50,000
60,000
70,000
80,000
90,000
100,000
150,000
200,000
Fuse Size
30
IRMS Ip
1
2
1
3
1
3
1
3
2
4
2
4
2
4
2
4
2
5
2
5
2
5
2
5
2
5
2
5
2
5
3
6
60
IRMS
1
2
2
2
3
3
3
3
3
3
3
3
4
4
5
5
Ip
3
4
4
5
6
6
7
7
8
8
8
8
9
9
11
12
100
IRMS
2
3
3
3
3
3
4
4
4
5
5
5
6
6
6
7
Ip
4
6
6
7
8
8
9
9
10
11
12
12
13
13
14
15
200
IRMS
3
4
4
5
6
6
6
7
7
7
8
8
9
9
9
10
Ip
7
9
10
12
13
13
14
15
16
17
18
18
19
19
21
22
400
IRMS
4
6
7
8
9
9
9
10
10
11
11
12
13
13
14
16
Ip
10
13
15
18
19
20
21
22
23
25
25
28
29
30
33
37
600
IRMS
5
9
10
11
12
13
13
14
15
16
17
17
18
18
22
24
Ip
12
19
22
25
28
30
30
32
35
37
39
39
41
42
50
55
31
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Buss Fuse Selection Chart (600 Volts or Less).
Circuit
Load
Ampere
Fuse
Rating
Symbol
Type
Voltage
Rating
(AC)
Class
Interrupting
Rating
(KA)
Remarks
All-purpose fuses.
Unequaled for combined
short-circuit and
overload protection.
(Specification grade product)
Conventional Dimensions–Class RK1, RK5 (1/10-600A), L (601-6000A)
Main,
Feeder
and
Branch
All type loads–
resistive or
inductive (optimum
overcurrent
protection).
1
/10
to
600A
LOW-PEAK
YELLOW™
(dual-element,
time-delay)
LPN-RK_SP
LPS-RK_SP
250V*
600V*
RK1††
300
601 to
6000A
LOW-PEAK
YELLOW™
(time-delay)
KRP-C_SP
600V
L
300
Motors, welders,
transformers,
capacitor banks
(circuits with heavy
inrush currents).
1
/10
to
600A
FUSETRON®
(dual-element,
time-delay)
FRN-R
FRS-R
250V*
600V*
RK5††
200
601 to
4000A
LIMITRON®
(time-delay)
KLU
600V
L
200
KTN-R
KTS-R
250V
600V
RK1††
200
Same short-circuit protection
as LOW-PEAK fuses, but
must be sized larger for
circuits with surge currents,
i.e., up to 300%.
KTU
600V
L
200
A fast-acting, highperformance fuse.
Non-motor loads
(circuits with no
heavy inrush
currents).
LIMITRON fuses
suited for circuit
breaker protection.
1
to
600A
LIMITRON®
(fast-acting)
601 to
6000A
Moderate degree of
current limitation. Time-delay
passes surge currents.
General purpose fuse.
Time-delay passes
surge-currents.
Reduced Dimensions For Installation in Restricted Space–Class J(1-600A), T(1-1200A), CC(1/10-30A), G(1/2-60A)
All type loads
(optimum
overcurrent
protection).
LOW-PEAK
YELLOW™
(dual-element,
time-delay)
LPJ_SP
600V*
J
300
All-purpose fuses.
Unequaled for combined
short-circuit and overload
protection. (Specification
grade product).
LIMITRON®
(quick-acting)
JKS
600V
J
200
Very similar to KTS-R
LIMITRON, but smaller.
1 to
1200A
T-TRON™
JJN
JJS
300V
600V
T
200
The space saver (1/3 the
size of KTN-R/KTS-R).
1
LIMITRON®
(fast-acting)
KTK-R
600V
CC
200
1
CC-TRON™
(time-delay)
FNQ-R
Very compact (13/32" x
11/2"); rejection feature.
Excellent for control
transformer protection .
LOW-PEAK
YELLOW™
(time-delay)
LP-CC
1
General purpose,
i.e., lighting
panelboards.
1
/2
to
60A
SC
SC
300V
G
100
Miscellaneous.
1
/8
to
ONE-TIME
NON
NOS
250V
600V
H or K5†
10
600A
SUPER-LAG®
RENEWABLE
REN
RES
250V
600V
H
10
1
FUSTAT®
(dual-element,
time-delay)
S
125V
S
10
FUSETRON®
(dual-element,
time-delay)
T
125V
**
10
Buss Type W
W
125V
**
10
Non-motor loads
(circuits with no
heavy inrush
currents).
Control transformer
circuits and lighting
ballasts, etc.
1
to
600A
/10 to 30A
/4 to 10A
Branch
All type loads especially small
HP motors
General
Purpose
(noncurrentlimiting
fuses)
Plug fuses can
be used for
branch circuits
and small
component
protection.
/2 to 30A
/4
to
30A
Current-limiting;
dia. x varying
lengths per ampere rating.
13/32"
Forerunners of
the modern
cartridge fuse.
Base threads of Type S
differ with ampere ratings.
T and W have Edison-base.
T & S fuses recommended
for motor circuits. W not
recommended for circuits
with motor loads.
* LPN-RK_SP, 125VDC; LPS-RK_SP, 300VDC. FRN-R, 125VDC; FRS-R, 300VDC; LPJ_SP, 300VDC.
** Listed as Edison-Base Plug Fuse.
† Some ampere ratings are available as Class K5 with a 50,000A interrupting rating.
†† RK1 and RK5 fuses fit standard switches, fuseblocks and holders; however, the rejection feature of Class R switches and fuseblocks designed specifically for rejection type fuses
(RK1 and RK5) prevent the insertion of the non-rejection fuses (K1, K5 and H).
Bussmann
Cooper Industries, Inc. Bussmann Division, P.O. Box 14460, St. Louis, MO 63178-4460
Sales Offices: U.S.A. 314-527-3877 • United Kingdom 44-1509-880737
Denmark 45-44850910 • Germany 49-6105-76968 • Singapore 65-227-5346
Australia 61-2-743-8333 • Mexico 525-352-0088 • India 91-80-225-1133
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
®
Bussmann
Table of Contents
Fuse Technology
Fuseology
199-204
Fuse Diagnostic Chart
205-207
Time-Current & Current Limitation Curves
208-218
Glossary of Terms
219-220
Now you can get current information about Bussmann products
at anytime, using BIF (Bussmann Information FAX) or visit us on
the World Wide Web.
Bussmann
Information Fax ~
314.527.1450
Bussmann
Worldwide Web ~
http://www.bussmann.com
BIF is a simple to use automated fax response system. All
you need is a touch-tone telephone and a fax machine to
get complete product specifications when you want it. BIF
document numbers are located throughout this catalog. To
get a detailed data sheet on the product of your choice, simply
dial 314-527-1450 and request the document number listed.
In a matter of minutes a data sheet will be faxed to you.
It’s that simple.
BIF documents can also be downloaded from the Internet. The
Bussmann web site is continuously updated with our newest
products and latest data on circuit protection solutions. Visit us
often at http://www.bussmann.com
©1997 Cooper Industries, Inc., Bussmann Division
Printed in U.S.A.
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Fuse Technology
Circuit Protection
Electrical distribution systems are often quite complicated. They
cannot be absolutely fail-safe. Circuits are subject to destructive
overcurrents. Harsh environments, general deterioration, accidental damage, damage from natural causes, excessive expansion, and/or overloading of the electrical distribution system are
factors which contribute to the occurrence of such overcurrents.
Reliable protective devices prevent or minimize costly damage to
transformers, conductors, motors, and the other many components and loads that make up the complete distribution system.
Reliable circuit protection is essential to avoid the severe monetary losses which can result from power blackouts and prolonged
downtime of facilities. It is the need for reliable protection, safety,
and freedom from fire hazards that has made the fuse a widely
used protective device.
Overcurrents
An overcurrent is either an overload current or a short-circuit current. The overload current is an excessive current relative to normal operating current, but one which is confined to the normal
conductive paths provided by the conductors and other components and loads of the distribution system. As the name implies,
a short-circuit current is one which flows outside the normal conducting paths.
Overloads
Overloads are most often between one and six times the normal
current level. Usually, they are caused by harmless temporary
surge currents that occur when motors are started-up or transformers are energized. Such overload currents, or transients, are
normal occurrences. Since they are of brief duration, any temperature rise is trivial and has no harmful effect on the circuit
components. (It is important that protective devices do not react
to them.)
Continuous overloads can result from defective motors (such as
worn motor bearings), overloaded equipment, or too many loads
on one circuit. Such sustained overloads are destructive and
must be cut off by protective devices before they damage the
distribution system or system loads. However, since they are of
relatively low magnitude compared to short-circuit currents,
removal of the overload current within minutes will generally prevent equipment damage. A sustained overload current results in
overheating of conductors and other components and will cause
deterioration of insulation, which may eventually result in severe
damage and short-circuits if not interrupted.
Short-Circuits
Whereas overload currents occur at rather modest levels, the
short-circuit or fault current can be many hundred times larger
than the normal operating current. A high level fault may be
50,000 amperes (or larger). If not cut off within a matter of a few
thousandths of a second, damage and destruction can become
Bussmann®
rampant—there can be severe insulation damage, melting of
conductors, vaporization of metal, ionization of gases, arcing,
and fires. Simultaneously, high level short-circuit currents can
develop huge magnetic-field stresses. The magnetic forces
between bus bars and other conductors can be many hundreds
of pounds per linear foot; even heavy bracing may not be adequate to keep them from being warped or distorted beyond
repair.
Fuses
The fuse is a reliable overcurrent protective device. A “fusible” link
or links encapsulated in a tube and connected to contact terminals comprise the fundamental elements of the basic fuse.
Electrical resistance of the link is so low that it simply acts as a
conductor. However, when destructive currents occur, the link
very quickly melts and opens the circuit to protect conductors
and other circuit components and loads. Fuse characteristics are
stable. Fuses do not require periodic maintenance or testing.
Fuses have three unique performance characteristics:
1. Modern fuses have an extremely “high interrupting
rating”—can withstand very high fault currents without
rupturing.
2. Properly applied, fuses prevent “blackouts.” Only the
fuse nearest a fault opens without upstream fuses
(feeders or mains) being affected—fuses thus provide
“selective coordination.” (These terms are precisely
defined in subsequent pages.)
3. Fuses provide optimum component protection by
keeping fault currents to a low value…They are said to
be “current limiting.”
Voltage Rating
The voltage rating of a fuse must be at least equal to or greater
than the circuit voltage. It can be higher but never lower. For
instance, a 600 volt fuse can be used in a 208 volt circuit.
The voltage rating of a fuse is a function of its capability to
open a circuit under an overcurrent condition. Specifically,
the voltage rating determines the ability of the fuse to suppress
the internal arcing that occurs after a fuse link melts and an arc
is produced. If a fuse is used with a voltage rating lower than the
circuit voltage, arc suppression will be impaired and, under some
fault current conditions, the fuse may not clear the overcurrent
safely. Special consideration is necessary for semiconductor fuse
and medium voltage fuse applications, where a fuse of a certain
voltage rating is used on a lower voltage circuit.
Ampere Rating
Every fuse has a specific ampere rating. In selecting the ampere
rating of a fuse, consideration must be given to the type of load
and code requirements. The ampere rating of a fuse normally
should not exceed the current carrying capacity of the circuit. For
199
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Fuse Technology
instance, if a conductor is rated to carry 20 amperes, a 20
ampere fuse is the largest that should be used. However, there
are some specific circumstances in which the ampere rating is
permitted to be greater than the current carrying capacity of the
circuit. A typical example is the motor circuit; dual-element fuses
generally are permitted to be sized up to 175% and non-timedelay fuses up to 300% of the motor full-load amperes. As a rule,
the ampere rating of a fuse and switch combination should be
selected at 125% of the continuous load current (this usually
corresponds to the circuit capacity, which is also selected at
125% of the load current). There are exceptions, such as when
the fuse-switch combination is approved for continuous operation at 100% of its rating.
Interrupting Rating
A protective device must be able to withstand the destructive
energy of short-circuit currents. If a fault current exceeds the
capability of the protective device, the device may actually rupture, causing additional damage. Thus, it is important when
applying a fuse or circuit breaker to use one which can sustain
the largest potential short-circuit currents. The rating which
defines the capacity of a protective device to maintain its integrity when reacting to fault currents is termed its “interrupting
rating”. The interrupting rating of most branch-circuit, molded
case, circuit breakers typically used in residential service entrance
panels is 10,000 amperes. (Please note that a molded case
circuit breaker’s interrupting capacity will typically be lower than
its interrupting rating.) Larger, more expensive circuit breakers
may have interrupting ratings of 14,000 amperes or higher. In
contrast, most modern, current-limiting fuses have an interrupting rating of 200,000 or 300,000 amperes and are commonly
used to protect the lower rated circuit breakers. The National
Electrical Code, Section 110-9, requires equipment intended to
break current at fault levels to have an interrupting rating sufficient
for the current that must be interrupted.
Selective Coordination – Prevention of Blackouts
The coordination of protective devices prevents system power
outages or blackouts caused by overcurrent conditions. When
only the protective device nearest a faulted circuit opens and
larger upstream fuses remain closed, the protective devices are
“selectively” coordinated (they discriminate). The word “selective”
is used to denote total coordination…isolation of a faulted circuit
by the opening of only the localized protective device.
KRP-C
1200SP
LPS-RK
600SP
LPS-RK
200SP
2:1 (or more)
2:1 (or more)
This diagram shows the minimum ratios of ampere ratings of LOW-PEAK
YELLOW fuses that are required to provide “selective coordination”
(discrimination) of upstream and downstream fuses.
200
Unlike electro-mechanical inertial devices (circuit breakers), it is a
simple matter to selectively coordinate fuses of modern design.
By maintaining a minimum ratio of fuse-ampere ratings between
an upstream and downstream fuse, selective coordination is
assured.
Current Limitation – Component Protection
Areas within waveform
loops represent destructive
energy impressed upon
circuit components
Normal
load current
Initiation of
short-circuit
current
Circuit breaker trips
and opens short-circuit
in about 1 cycle
A non-current-limiting protective device, by permitting a shortcircuit current to build up to its full value, can let an immense amount of
destructive short-circuit heat energy through before opening the circuit.
Fuse opens and clears
short-circuit in less
than cycle
A current-limiting fuse has such a high speed of response that it cuts off a
short-circuit long before it can build up to its full peak value.
If a protective device cuts off a short-circuit current in less than
one-quarter cycle, before it reaches its total available (and highly
destructive) value, the device is a “current-limiting” device. Most
modern fuses are current-limiting. They restrict fault currents to
such low values that a high degree of protection is given to circuit
components against even very high short-circuit currents. They
permit breakers with lower interrupting ratings to be used. They
can reduce bracing of bus structures. They minimize the need of
other components to have high short-circuit current “withstand”
ratings. If not limited, short-circuit currents can reach levels of
30,000 or 40,000 amperes or higher in the first half cycle (.008
seconds, 60 hz) after the start of a short-circuit. The heat that
can be produced in circuit components by the immense energy
of short-circuit currents can cause severe insulation damage or
even explosion. At the same time, huge magnetic forces
developed between conductors can crack insulators and distort
and destroy bracing structures. Thus, it is important that
a protective device limit fault currents before they reach their
full potential level.
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Fuse Technology
Bussmann®
Operating Principles of Bussmann® Fuses
The principles of operation of the modern, current-limiting Buss
fuses are covered in the following paragraphs.
Non-Time-Delay Fuses
The basic component of a fuse is the link. Depending upon the
ampere rating of the fuse, the single-element fuse may have one
or more links. They are electrically connected to the end blades (or
ferrules) (see Figure 1) and enclosed in a tube or cartridge surrounded by an arc quenching filler material. BUSS® LIMITRON®
and T-TRON® fuses are both single-element fuses.
Under normal operation, when the fuse is operating at or near its
ampere rating, it simply functions as a conductor. However, as
illustrated in Figure 2, if an overload current occurs and persists for
more than a short interval of time, the temperature of the link eventually reaches a level which causes a restricted segment of the link
to melt. As a result, a gap is formed and an electric arc established. However, as the arc causes the link metal to burn back, the
gap becomes progressively larger. Electrical resistance of the arc
eventually reaches such a high level that the arc cannot be sustained and is extinguished. The fuse will have then completely cut
off all current flow in the circuit. Suppression or quenching of the
arc is accelerated by the filler material. (See Figure 3.)
Single-element fuses of present day design have a very high
speed of response to overcurrents. They provide excellent shortcircuit component protection. However, temporary, harmless
overloads or surge currents may cause nuisance openings unless
these fuses are oversized. They are best used, therefore, in circuits not subject to heavy transient surge currents and the temporary over-load of circuits with inductive loads such as motors,
transformers, solenoids, etc. Because single-element, fast-acting
fuses such as LIMITRON and T-TRON fuses have a high speed of
response to short-circuit currents, they are particularly suited for
the protection of circuit breakers with low interrupting ratings.
Whereas an overload current normally falls between one and
six times normal current, short-circuit currents are quite high.
The fuse may be subjected to short-circuit currents of 30,000
or 40,000 amperes or higher. Response of current limiting fuses
to such currents is extremely fast. The restricted sections of the
fuse link will simultaneously melt (within a matter of two or threethousandths of a second in the event of a high-level fault current).
The high total resistance of the multiple arcs, together with the
quenching effects of the filler particles, results in rapid arc suppression and clearing of the circuit. (Refer to Figures 4 & 5) Shortcircuit current is cut off in less than a half-cycle, long before the
short-circuit current can reach its full value (fuse operating in its
current limiting range).
Figure 1. Cutaway view of typical single-element fuse.
Figure 2. Under sustained overload, a section of the link melts and an
arc is established.
Figure 3. The “open” single-element fuse after opening a circuit
overload.
Figure 4. When subjected to a short-circuit current, several
sections of the fuse link melt almost instantly.
Figure 5. The “open” single-element fuse after opening a short circuit.
201
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Fuse Technology
Dual-Element, Time-Delay Fuses as Manufactured
by Bussmann
Unlike single-element fuses, the dual-element, time-delay fuse
can be applied in circuits subject to temporary motor overloads
and surge currents to provide both high performance shortcircuit and overload protection. Oversizing in order to prevent
nuisance openings is not necessary. The dual-element, timedelay fuse contains two distinctly separate types of elements
(Figure 6). Electrically, the two elements are series connected.
The fuse links similar to those used in the non-time-delay fuse
perform the short-circuit protection function; the overload element provides protection against low-level overcurrents or overloads and will hold an overload which is five times greater than
the ampere rating of the fuse for a minimum time of 10 seconds.
As shown in Figure 6, the overload section consists of a copper
heat absorber and a spring operated trigger assembly. The heat
absorber bar is permanently connected to the heat absorber
extension (left end of illustration) and to the short-circuit link on
the opposite end of the fuse by the “S”-shaped connector of the
trigger assembly. The connector electrically joins the short-circuit
link to the heat absorber in the overload section of the fuse.
These elements are joined by a “calibrated” fusing alloy. As
depicted in Figure 7, an overload current causes heating of the
short-circuit link connected to the trigger assembly. Transfer of
heat from the short-circuit link to the heat absorbing bar in the
mid-section of the fuse begins to raise the temperature of the
heat absorber. If the overload is sustained, the temperature of the
heat absorber eventually reaches a level which permits the trigger spring to “fracture” the calibrated fusing alloy and pull the
connector free of the short-circuit link and the heat absorber. As
a result, the short-circuit link is electrically disconnected from the
heat absorber, the conducting path through the fuse is opened,
and overload current is interrupted (See Figure 8.). A critical
aspect of the fusing alloy is that it retains its original characteristic after repeated temporary overloads without degradation.
When subjected to a short circuit current, the restricted sections
of the short-circuit link will simultaneously melt (within a matter of
two or three-thousandths of a second in the event of a high-level
fault current). The high total resistance of the multiple arcs,
together with the quenching effects of the filler particles, results in
rapid arc suppression and clearing of the circuit. (Refer to Figures
9 & 10.)
BUSS dual-element fuses, typically LOW-PEAK YELLOW™ and
FUSETRON ® fuses, utilize the spring-loaded design in the overload element.
202
Overload
Element
Trigger Assembly
Spring
Heat Absorber
Short-Circuit
Element
Short-Circuit
Link
Calibrated Fusing Alloy
and “S” Connector
Figure 6. The true dual-element fuse has distinct and separate overload
and short-circuit elements.
Figure 7. Under sustained overload conditions, the trigger spring fractures
the calibrated fusing alloy and releases the “connector”.
Figure 8. The “open” dual-element fuse after opening under an overload
condition.
Figure 9. Like the single element fuse, a short-circuit current causes the
restricted portions of the short-circuit elements to melt. Arcing to burn
back the resulting gaps occurs until the arcs are suppressed by the arc
quenching material and the increased arc resistance.
Figure 10. The “open” dual-element fuse after opening under a shortcircuit condition.
This particular plot reflects the characteristics of a 200 ampere,
250 volt, LOW-PEAK YELLOW dual-element fuse. Note that at
the 1,000 ampere overload level, the time interval which is
required for the fuse to open is 10 seconds. Yet, at approximately the 2,200 ampere overcurrent level, the opening (melt) time of
a fuse is only 0.01 seconds. It is apparent that the time intervals
become shorter as the overcurrent levels become larger. This
relationship is termed an inverse time-to-current characteristic.
Time-current curves are published or are available on most commonly used fuses showing “minimum melt,” “average melt”
and/or “total clear” characteristics. Although upstream and
downstream fuses are easily coordinated by adhering to simple
ampere ratios, these time-current curves permit close or critical
analysis of coordination.
Better Motor Protection in Elevated Ambients
The derating of dual-element fuses based on increased ambient
temperatures closely parallels the derating curve of motors in elevated ambient. This unique feature allows for optimum protection
of motors, even in high temperatures.
400
300
200
LOW-PEAK YELLOW
LPN-RK200 SP (RK1)
100
80
60
40
30
20
10
8
6
TIME IN SECONDS
Fuse Time-Current Curves
When a low level overcurrent occurs, a long interval of time will
be required for a fuse to open (melt) and clear the fault. On the
other hand, if the overcurrent is large, the fuse will open very
quickly. The opening time is a function of the magnitude of the
level of overcurrent. Overcurrent levels and the corresponding
intervals of opening times are logarithmically plotted in graph
form as shown to the right. Levels of overcurrent are scaled on
the horizontal axis; time intervals on the vertical axis. The curve is
thus called a “time-current” curve.
4
3
2
1
.8
.6
.4
.3
.2
150
140
130
PERCENT OF RATING OR
OPENING TIME
.1
.08
.06
Affect on Carrying
Capacity Rating
120
110
100
.04
90
80
70
.03
Affect on
Opening Time
.02
60
50
140°F
(60°C)
176°F
(80°C)
212°F
(100°C)
AMBIENT
6,000
8,000
10,000
104°F
(40°C)
3,000
4,000
68°F
(32°C)
2,000
–32°F
(0°C)
600
800
1,000
–76°F
–40°F
–4°F
(–60°C) (–40°C) (–20°C)
300
400
30
200
.01
40
100
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Fuse Technology
CURRENT IN AMPERES
Affect of ambient temperature on operating characteristics of FUSETRON
and LOW-PEAK YELLOW Dual-Element Fuses.
203
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Fuse Technology
Bussmann®
Better Protection Against Motor Single Phasing
When secondary single-phasing occurs, the current in the remaining phases increases to approximately 200% rated full load
current. (Theoretically 173%, but change in efficiency and power
factor make it about 200%.) When primary single-phasing
occurs, unbalanced voltages occur on the motor circuit causing
currents to rise to 115%, and 230% of normal running currents
in delta-wye systems.
Dual-element fuses sized for motor running overload protection
will help to protect motors against the possible damages of
single-phasing.
Classes of Fuses
Safety is the industry mandate. However, proper selection, overall
functional performance and reliability of a product are factors
which are not within the basic scope of listing agency activities.
In order to develop its safety test procedures, listing agencies
develop basic performance and physical specifications or standards for a product. In the case of fuses, these standards have
culminated in the establishment of distinct classes of low-voltage
(600 volts or less) fuses; classes RK1, RK5, G, L, T, J, H and CC
being the more important.
The fact that a particular type of fuse has, for instance, a classification of RK1, does not signify that it has the identical function or
performance characteristics as other RK1 fuses. In fact, the LIMITRON® non-time-delay fuse and the LOW-PEAK YELLOW™
dual-element, time-delay fuse are both classified as RK1.
Substantial differences in these two RK1 fuses usually requires
considerable difference in sizing. Dimensional specifications of
each class of fuse does serve as a uniform standard.
Class R Fuses
Class R (“R” for rejection) fuses are high performance, ⁄Ω¡º to 600
ampere units, 250 volt and 600 volt, having a high degree of current limitation and a short-circuit interrupting rating of up to
300,000 amperes (rms symmetrical). BUSS Class R's include
Classes RK1 LOW-PEAK YELLOW™ and LIMITRON® fuses, and
RK5 FUSETRON® fuses. They have replaced BUSS K1 LOWPEAK and LIMITRON fuses and K5 FUSETRON fuses. These
fuses are identical, with the exception of a modification in the
mounting configuration called a “rejection feature”. This feature
permits Class R fuses to be mounted in rejection type fuseclips.
“R” type fuseclips prevent older type Class H, ONE-TIME and
RENEWABLE fuses from being installed. The use of Class R fuseholders is thus an important safeguard. The application of Class R
fuses in such equipment as disconnect switches permits the
equipment to have a high interrupting rating. NEC Articles 110-9
and 230-65 require that protective devices have adequate capacity to interrupt short-circuit currents. Article 240-60(b) requires
fuseholders for current-limiting fuses to reject non-current-limiting
type fuses.
204
In the above illustration, a grooved ring in one ferrule provides the
rejection feature of the Class R fuse in contrast to the lower
interrupting rating, non-rejection type.
Branch-Circuit Listed Fuses
Branch-circuit listed fuses are designed to prevent the installation
of fuses that cannot provide a comparable level of protection to
equipment.
The characteristics of Branch-circuit fuses are:
1. They must have a minimum interrupting rating of 10,000
amps.
2. They must have a minimum voltage rating of 125 volts.
3. They must be size rejecting such that a fuse of a lower
voltage rating cannot be installed in the circuit.
4. They must be size rejecting such that a fuse with a current
rating higher than the fuseholder rating cannot be installed.
Previous
Primary at code max. of
250% or next standard size
if 250% does not correspond
to a standard rating.
Primary
Protection
Only
Supervised
Installations
Primary
and
Secondary
Protection
Over
600V
Nominal
Un-Supervised
Installations
Transformer Impedance
Less Than or Equal to 6%.
Primary at code max.
of 300%
Transformer Impedance
Greater Than 6% But Less
Than 10%.
Primary at code max.
of 300%
Transformer Impedance
Less Than or Equal to 6%.
Transformer Impedance
Greater Than 6% But Less
Than 10%.
Primary at code max. of 300%
or next standard size if 300%
does not correspond to a
standard rating.
Primary at code max. of 300%
or next standard size if 300%
does not correspond to a
standard rating.
(Note: Components on the secondary still need overcurrent protection.)
Secondary Over 600V
Secondary at code max.
of 250%.
Secondary 600V or
Below
Secondary at code max.
of 250%.
Secondary Over 600V
Secondary at code max.
of 225%.
Secondary 600V or
Below
Secondary at code max.
of 250%.
Secondary Over 600V
Secondary at code max. of
250% or next standard size if
250% does not correspond to
a standard rating.
Secondary 600V or
Below
Secondary at code max. of
125% or next standard size if
125% does not correspond to
a standard rating.
Secondary Over 600V
Secondary at code max. of
225% or next standard size if
225% does not correspond to
a standard rating.
Secondary 600V or
Below
Secondary at code max. of
125% or next standard size if
125% does not correspond to
a standard rating.
OPTIMUM PROTECTION
Primary
Protection
Only
600V
Nominal
or Less
(Note: Components on
the secondary still need
overcurrent protection.)
Without
Thermal
Overload
Protection
Transformer
Impedance of
6% or Less
With
Thermal
Overload
Protection
Transformer
Impedance of
More Than 6%
But Less
Than 10%
205
Based on 1996 N.E.C.®
125% or next size larger
Rated primary current
greater than or equal to
2 amps but less than 9 amps.
125% or next size larger
Max. 300% or next size smaller. (See
N.E.C. 430-72(c) for control circuit
transformer maximum of 500%.
Max. 167% or next size smaller.
Rated primary current greater
than or equal to 9 amps.
125% or next size larger
Max. of 125% or next larger*.
Rated secondary current less
than 9 amps.
A
Rated secondary current 9
amps or greater.
B
Rated secondary current less
than 9 amps.
C
Rated secondary current 9
amps or greater.
D
Rated secondary current less
than 9 amps.
E
Rated secondary current 9
amps or greater.
F
A
B
C
Primary and secondary
fuses at 125% of
primary and secondary
F.L.A. or next size
larger.
D
E
F
Maximum Fuse Size
% of Primary
% of Secondary
F.L.A. (Or next
F.L.A.
size smaller.)
A
B
C
D
E
F
250%
250%
600%
600%
400%
400%
167% or next size smaller.
125% or next size larger.*
167% or next size smaller.
125% or next size larger.*
167% or next size smaller.
125% or next size larger.*
*When 125% of F.L.A. corresponds to
a standard rating, the next larger size
is not permitted.
Fuse
250V LPN-RK_SP, FRN-R
600V KRP-C_SP, LPJ_SP,
LPS-RK_SP, FNQ-R, FRS-R
Bussmann®
Primary
and
Secondary
Protection
N.E.C. MAXIMUMS
(All Fuse Types Shown)
(LPN-RK_SP, LPS-RK_SP,
FRN-R, FRS-R)
Rated primary current
less than 2 amps.
Fuse
250V LPN-RK_SP, FRN-R
600V LPS-RK_SP, LPJ_SP,
KRP-C_SP, FNQ-R, FRS-R
2475V JCD
2750V JCX
2750/5500V JCW
5500V JCE, JCQ, JCY, JCU,
5.5 ABWNA, 5.5 AMWNA, 5.5 FFN
7200V 7.2 ABWNA, 7.2 SDLSJ, 7.2 SFLSJ
8300V JCZ, JDZ, 8.25 FFN
15500V JCN, JDN, JDM, 15.5 CAVH
17500V 17.5 CAV, 17.5 SDM
24000V 24 SDM, 24 SFM, 24 FFM
36000V 36 CAV, 36 SDQ, 36 SFQ
38000V 38 CAV
Fuse Diagnostic Chart
Transformers
(N.E.C. 450-3)
Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengine
Contents
Solid State Devices
(Diodes, SCR-s,
Triacs, Transistors)
Solenoids
(Coils)
FUSE SIZED FOR:
Protected by
Time-Delay Fuses
600V & Less
Protected by NonTime-Delay Fuses
& all Class CC Fuses
Backup Overload
w/ Motor Starter
& Short-Circuit
Protection
125% of motor F.L.A. or next size larger.
Short-Circuit
Only
175%* of motor F.L.A. or next size larger. If this will not allow motor to start, due to higher
than normal inrush currents or longer than normal acceleration times (5 sec. or greater),
fuse may be sized up to 225% or next size smaller.
Short-Circuit
Only
Max. of 300%* of motor F.L.A. or next size larger. If this will not allow motor to start due
to higher than normal inrush currents or longer than normal acceleration times (5 sec. or
greater), fuses through 600 amps may be sized up to 400% or next size smaller.
Fuse
2400V JCK, JCK-A, JCH
4800V JCL, JCL-A, JCG
7200V JCR, 7.2 WKMSJ
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-600V LPJ_SP
0-250V KTN-R, NON
0-300V JJN
251-600V KTS-R, NOS
301-600V JJS
0-600V LP-CC, LPT, JKS, KTK-R
*150% for wound rotor and all DC motors.
Feeder Circuits
(600 Amps & Less)
No Motor
Load
100% of non-continuous load plus 125% of continuous load.
Combination
Motor Loads
and other
Loads
150%* of the F.L.A. of largest motor (if there are two or more motors of same size, one is considered to be the largest)
plus the sum of all the F.L.A. for all other motors plus 100% of non-continuous, non-motor load plus 125% of
continuous, non-motor load.
Motor
Loads
150%* of the F.L.A. of largest motor (if there are two or more motors of same size, one is considered to be the largest)
plus the sum of all the F.L.A. for all other motors.
0-250V LPN-RK_SP, FRN-R
0-300V JJN
251-600V LPS-RK_SP, FRS-R
301-600V JJS
0-600V JKS, LPJ_SP, KTK-R, LP-CC, LPT
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-600V LPJ_ SP, LP-CC
*A max. of 175% (or the next standard size if 175% does not correspond to a standard size) is allowed for all but wound rotor and
all D.C. motors.
Main, Branch &
Feeder Circuits
(601-6000 Amps)
150% to 225% of full load current of largest motor plus 100% of full load current of all other motors plus 125% of
continuous non-motor load plus 100% of non-continuous non-motor load.
0-600V KRP-C_SP
Short-Circuit
Protection
Only
“F”, “S”, “K” & 170M Series fuses sized up to several sizes larger than full load RMS or DC rating of device.
0-130V FWA
0-250V FWX
0-500V FWH
0-600V FWC, KAC, KBC
0-700V FWP, 170M Series, SPP
0-1000V FWJ, 170M Series, SPJ
Branch Circuit
Fuses
Size at 125% or next size smaller.
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-600V LPJ_SP, LP-CC
0-32V MDL 9-30, FNM 20-30
0-125V MDA 25-30, FNM 12-15
Supplementary
Fuses
Based on 1996 N.E.C.®
0-250V MDL ⁄Ω¡§-8, MDA ¤Ω¡º-20, FNM ⁄Ω¡º-10, FNW 12-30, MDQ ⁄Ω¡ºº-7
Size at 125% or next size larger.
0-500 FNQ ⁄Ω¡º-30
Bussmann®
Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengine
Mains
Feeders
Branches
Above 600V
Fuse Diagnostic Chart
206
Motor
Loads
(N.E.C. 430)
Compare the min. melting time-current characteristics
of the fuses with the time-current characteristics of the
overload relay curve. The size fuse which is selected
should be such that short-circuit protection is provided
by the fuse and overload protection is provided by the
controller overload relays.
Electric Boilers with
Resistance Type Immersion
Heating Elements in an ASME
Rated and Stamped Vessel.
Size at 125% or next size larger but in no case larger than 150 amperes for each subdivided load.
Fluorescent
Consult fixture manufacturer for size and type.
All Other
(Mercury,
Sodium, etc.)
Consult fixture manufacturer for size and type.
Indoor
Ballasts
Capacitors
(N.E.C. 460)
Outdoor
Mercury,
Sodium, etc.
Consult fixture manufacturer for size and type.
On load side of
motor running
overcurrent device
Protection recommended
as shown below, but not
required
Protected by
Time-Delay
Fuses
150% to 175%
of full load current
Fuse
0-250V LPN-RK_SP, FRN-R
251-600V LPS-RK_SP, FRS-R
0-600V FNQ-R, LPJ_SP, LP-CC
Protected by
Non-Time Delay
Fuses
250% to 300%
of full load current
0-250V KTN-R, NON
0-300V JJN
251-600V KTS-R, NOS
0-600V JKS, KTK-R
301-600V JJS
Based on 1996 N.E.C.®
Fuse
0-250V LPN-RK_SP, FRN-R, NON
0-300V JJN
0-480V SC
251-600V LPS-RK_SP, FRS-R, NOS
301-600V JJS
0-600V LPJ_SP, LP-CC, FNQ-R, JKS, KTK-R
Fuse
Holder
Fuse
Holder
GLR
GMF
GRF
HLR
GLQ
GMQ
HLQ
BAF
BAN
KTK
FNM
FNQ
FNW
HPF
HPS
KTK-R
FNQ-R
LP-CC
HPS-RR
HPF-RR
KTQ
BBS
HPS-L
HPF-L
BAF
BAN
KTK
FNM
FNQ
FNW
HEB
HEX
HPC-D
KTK-R
FNQ-R
LP-CC
HEY
Fuse
Holder
HPF-EE
HPS-EE
HPF-JJ
SC 20
HPS-JJ
HPF-FF
SC 25-30
HPS-FF
SC -0-15
207
Bussmann®
Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengine
Size at 125% or next size larger but in no case larger than 60 amperes for each subdivided load.
Fuse Diagnostic Chart
Electric
Heat
(N.E.C. 424)
Electric Space Heating
.01
208
CURRENT IN AMPERES
200,000
100,000
1
.1
AMPERE
RATING
400,000
1,000
PROSPECTIVE SHORT-CIRCUIT CURRENT
SYMMETRICAL RMS AMPERES
200,000
10
100,000
AMPERE
RATING
10,000
KRP-C Time-Current Characteristic Curves—
Average Melt
1,000
100
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
4000A
5000A
6000A
1600A
2000A
2500A
3000A
1200A
800A
300
10,000
1,000
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Time-Current & Current Limitation Curves
Bussmann®
KRP-C, Class L Fuses
KRP-C Current Limitation Curves
B
100,000
10,000
A
6,000
5,000
4,000
3,000
2,500
2,000
1,600
1,200
800
601
LPN-RK (250V) Class RK1 Fuses
Time-Current Characteristic Curves—Average Melt
600A
400A
200A
100A
60A
30A
AMPERE
RATING
LPN-RK_SP (250V)
300
1/10
300
15A
20A
Time-Current Characteristic Curves - Average Melt
15/100
2/10
3/10
4/10 1/2
8/10 6/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
4
5
6-1/4
8
10
12
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
LPN-RK_SP (250V)
100
TIME IN SECONDS
10
10
TIME IN SECONDS
1
1
1,000
100
1
.1
.01
.1
10
.1
CURRENT IN AMPERES
Current Limitation Curves
B
400,000
AMPERE
RATING
LPN-RK_SP (250V)
100,000
600
400
200
100
60
10,000
30
200,000
100,000
1,000
10,000
A
1,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
RMS SYMMETRICAL CURRENT IN AMPHERES
10,000
1,000
100
.01
20
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
209
210
1,000
LPS-RK
(600V)
100,000
AMPERE
RATING
CURRENT IN AMPERES
1,000
100
100
400,000
200,000
.01
10
100
100,000
.01
1/10
15/100
2/10
3/10
4/10 1/2
6/10
8/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
4
5
6-1/4
8
10
12
300
1
10
TIME IN SECONDS
LPS-RK
(600V)
10,000
.1
.1
AMPERE
RATING
1,000
20,000
600A
400A
Time-Current Characteristic Curves—Average Melt
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
CURRENT IN AMPERES
10,000
1,000
200A
100A
60A
30A
20A
300
100
30
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Time-Current & Current Limitation Curves
Bussmann®
LPS-RK (600V) Class RK1 Fuses
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
LPS-RK
(600V)
10
1
1
.1
Current Limitation Curves
B
600
400
200
100
10,000
60
30
A
1,000
FRN-R (250V)
100,000
AMPERE
RATING
400,000
200,000
CURRENT IN AMPERES
1,000
100
100
100,000
.01
10
1
TIME IN SECONDS
10
1
.1
FRN-R
(250V)
15/100
2/10
3/10
4/10 1/2
8/10 6/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
4
5
6-1/4
8
10
12
1/10
300
10,000
20,000
600A
400A
200A
100A
60A
30A
AMPERE
RATING
1,000
CURRENT IN AMPERES
10,000
1,000
100
300
15A
Time-Current Characteristic Curves—Average Melt
INSTANTANEOUS PEAK LET-THRU CURRENT AMPERES
.01
20
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Time-Current & Current Limitation Curves
Bussmann®
FRN-R (250V) Class RK5 Fuses
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
100
FRN-R (250V)
10
1
.1
.1
Current Limitation Curves
B
600
400
200
100
60
10,000
30
A
PROSPECTIVE SHORT CIRCUIT CURRENT
SYMMETRICAL RMS AMPERES
211
212
400,000
FRS-R (600V)
1,000
AMPERE
RATING
CURRENT IN AMPERES
1,000
100
100,000
PROSPECTIVE SHORT CIRCUIT CURRENT
SYMMETRICAL RMS AMPERES
200,000
100
100,000
.01
100
10
10
1
TIME IN SECONDS
1/10
15/100
2/10
3/10
4/10 1/2
6/10
8/10
1
1-1/4
1-6/10
2
2-1/2
3-2/10
4
5
6-1/4
8
10
12
300
1
.1
AMPERE
RATING
10,000
30,000
FRS-R
600V
INSTANTANEOUS PEAK LET THRU CURRENT AMPERES
CURRENT IN AMPERES
10,000
600A
400A
200A
100A
60A
30A
15A
Time-Current Characteristic Curves—Average Melt
1,000
.01
1,000
TIME IN SECONDS
300
100
20
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Time-Current & Current Limitation Curves
Bussmann®
FRS-R (600V) Class RK5 Fuses
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
FRS-R
(600V)
10
1
.1
.1
Current Limitation Curves
B
600
400
200
100
60
10,000
30
A
KTN-R (250V) Class RK1 Fuses
Current Limitation Curves
B
400,000
10
1
AMPERE
RATING
100
KTN-R
(250V)
100,000
600
400
200
100
10,000
60
30
A
1,000
100,000
KTN-R
(250V)
10,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
300
200,000
AMPERE
RATING
1,000
600A
400A
200A
100A
60A
30A
Time-Current Characteristic Curves—Average Melt
TIME IN SECONDS
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
.1
10,000
1,000
100
.01
40
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
RMS SYMMETRICAL CURRENT IN AMPERES
213
KTS-R (600V) Class RK1 Fuses
10
600
400
200
100
10,000
60
30
A
1,000
1
AMPERE
RATING
100,000
200,000
100
100,000
KTS-R
(600V)
KTS-R
(600V)
10,000
300
B
400,000
1,000
AMPERE
RATING
Current Limitation Curves
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
600A
400A
200A
100A
60A
30A
Time-Current Characteristic Curves—Average Melt
TIME IN SECONDS
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
.1
RMS SYMMETRICAL CURRENT IN AMPERES
214
10,000
1,000
100
.01
40
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
.01
10,000
1,000
100
600A
10A
15A
20A
30A
40A
50A
60A
100A
125A
200A
225A
400A
5A
3A
1A
300
10
1
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
AMPERE
RATING
LPJ
100
10
1
200,000
1,000
AMPERE
RATING
100,000
100,000
10,000
1,000
.1
100
100
Time-Current Characteristic Curves—
Average Melt
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Time-Current & Current Limitation Curves
Bussmann®
LPJ (600V), Class J Fuses
Current Limitation Curves
B
LPJ
10,000
600A
400A
200A
100A
60A
50A
40A
30A
20A
15A
A
PROSPECTIVE SHORT-CIRCUIT CURRENT
SYMMETRICAL RMS AMPS
RMS SYMMETRICAL CURRENT IN AMPERES
215
JJN & JJS, Class T Fuses
400A
500A
800A
200A
100A
60A
15A
30A
10A
1A
AMPERE
RATING
5A
Time-Current Characteristic Curves—Average Melt
3A
600A
400A
200A
100A
60A
30A
300
15A
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
300
JJN
(300V)
JJS
(600V)
100
100
TIME IN SECONDS
10
10
1
.1
CURRENT IN AMPERES
10,000
10
1
.01
1,000
1
100
TIME IN SECONDS
1,000
100
20
.01
10,000
.1
RMS SYMMETRICAL CURRENT IN AMPERES
Current Limitation Curves
400
200
100
10,000
60
30
15
100,000
200,000
10,000
100
A
200
1,000
1,000
RMS SYMMETRICAL CURRENTS IN AMPERES
A-B = ASYMMETRICAL AVAILABLE PEAK (2.3 x SYMM RMS AMPS)
216
AMPERE
RATING
100,000
1200
800
600
400
200
100
60
30
10,000
1,000
A
200
200,000
1200
800
600
100,000
100,000
JJS
(600V)
10,000
AMPERE
RATING
JJN
(300V)
B
400,000
100
400,000
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
B
1,000
Current Limitation Curves
INSTANTANEOUS PEAK LET-THRU CURRENT IN AMPS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Time-Current & Current Limitation Curves
RMS SYMMETRICAL CURRENTS IN AMPERES
A–B=ASYMMETRICAL AVAILABLE PEAK (2.3 X SYMM RMS AMPS)
.01
CURRENT IN AMPERES
.01
200
LP-CC
100
10
100
7-1/2
5
3
1
1/2
AMPERE
RATING
1
3
3-1/2
4
4-1/2
6
8
10
12
15
20
25
30
1/2
6/10
8/10
1
1-1/4
Time-Current Characteristic Curves—Average Melt
.4
1
TIME IN SECONDS
100
1000
200
100
10
1
.4
TIME IN SECONDS
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Time-Current & Current Limitation Curves
Bussmann®
LP-CC & FNQ-R Class CC Fuses
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
FNQ-R
10
10
1
.1
.1
RMS SYMMETRICAL CURRENT IN AMPERES
217
.01
218
300
RMS SYMMETRICAL CURRENT IN AMPERES
400
100
10
1
TIME IN SECONDS
8A
10A
15A
20A
30A
5A
3A
2A
1A
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Time-Current & Current Limitation Curves
KTK-R, Class CC Fuses
Time-Current Characteristic Curves—Average Melt
AMPERE
RATING
KTK-R
100
10
1
.1
Bussmann®
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Glossary of Terms
Ampere
The measurement of intensity of rate of flow of
electrons in an electric circuit. An ampere is
the amount of current that will flow through a
resistance of one ohm under a pressure of one
volt.
Ampere Rating
The current-carrying capacity of a fuse. When
a fuse is subjected to a current above its
ampere rating, it will open the circuit after a
predetermined period of time.
Ampere Squared Seconds, l2t
The measure of heat energy developed within
a circuit during the fuse’s clearing. It can be
expressed as “melting l2t”, “arcing l2t” or the
sum of them as “Clearing l2t”. “l” stands for
effective let-through current (RMS), which is
squared, and “t” stands for time of opening, in
seconds.
Arcing Time
The amount of time from the instant the fuse
link has melted until the overcurrent is interrupted, or cleared.
Breaking Capacity
(See Interrupting Rating)
Cartridge Fuse
A fuse consisting of a current responsive element inside a fuse tube with terminals on both
ends.
Class CC Fuses
600V, 200,000 ampere interrupting rating,
branch circuit fuses with overall dimensions of
⁄‹Ω£™∑ ≈ 1⁄Ω™∑. Their design incorporates a rejection feature that allows them to be inserted
into rejection fuse holders and fuse blocks
that reject all lower voltage, lower interrupting
rating ⁄‹Ω£™∑ ≈ 1⁄Ω™∑ fuses. They are available
from ⁄Ω¡º amp through 30 amps.
Class G Fuses
480V, 100,000 ampere interrupting rating
branch circuit fuses that are size rejecting to
eliminate overfusing. The fuse diameter is
⁄‹Ω£™∑ while the length varies from 1fiΩ¡§∑ to 2⁄Ω¢∑.
These are available in ratings from 1 amp
through 60 amps.
Class H Fuses
250V and 600V, 10,000 ampere interrupting
rating branch circuit fuses that may be renewable or non-renewable. These are available in
ampere ratings of 1 amp through 600 amps.
Class J Fuses
These fuses are rated to interrupt a minimum
of 200,000 amperes AC. They are labelled
as “Current-Limiting”, are rated for 600 volts
AC, and are not interchangeable with other
classes.
Class K Fuses
These are fuses listed as K-1, K-5, or K-9
fuses. Each subclass has designated I2t and lp
maximums. These are dimensionally the same
as Class H fuses, and they can have interrupting ratings of 50,000, 100,000, or 200,000
amps. These fuses are current-limiting.
However, they are not marked “current-limiting” on their label since they do not have a
rejection feature.
Class L Fuses
These fuses are rated for 601 through 6000
amperes, and are rated to interrupt a minimum
of 200,000 amperes AC. They are labelled
“Current-Limiting” and are rated for 600 volts
AC. They are intended to be bolted into their
mountings and are not normally used in clips.
Some Class L fuses have designed in time-delay
features for all purpose use.
Class R Fuses
These are high performance fuses rated ⁄Ω¡º600 amps in 250 volt and 600 volt ratings. All
are marked “Current Limiting” on their label
and all have a minimum of 200,000 amp interrupting rating. They have identical outline
dimensions with the Class H fuses but have a
rejection feature which prevents the user from
mounting a fuse of lesser capabilities (lower
interrupting capacity) when used with special
Class R Clips. Class R fuses will fit into either
rejection or non-rejection clips.
Class T Fuses
An industry class of fuses in 300 volt and 600
volt ratings from 1 amp through 1200 amps.
They are physically very small and can be
applied where space is at a premium. They are
fast acting and time-lag fuses, with an interrupting rating of 200,000 amps RMS.
Classes of Fuses
The industry has developed basic physical
specifications and electrical performance
requirements for fuses with voltage ratings of
600 volts or less. These are known as standards. If a type of fuse meets the requirements
of a standard, it can fall into that class. Typical
classes are K, RK1, RK5, G, L, H, T, CC, and J.
Clearing Time
The total time between the beginning of the
overcurrent and the final opening of the circuit
at rated voltage by an overcurrent protective
device. Clearing time is the total of the melting time and the arcing time.
Current Limitation
A fuse operation relating to short circuits only.
When a fuse operates in its current-limiting
range, it will clear a short circuit in less than ⁄Ω™
cycle. Also, it will limit the instantaneous peak
let-through current to a value substantially
less than that obtainable in the same circuit if
that fuse were replaced with a solid conductor of equal impedance.
Dual Element Fuse
Fuse with a special design that utilizes two
individual elements in series inside the fuse
tube. One element, the spring actuated trigger assembly, operates on overloads up to
5-6 times the fuse current rating. The other
element, the short circuit section, operates on
short circuits up to their interrupting rating.
Electrical Load
That part of the electrical system which actually uses the energy or does the work
required.
Fast Acting Fuse
A fuse which opens on overload and short
circuits very quickly. This type of fuse is not
designed to withstand temporary overload
currents associated with some electrical
loads.
Fuse
An overcurrent protective device with a fusible
link that operates and opens the circuit on an
overcurrent condition.
High Speed Fuses
Fuses with no intentional time-delay in the
overload range and designed to open as
quickly as possible in the short-circuit range.
These fuses are often used to protect solidstate devices.
Inductive Load
An electrical load which pulls a large amount
of current—an inrush current—when first
energized. After a few cycles or seconds the
current “settles down” to the full-load running
current.
Interrupting Capacity
See Interrupting Rating
Interrupting Rating
(Breaking Capacity)
The rating which defines a fuse’s ability to
safely interrupt and clear short circuits. This
rating is much greater than the ampere rating
of a fuse. The NEC® defines Interrupting
Rating as “The highest current at rated voltage that an overcurrent protective device is
intended to interrupt under standard test
conditions.”
Melting Time
The amount of time required to melt the fuse
link during a specified overcurrent. (See
Arcing Time and Clearing Time.)
“NEC” Dimensions
These are dimensions once referenced in the
National Electrical Code. They are common to
Class H and K fuses and provide interchangeability between manufacturers for
fuses and fusible equipment of given ampere
and voltage ratings.
Ohm
The unit of measure for electric resistance. An
ohm is the amount of resistance that will allow
one ampere to flow under a pressure of one
volt.
219
Courtesy of Steven Engineering, Inc. Ÿ 230 Ryan Way, South San Francisco, CA, 94080-6370 Ÿ Main Office: (650) 588-9200 Ÿ Outside Local Area: (800) 258-9200 Ÿ www.stevenengineering.com
Bussmann®
Glossary of Terms
Ohm’s Law
The relationship between voltage, current,
and resistance, expressed by the equation E
= IR, where E is the voltage in volts, I is the
current in amperes, and R is the resistance in
ohms.
One Time Fuses
Generic term used to describe a Class H
nonrenewable cartridge fuse, with a single
element.
Overcurrent
A condition which exists on an electrical
circuit when the normal load current is
exceeded. Overcurrents take on two separate
characteristics—overloads and short circuits.
Overload
Can be classified as an overcurrent which
exceeds the normal full load current of a circuit.
Also characteristic of this type of overcurrent is
that it does not leave the normal current carrying path of the circuit—that is, it flows from
the source, through the conductors, through
the load, back through the conductors, to the
source again.
Peak Let-Through Current, lp
The instantaneous value of peak current letthrough by a current-limiting fuse, when it
operates in its current-limiting range.
Renewable Fuse (600V & below)
A fuse in which the element, typically a zinc
link, may be replaced after the fuse has
opened, and then reused. Renewable fuses
are made to Class H standards.
Resistive Load
An electrical load which is characteristic of
not having any significant inrush current.
When a resistive load is energized, the current
rises instantly to its steady-state value, without first rising to a higher value.
220
R.M.S. Current
The R.M.S. (root-mean-square) value of any
periodic current is equal to the value of the
direct current which, flowing through a resistance, produces the same heating effect in
the resistance as the periodic current does.
Semiconductor Fuses
Fuses used to protect solid-state devices.
See “High Speed Fuses”.
Short Circuit
Can be classified as an overcurrent which
exceeds the normal full load current of a circuit by a factor many times (tens, hundreds or
thousands greater). Also characteristic of this
type of overcurrent is that it leaves the normal
current carrying path of the circuit—it takes a
“short cut” around the load and back to the
source.
Short-Circuit Rating
The maximum short-circuit current an electrical component can sustain without the occurrence of excessive damage when protected
with an overcurrent protective device.
Short-Circuit Withstand Rating
Same definition as short-circuit rating.
Single Phasing
That condition which occurs when one phase
of a three phase system opens, either in a low
voltage (secondary) or high voltage (primary)
distribution system. Primary or secondary single phasing can be caused by any number of
events. This condition results in unbalanced
currents in polyphase motors and unless protective measures are taken, causes overheating and failure.
Threshold Current
The symmetrical RMS available current at the
threshold of the current-limiting range, where
the fuse becomes current-limiting when tested to the industry standard. This value can be
read off of a peak let-through chart where the
fuse curve intersects the A-B line. A threshold
ratio is the relationship of the threshold current to the fuse’s continuous current rating.
Time-Delay Fuse
A fuse with a built-in delay that allows temporary and harmless inrush currents to pass
without opening, but is so designed to open
on sustained overloads and short circuits.
Voltage Rating
The maximum open circuit voltage in which a
fuse can be used, yet safely interrupt an overcurrent. Exceeding the voltage rating of a fuse
impairs its ability to clear an overload or short
circuit safely.
Withstand Rating
The maximum current that an unprotected
electrical component can sustain for a specified period of time without the occurrence of
extensive damage.