Applying Interrupting Rating: Circuit Breakers
Interrupting Rating Vs. Interrupting Capacity
Interrupting Rating
“Standard” Test Conditions - Circuit Breakers
It is the maximum short-circuit current that an overcurrent protective device
can safely interrupt under standard test conditions. The phrase “under
standard test conditions” means it is important to know how the overcurrent
protective device is tested in order to assure it is properly applied. This can be
very important when it comes to the application of circuit breakers, mainly
ratings of 100A and less.
This is not the case with circuit breakers. Because of the way circuit breakers
are short circuit tested (with additional conductor impedance), their interrupting
capacity can be less than their interrupting rating. When the test circuit is
calibrated for the circuit breaker interrupting rating tests, the circuit breaker is
not in the circuit. After the test circuit has been verified to the proper level of
short-circuit current, the circuit breaker is placed into the circuit. However, in
addition to the circuit breaker, significant lengths of conductor are permitted to
be added to the circuit after the calibration. This additional conductor
impedance can result in a significantly lower short-circuit current. So a circuit
breaker marked with an interrupting rating of 22,000A may in fact have an
interrupting capacity of only 9,900A.
To better understand this, it is necessary to review the standard interrupting
rating test procedures for circuit breakers: Molded Case Circuit Breakers - UL
489 and CSA 5 Test Procedures. UL 489 requires a unique test set-up for
testing circuit breaker interrupting ratings. The diagram below illustrates a
typical calibrated test circuit waveform for a 20A, 240V, 2-pole molded case
circuit breaker, with a marked interrupting rating of 22,000A, RMS symmetrical.
Interrupting Capacity
The highest current at rated voltage that the device can interrupt. This
definition is from the IEEE Standard Dictionary of Electrical and Electronic
Terms.
Standard Test Conditions - Fuses
Branch circuit fuses are tested without any additional conductor in the test
circuit. For instance, if a fuse has an interrupting rating of 300,000A, the test
circuit is calibrated to have at least 300,000A at the rated fuse voltage. During
the test circuit calibration, a bus bar is used in place of the fuse to verify the
proper short-circuit current. Then the bus bar is removed and the fuse is
inserted; the test is then conducted. If the fuse passes the test, the fuse is
marked with this interrupting rating (300,000A). In the procedures just outlined
for fuses, there are no extra conductors inserted into the test circuit after the
short-circuit current is calibrated. A major point is that the fuse interrupts an
available short-circuit current at least equal to or greater than its marked
interrupting rating. In other words, because of the way fuses are short-circuit
tested (without additional conductor impedance), their interrupting capacity is
equal to or greater than their marked interrupting rating.
26
©2005 Cooper Bussmann
Applying Interrupting Rating: Circuit Breakers
Interrupting Rating Vs. Interrupting Capacity
The diagram below illustrates the test circuit as allowed by UL 489.
Standard interrupting rating tests for a 22,000A sym. RMS interrupting rated
circuit breaker will allow for a maximum 4 feet rated wire on the line side for
each lead, and 10 inch rated wire on the load side for each lead of the circuit
breaker. See the following diagrams and table, that provide a short circuit
analysis of this test circuit as seen by the circuit breaker.
S.C.P .F. = 20%
S.C. A vail. = 22,000A
R LINE
20A
XLINE
R CB
XCB
R LOAD
XLOAD
RS
Conclusion (refer to table above and graphs below)
XS
SOURCE:
4' Rated W ire (12 AWG Cu)
10" Rated Wire (12 AWG Cu)
Note: For calculations, R CB and X CB are assum ed negligible.
Test station source impedance is adjusted to achieve a calibrated 22,000 RMS
symmetrical amps at 20% or less power factor. This circuit can achieve a peak
current of 48,026 amps. For the calibration test, a bus bar (shorting bar) is
inserted between the test station terminals.
This 22,000A (with short circuit power factor of 20%) interrupting rated circuit
breaker has an interrupting capacity of 9900A at a short circuit power factor of
88%. Unless there is a guarantee that no fault will ever occur at less than 4
feet 10 inches from the load terminals of the circuit breaker, this circuit breaker
must only be applied where there are 9,900A or less available on its line side.
A graphic analysis of this actual short circuit follows.
Test station
source leads
Shorting bar
After the circuit calibration is verified, the shorting bar is removed and the
circuit breaker is inserted. In addition, lengths of rated conductor are permitted
to be added as shown. This extra rated conductor has a high impedance and
effectively restricts the current to 9900 RMS symmetrical amps. The power
factor increases to 88% due to small conductor high resistance versus its
reactance.
This circuit can now only achieve a peak current of 14,001 amps.
Test station
source leads
Shorting bar
removed, circuit
breaker &
conductors added
Each
4 feet 12 AWG
20 A
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 9900A at 88% power
factor, not 22,000A at 20% power factor. The following graph 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.)
20A, 240V, 2-Pole
Circuit Breaker
marked 22,000 A.I.R.
Each
10 inches 12 AWG
©2005 Cooper Bussmann
27
Applying Interrupting Rating: Circuit Breakers
Interrupting Rating Vs. Interrupting Capacity
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.
Following is an example of a partial table showing the actual Ip and IRMS
values to which circuit breakers are tested.
“Bus Bar Conditions”- Circuit Breakers
Beginning October 31, 2000, UL 489 requires circuit breakers rated 100A and
less to additionally be tested under “bus bar conditions.” However, this does
not assure that the circuit breaker’s interrupting capacity equals its interrupting
rating nor even that the circuit breaker is reusable. In this test, line and load
terminals are connected to 10 inches of rated conductor. For single pole circuit
breakers, these 10 inch leads are then connected to 4 feet of 1 AWG for
connection to the test station. For multi-pole circuit breakers, the 10 inch line
side leads are connected to the test station through 4 feet of 1 AWG. The load
side is shorted by 10 inch leads of rated conductor per pole. These “bus bar
condition” tests still do not fully address the situation where a fault can occur
less than 4 feet 10 inches from the circuit breaker.
One point to be made is that acceptable bus shot test results per the product
standard do not meet the NEC® definition for a circuit breaker. For example,
7.1.11.6.3.1 of UL 489 states “The inability to relatch, reclose, or otherwise
reestablish continuity ... shall be considered acceptable for circuit breakers
which are tested under bus bar conditions”. In practical terms, this means the
circuit breaker doesn’t have to work after a fault near the circuit breaker
occurs. This is in violation of the NEC® definition for a circuit breaker: “A
device designed to open and close a circuit by nonautomatic means and to
open the circuit automatically on a predetermined overcurrent without damage
to itself when properly applied within its rating.” In addition, under “bus bar
condition” tests the circuit breaker is required to only interrupt one short-circuit
current. For this one short circuit test shot, the circuit breaker is in its closed
position and the short-circuit current is initiated by the test station switch closing randomly. The “bus bar conditions” test procedures do not evaluate the
circuit breaker for “closing-on” the short circuit. “Closing-on” a short circuit is
an important criteria for safety.
28
©2005 Cooper Bussmann
Applying Interrupting Rating: Circuit Breakers
Single-Pole Interrupting Capability
An overcurrent protective device must have an interrupting rating equal to or
greater than the fault current available at its line terminals for both three-phase
bolted faults and for one or more phase-to-ground faults (110.9). Although most
electrical systems are designed with overcurrent devices having adequate
three-phase interrupting ratings, the single-pole interrupting capabilities are
easily overlooked. This section will examine single-pole interrupting capability
(also referred to as individual pole interrupting capability).
This section will show how single-pole interrupting capabilities must be
considered in some applications. It will also show there are simple solutions
that exist to provide adequate interrupting ratings if molded case circuit
breakers, self protected starters and other overcurrent protective devices are
found to have insufficient single-pole interrupting capabilities.
A Fine Print Note was added to 430.52(C)(6) of the 2005 NEC® and 240.85 of
the 2002 NEC®. These Fine Print Notes alert users that mechanical devices,
such as self-protected combination controllers and circuit breakers, have
single-pole interrupting capabilities that must be considered for proper
application. They state:
240.85 FPN: Proper application of molded case circuit breakers on
3-phase systems, other than solidly grounded wye, particularly on
corner grounded delta systems, considers the circuit breakers’
individual pole interrupting capability.
430.52(C)(6) FPN: Proper application of self protected combination
controllers on 3-phase systems, other than solidly grounded wye,
particularly on corner grounded delta systems, considers the
circuit breakers’ individual pole interrupting capability.
As will be shown, there are other overcurrent device types and other
grounding system types where individual pole interrupting capability must be
analyzed.
The single-pole interrupting capability of a circuit breaker, self protected starter
and other similar mechanical overcurrent protective device is its ability to open
an overcurrent at a specified voltage utilizing only one pole of the multi-pole
device. Multi-pole mechanical overcurrent protective devices are typically
marked with an interrupting rating. This marked interrupting rating applies to all
three poles interrupting a three-phase fault for a three-pole device. The
marked interrupting rating of a three-pole device does not apply to a single
pole that must interrupt a fault current at rated voltage.
Single Pole Interrupting Capabilities
A circuit breaker’s, self protected starter’s, or other
mechanical protective device’s ability to open an overcurrent
at a specified voltage utilizing only one pole of the device.
Single-Pole Interrupting Capabilities
For Overcurrent Devices
Current-Limiting Fuses: the marked interrupting rating is the tested singlepole interrupting rating. So single-pole interrupting capability is not an issue
with fuse applications.
Airframe/Power Circuit Breaker: per ANSI C37.13 and C37.16 the singlepole interrupting rating is 87% of its three-pole interrupting rating.
Molded Case Circuit Breakers: Listed three-pole molded case circuit breakers
have minimum single-pole interrupting capabilities according to Table 7.1.7.2 of UL
489. Table 1 on this page indicates the single-pole test value for various three-pole
molded case circuit breakers taken from Table 7.1.7.2 of UL 489. A
similar table is shown on page 54 of the IEEE “Blue Book,” Recommended
©2005 Cooper Bussmann
Practice for Applying Low-Voltage Circuit Breakers Used in Industrial and
Commercial Power Systems, (Std 1015-1997).
Self Protected Starters: UL 508, Table 82A.3 specifies the short circuit test
values on one pole as 4320A for 0 to 10Hp devices rated 200 to 250 volts and
8660A for 0 to 200Hp devices rated 600 volts maximum.
Molded case circuit breakers and self protected starters may not be able to
safely interrupt single-pole faults above these respective values shown in
previous paragraphs. Per 110.9, all overcurrent protective devices that are
intended to interrupt fault currents must have single-pole interrupting
capabilities for the maximum single-pole fault current that is available. And
typically, engineers, contractors and inspectors (AHJs) rely on the applicable
product standard testing and listing criteria to verify device ratings as being
suitable for specific applications.
TABLE 1: “Standard” UL 489 Interrupting Tests For 3-Pole Molded
Case Circuit Breakers
Molded Case Circuit Breaker Testing - UL 489
Devices must be applied within the limitations of their listing. UL 489 is the
standard for molded case circuit breakers. UL 489 has tests which it refers to
as “standard” interrupting tests for molded case circuit breakers. A more
appropriate term would be “base” or “lowest” interrupting level that any circuit
breaker of a given rated voltage and amp rating must meet. There are circuit
breakers on the market that just test to these “standard” or “base” interrupting
tests. However, because these base interrupting ratings are rather modest
values, some circuit breakers are listed with higher interrupting ratings than
the standard or base levels; for these circuit breakers, there are additional
procedures for higher level interrupting tests.
These “standard” or “base” interrupting tests for three-pole circuit breakers
involve individual single-pole interrupting tests and multi-pole interrupting tests.
Table 7.1.7.2 of UL 489 provides the single-pole (individual) and multi-pole
interrupting current values for various voltage rating and amp rating circuit
breakers. Table 1 shows the single-pole short-circuit current values (from
Table 7.1.7.2 of UL 489) for which all three-pole circuit breakers are tested and
evaluated under single-pole interrupting capabilities. The far right column of
Table 1 shows the three-phase short-circuit current values (from Table 7.1.7.2
of UL 489) for which all three-pole circuit breakers are tested and evaluated.
These “standard” circuit breakers would be marked with an Interrupting Rating
(if above 5000A) corresponding to the three-phase short-circuit current value.
The “standard” circuit breaker is not marked with a single-pole interrupting
rating which would correspond to the single-pole interrupting test value.
29
Applying Interrupting Rating: Circuit Breakers
Single-Pole Interrupting Capability
Circuit breakers with interrupting ratings higher than the “standard” interrupting
values are needed in today’s systems, so additional provisions are in UL 489.
Higher interrupting rated molded case circuit breakers are additionally tested
and evaluated per 7.1.11 of UL 489 to a “High Short Circuit Test” procedure in
order to be marked with a higher interrupting rating. This test procedure does
not include a single-pole test of higher short-circuit current value than the
“standard” test provisions. The three-pole test current value can be equal to
any value listed in Table 8.1 of UL 489, from 7500A to 200,000A. This threephase test value must be greater than the values in the far right column of
Table 1. If a circuit breaker successfully tested to a higher three-pole
interrupting value per the “High Short Circuit Tests,” the molded case circuit
breaker is marked with this higher interrupting rating which corresponds to the
three-pole “high short circuit” current test value.
As mentioned, a single-pole interrupting test at a higher value than shown in
Table 1 is not required in these optional “High Short Circuit Test” procedures.
Because of this, the marked three-pole interrupting rating can be much higher
than the tested individual pole interrupting capability. In addition, the singlepole capability is not required to be marked on the molded case circuit
breaker; it can only be determined by reviewing the UL 489 standard.
Shown below are three still photos from a videotaping of a single-pole fault
interruption test on a three-pole circuit breaker rated 480V. This circuit breaker
is marked with a three-pole interrupting rating of 35,000A at 480V. This
marked interrupting rating is per UL 489 test procedures. This circuit breaker is
tested for individual single-pole interrupting capabilities in UL 489 at an
available fault current of 8660A (Table 1 prior page). The test that is shown
below is with an available fault current of 25,000A.
This device is tested for three-pole interruption with available
fault of 35,000 amps and is tested for individual single-pole
interruption with available fault of only 8,660 amps (UL 489)
Actual Example
The diagram below illustrates the UL 489 test procedure for a 100A, 480V,
three phase circuit breaker that gets listed for a high interrupting rating. Test A
and B are the required standard or base interrupting tests. Test A is a threepole interrupting test at 10,000A (Table 1, right column), which is a modest
three-phase available short-circuit current, and Test B is a single-pole interrupting test at a modest single-phase available short-circuit current of 8660A (Table
1). Then, in addition, to be listed and marked with the higher 65,000A interrupting rating, the circuit breaker must pass the criteria for Test C. Test C is a threepole interrupting test at 65,000A three-phase available short-circuit current. Test
D is not conducted; it is not part of the UL 489 evaluation procedure. This
higher three-phase interrupting rated circuit breaker does not have to undergo
any test criteria at a corresponding higher single-pole short-circuit current.
Test set up prior to closure of test station switch.
Three-Pole Interrupting Rating & Single-Pole Interrupting
Capabilities Test Procedures for Molded Case Circuit
Breakers – UL 489
100A, 480V, 3-Pole CB
Interrupting Rating = 65,000 A (3-Pole)
Base Interrupting Rating Procedure
Test A
Test B
3-Phase Test
10,000A
1-Pole Test
8,660A
From Table 1
From Table 1
Photo of 3-pole circuit breaker during test of individual single-pole interruption of a
fault current beyond the value in Table 1. Magnetic forces of short-circuit current
caused test board to move.
High Interrupting Rating Procedure
Test C
Test D
3-Phase Test
65,000A
1-Pole Test
NONE
As an example of single-pole interrupting capability in a typical installation,
consider this three-pole, 100A, 480V circuit breaker with a three-pole
interrupting rating of 65,000A. Referring to Table 1, this breaker has an 8660A
single-pole interrupting capability for 480V faults across one pole. If the
available line-to-ground fault current exceeds 8660A at 480V, such as might
occur on the secondary of a 1000kVA, 480V, corner-grounded, delta transformer, the circuit breaker may be misapplied.
30
Photo (later in sequence) of 3-pole circuit breaker during test of individual single-pole
interruption of a fault current beyond its single-pole interrupting capability - it violently
exploded.
©2005 Cooper Bussmann
Applying Interrupting Rating: Circuit Breakers
Single-Pole Interrupting Capability
Possible Fault Currents
During A Ground Fault Condition
Solidly Grounded WYE System
The magnitude of a ground fault current is dependent upon the location of the
fault with respect to the transformer secondary. Referring to Figure 2, the
ground fault current flows through one coil of the wye transformer secondary
and through the phase conductor to the point of the fault. The return path is
through the enclosure and conduit to the bonding jumper and back to the
secondary through the grounded neutral. Unlike three-phase faults, the
impedance of the return path must be used in determining the magnitude of
ground fault current. This ground return impedance is usually difficult to
calculate. If the ground return path is relatively short (i.e. close to the center
tap of the transformer), the ground fault current will approach the three phase
short-circuit current.
Theoretically, a bolted line-to-ground fault may be higher than a three-phase
bolted fault since the zero-sequence impedance can be less than the positive
sequence impedance. The ground fault location will determine the level of
short-circuit current. The prudent design engineer assumes that the ground
fault current equals at least the available three-phase bolted fault current and
makes sure that the overcurrent devices are rated accordingly.
Type of Ground System Affect
On Single-Pole Interruption
The method in which a system is grounded can be a significant factor in the
performance of multi-pole, mechanical overcurrent protective devices used in
three phase systems. To illustrate this, several different grounding systems
with molded case circuit breakers will be analyzed.
Solidly Grounded WYE Systems
The solidly grounded, wye system shown in Figure 1 is by far the most
common type of electrical system. This system is typically delta connected on
the primary and has an intentional solid connection between the ground and
the center of the wye connected secondary (neutral). The grounded neutral
conductor carries single-phase or unbalanced three-phase current. This
system lends itself well to commercial and industrial applications where 480V
(L-L-L) three-phase motor loads and 277V (L-N) lighting is needed.
Solidly Grounded WYE System
SERVICE
PANEL
A
277V
7V
SERVICE
PANEL
Steel Conduit
A
480V
B
C
480V
C
N
N
Figure 2. Single-Pole Fault to Ground in Solidly Grounded Wye
System
In solidly grounded wye systems, the first low impedance fault to ground is
generally sufficient to open the overcurrent device on the faulted leg. In Figure
2, this fault current causes the branch circuit overcurrent device to clear the
277V fault. This system requires compliance with single-pole interrupting
capability for 277V faults on one pole. If the overcurrent devices have a singlepole interrupting capability adequate for the available short-circuit current, then
the system meets NEC® 110.9.
Although not as common as the solidly grounded wye connection, the
following three systems are typically found in industrial installations where
continuous operation is essential. Whenever these systems are encountered,
it is absolutely essential that the proper application of single-pole interrupting
capabilities be assured. This is due to the fact that full phase-to-phase voltage
can appear across just one pole. Phase-to-phase voltage across one pole is
much more difficult for an overcurrent device to clear than the line-to-neutral
voltage associated with the solidly grounded wye systems.
Corner-Grounded-Delta Systems (Solidly Grounded)
The system of Figure 3 has a delta-connected secondary and is solidly
grounded on the B-phase. If the B-phase should short to ground, no fault
current will flow because it is already solidly grounded.
Corner Grounded Delta System
Steel Conduit
SERVICE
PANEL
BRANCH
PANEL
B
48
0V
N
V
C
0
48
480V
Steel Conduit
A
A
B
N
Fault to
conduit
B
480V
7V
BRANCH
PANEL
A
A
27
C 27
BRANCH
PANEL
Single pole must
interrupt fault current
B
C
480V
B
C
Figure 1. Solidly Grounded WYE System
If a fault occurs between any phase conductor and ground (Figure 2), the
available short-circuit current is limited only by the combined impedance of the
transformer winding, the phase conductor and the equipment ground path
from the point of the fault back to the source. Some current (typically 5%) will
flow in the parallel earth ground path. Since the earth impedance is typically
much greater than the equipment ground path, current flow through earth
ground is generally negligible.
©2005 Cooper Bussmann
Figure 3. Corner-Grounded Delta System (Solidly Grounded)
If either Phase A or C is shorted to ground, only one pole of the branch-circuit
overcurrent device will see the 480V fault as shown in Figure 4. This system
requires compliance with single-pole interrupting capabilities for 480V faults on
one pole because the branch-circuit circuit breaker would be required to
interrupt 480V with only one pole.
31
Applying Interrupting Rating: Circuit Breakers
Single-Pole Interrupting Capability
Corner Grounded Delta System
Single pole must
interrupt
SERVICEfault current
BRANCH
PANEL
PANEL
Steel Conduit
A
A
Fault to
conduit
B
C
480V
B
When the first fault occurs from phase to ground as shown in Figure 6, the
current path is through the grounding resistor. Because of this inserted
resistance, the fault current is not high enough to open protective devices.
This allows the plant to continue “on line.” NEC® 250.36(3) requires ground
detectors to be installed on these systems, so that the first fault can be found
and fixed before a second fault occurs on another phase.
High Impedance Grounded System
SERVICE
PANEL
C
BRANCH
PANEL
277V
Steel Conduit
A
A
480V
27
7
C 27
7V
V
B
First fault
to steel
conduit
B
480V
C
Figure 4. Fault to Ground on a Corner-Grounded Delta System
A disadvantage of corner-grounded delta systems is the inability to readily
supply voltage levels for fluorescent or HID lighting (277V). Installations with
this system require a 480-120V transformer to supply 120V lighting. Another
disadvantage, as given on page 33 of IEEE Std 142-1991, Section 1.5.1(4)
(Green Book) is “the possibility of exceeding interrupting capabilities of marginally applied circuit breakers, because for a ground fault, the interrupting
duty on the affected circuit breaker pole exceeds the three-phase fault duty.” A
line-to-ground fault with this type grounding system is essentially a line-to-line
fault where the one line is grounded. The maximum line-to-line bolted shortcircuit current is 87% of the three phase bolted short-circuit current. Review
the photo sequence testing (page 30) of the 225A, three phase circuit breaker
with a 35,000A interrupting rating (three-pole rating). 87% of 35,000A is
30,450A. The single-pole test was run with an available of only 25,000A.
Impedance Grounded System
High Impedance Grounded System
SERVICE
PANEL
BRANCH
PANEL
Steel Conduit
A
277V
A
V
7
C 27
27
480V
7V
Low Value of Fault Current
Because of Ground Resistor in
Short-Circuit Path
Figure 6. First Fault in Impedance Grounded System
Even though the system is equipped with a ground alarm, the exact location of
the ground fault may be difficult to determine. The first fault to ground MUST
be removed before a second phase goes to ground, creating a 480V fault
across only one pole of the affected branch circuit device. Figure 7 shows how
the 480V fault can occur across one pole of the branch circuit device. It is
exactly because of this possibility that single-pole interrupting capabilities must
be considered for mechanical overcurrent protective devices.
High Impedance Grounded System
Single
pole must
SERVICE
interrupt
fault current
PANEL
277V
“Low or High” impedance grounding schemes are found primarily in industrial
installations. These systems are used to limit, to varying degrees, the amount
of current that will flow in a phase to ground fault. “Low” impedance grounding
is used to limit ground fault current to values acceptable for relaying schemes.
This type of grounding is used mainly in medium voltage systems and is not
widely installed in low voltage applications (600V or below). The “High”
impedance grounded system offers the advantage that the first fault to ground
will not draw enough current to cause the overcurrent device to open. This
system will reduce the stresses, voltage dips, heating effects, etc. normally
associated with high short-circuit current. Referring to Figure 5, high
impedance grounded systems have a resistor between the center tap of the
wye transformer and ground. High impedance grounding systems are used in
low voltage systems (600V or less). With high impedance grounded systems,
line-to-neutral loads are not permitted per National Electrical Code®,
250.36(4).
Resistor
Resistor keeps first
fault current low:
5 Amps or so
A
Steel Conduit
A
27
480V
7V
C 27
BRANCH
PANEL
7V
B
B
480V
C
High Value of Fault
Current Because
Ground Resistor No
Longer in Path
First fault
to steel
conduit
Second Fault
To Enclosure
Figure 7. Second Fault in Impedance Grounded System
The magnitude of this fault current can approach 87% of the L-L-L short-circuit
current. Because of the possibility that a second fault will occur, single-pole
interrupting capability must be investigated. The IEEE “Red Book,” Std 1411993, page 367, supports this requirement, “One final consideration for
impedance-grounded systems is the necessity to apply overcurrent devices
based upon their “single-pole” short-circuit interrupting rating, which can be
equal to or in some cases less than their ‘normal rating’.”
B
B
480V
C
Figure 5. Impedance Grounded System
32
©2005 Cooper Bussmann
Applying Interrupting Rating: Circuit Breakers
Single-Pole Interrupting Capability
Ungrounded Systems
Ungrounded System
The Ungrounded System of Figure 8 offers the same advantage for continuity
of service that is characteristic of high impedance grounded systems.
Single pole must
interrupt
fault current
SERVICE
Ungrounded System
SERVICE
PANEL
Steel
Conduit
480V
C
B
B
First fault
to steel
conduit
480V
C
480V
Steel
Conduit
A
A
A
A
C
BRANCH
PANEL
BRANCH
PANEL
PANEL
B
B
480V
C
High Value of Fault
Current Because
Large Impedance is
No Longer in Path
Figure 10. Second Fault to Conduit in Ungrounded System
Figure 8. Ungrounded System
Although not physically connected, the phase conductors are capacitively
coupled to ground. The first fault to ground is limited by the large impedance
through which the current has to flow (Figure 9). Since the fault current is
reduced to such a low level, the overcurrent devices do not open and the plant
continues to “run.”
Ungrounded System
SERVICE
PANEL
480V
C
BRANCH
PANEL
Steel
Conduit
A
A
Second Fault
To Enclosure
B
B
480V
C
First fault
to steel
conduit
Low Value of Fault Current
Because of Large Capacitively
Coupled Impedance to Ground
Figure 9. First Fault to Conduit in Ungrounded System
As with High Impedance Grounded Systems, ground detectors should be
installed (but are not required by the NEC®), to warn the maintenance crew to
find and fix the fault before a second fault from another phase also goes to
ground (Figure 10).
The second fault from Phase B to ground (in Figure 10) will create a 480 volt
fault across only one pole at the branch circuit overcurrent device. Again, the
values from Table 1 for single pole interrupting capabilities must be used for
molded case circuit breaker systems as the tradeoff for the increased
continuity of service. The IEEE “Red Book,” Std 141-1993, page 366, supports
this requirement, “One final consideration for ungrounded systems is the
necessity to apply overcurrent devices based upon their “single-pole” short
circuit interrupting rating, which can be equal to or in some cases less than
their normal rating.”
In the NEC® 250.4(B) Ungrounded Systems (4) Path for Fault Current, it is
required that the impedance path through the equipment be low so that the
fault current is high when a second fault occurs on an ungrounded system.
What Are Single-Pole
Interrupting Capabilities For Fuses?
By their inherent design a fuse’s marked interrupting rating is its single-pole
interrupting rating. Per UL/CSA/ANCE 248 Fuse Standards, fuses are tested
and evaluated as single-pole devices. Therefore, a fuse’s marked interrupting
rating is its single-pole interrupting rating. So it is simple, fuses can be applied
on single phase or three phase circuits without extra concern for single-pole
interrupting capabilities. There is no need to perform any special calculations
because of the grounding system utilized. Just be sure the fuses’ interrupting
ratings are equal to or greater than the available short-circuit currents. Modern
current-limiting fuses are available with tested and marked single-pole interrupting ratings of 200,000 or 300,000A. Low-Peak LPJ_SP, KRP-C_SP, LPSRK_SP and LPN-RK_SP fuses all have UL Listed 300,000A single-pole
interrupting ratings. This is a simple solution to assure adequate interrupting
ratings for present and future systems no matter what the grounding scheme.
Review the three drawings for a fusible, high impedance grounded system.
High Impedance Grounded System
SERVICE
PANEL
Resistor
BRANCH
PANEL
Steel Conduit
A
A
480V
27
C
7V
27
7V
B
B
480V
C
Figure 11. Fusible high impedance grounded system.
©2005 Cooper Bussmann
33
Applying Interrupting Rating: Circuit Breakers
Single-Pole Interrupting Capability
High Impedance Grounded System
Resistor Keeps First
Fault Current Low:
5 Amps or So
A
BRANCH
PANEL
Steel Conduit
A
480V
27
7V
C
SERVICE
PANEL
B
7V
27
B
First Fault
to Steel
Conduit
480V
C
Figure 12. Upon first fault, the fault current is low due to resistor. As
intended the fuse does not open.
High Impedance Grounded System
Single Pole Must Interrupt Fault Current:
Fuse’s Marked Interrupting Rating Is Its SingleSERVICE
Pole Interrupting
Rating: SimpleBRANCH
Solution
PANEL
PANEL
Steel Conduit
A
A
27
7V
C
480V
B
7V
27
B
480V
C
High Value of Fault
Current Because
Ground Resistor No
Longer in Path
First Fault
to Steel
Conduit
Second Fault
to Enclosure
Figure 13. Upon the second fault, the fault is essentially a line-line fault
with the impedance of the conductors and the ground path.
The fuse must interrupt this fault. Since a fuse’s interrupting
rating is the same as its single-pole interrupting capability,
modern fuses with 200,000A or 300,000A interrupting rating
can be applied without further analysis for single pole
interrupting capabilities.
34
©2005 Cooper Bussmann