Suggestion on How to Use
• Industry Trainers are encouraged to use this
material in their sessions
• Download the presentation file
• Print the Notes pages and read them as you
view the presentation in the “Slide Show”
view. In this way you see the slides in large
format and have animation (when available)
© 2015 Eaton. All Rights Reserved..
1
Component Protection
© 2015 Eaton. All Rights Reserved..
Component Protection
• Electrical system components must be applied within
their short-circuit current ratings.
• Electrical system components can be destroyed if the
overcurrent devices do not limit the short-circuit current
to within the short-circuit current rating of the system’s
components.
• NEC 110.10 covers protection of electrical system
components.
© 2015 Eaton. All Rights Reserved..
3
Component Protection
• NEC 110.10 – Circuit Impedance, Short-Circuit Current
Ratings, and Other Characteristics.
• “The overcurrent protective devices, the total
impedance, the equipment short-circuit current ratings,
and other characteristics of the circuit to be protected
shall be so selected and coordinated to permit the
circuit protective devices used to clear a fault to do so
without extensive damage to the electrical equipment of
the circuit.”
• The specified OCPD must protect the electrical
components/equipment from damage under short
circuit conditions.
© 2015 Eaton. All Rights Reserved..
4
Component Protection
• NEC 110.10 – Circuit Impedance, Short-Circuit Current
Ratings, and Other Characteristics.
• “This fault shall be assumed to be either between two or
more of the circuit conductors or between any circuit
conductor and the equipment grounding conductor(s)
permitted in 250.118.”
• The equipment must be protected for the various
types of potential faults, including phase to phase,
phase-to-ground, phase-to-neutral
© 2015 Eaton. All Rights Reserved..
5
Component Protection
• NEC 110.10 - Component Protection
• “Listed equipment applied in accordance with their
listing shall be considered to meet the requirements of
this section.”
• The testing requirements of electrical components
must be understood to assure proper application
and protection.
© 2015 Eaton. All Rights Reserved..
6
Component Protection
•
Component short-circuit current ratings depends upon:
1) Amount of fault current (RMS sym. short-circuit current, and in
some cases, peak let-through current) and length of time, or
2) I2t Withstand Ratings
•
These values have been established through short-circuit
testing of the components.
•
If both the let-through currents (IRMS and Ip) and the time it
takes to clear the fault, or the let-through I2t, of the currentlimiting overcurrent protective device are less than the
withstand rating of the electrical component, then that
component will be protected from short-circuit damage.
© 2015 Eaton. All Rights Reserved..
7
Component Protection
• The following components can be analyzed by
establishing the short-circuit withstand data of each
component and then selecting the proper overcurrent
protective device:
• Wire/Cable
• Bus (Busway, Switchboards, Motor Control Centers and
Panelboards)
• Low Voltage Motor Controllers
• Transfer Switches
© 2015 Eaton. All Rights Reserved..
8
Conductor Protection
• Proper protection of conductors will improve reliability and
reduce the possibility of injury.
• Overcurrent devices must limit the short-circuit current to
within the short-circuit current rating of the conductor.
• Merely matching the ampacity rating of the conductor with
the ampere rating of a protective device may not assure
component protection of the conductor under short-circuit
conditions, especially when the overcurrent device is
sized up to 10 times the ampacity of the conductor (or
even higher).
© 2015 Eaton. All Rights Reserved..
9
Conductor Protection
• Withstand ratings for conductors (5 sec & I2t):
• ICEA - Insulation Damage (150 Deg C)
• Soares - Cable to become loose under lug (Annealing
point of copper - 250 Deg C)
• Onderdonk - Melting of Cable (1,083 Deg C)
© 2015 Eaton. All Rights Reserved..
10
ICEA Chart
• Short Circuit Withstand
Chart for Copper
Conductors with
Thermoplastic
insulation
• Allowable Short Circuit
Current Based on
Clearing Time and
Conductor Size
• Based on physics
formula
© 2015 Eaton. All Rights Reserved..
11
Conductor Protection
• ICEA insulation damage tables
• Maximum Short Circuit Withstand Current (A)
© 2015 Eaton. All Rights Reserved..
12
Overcurrent Device Selection
• The next step to analyzing protection of
conductors against short circuits depends upon
the overcurrent device selected and available
fault current
• The overcurrent protective device used to
protect the conductor can either be a current
limiting fuse or circuit breaker (current limiting or
non-current limiting).
© 2015 Eaton. All Rights Reserved..
13
Conductor Protection
60A Class RK1 Fuse - LPS-RK60SP
40,000A
RMS Sym.
LOW-PEAK® Dual-Element Fuse
Short-Circuit
20HP
Available
460V, 3Ø
10 AWG THW Copper
• The circuit shown originates at a distribution panel where 40,000
amperes RMS symmetrical are available.
• The 10 AWG THW copper conductor is protected against shortcircuits by a 60A Class RK1 fuse.
© 2015 Eaton. All Rights Reserved..
14
Conductor Protection
• 10 AWG conductor short circuit withstand is 4,300A for
one cycle (based upon ICEA).
• The OCPD (LPS-RK-60SP) must limit the 40,000A RMS
fault to 4,300A and clear within one cycle or less in order
to properly protect the 10AWG conductor.
• The LPS-RK-60SP let-thru chart shows: that a 40,000A
fault is reduced to 3,000A (w/in 1/2 cycle)
• Since the amount of current
let-through by the fuse is less
than the short-circuit withstand
for the given period of time,
the conductor is protected.
© 2015 Eaton. All Rights Reserved..
15
Busway Protection
KRP-C800SP 800 Amp LOW-PEAK
Time-Delay Fuses
Short-Circuit
65,000A
RMS Sym.
Available
•
•
•
800A Switch
800 Amp Plug-in Bus
Bracing Required?
Available Fault current equals 65,000A
With KRP-C800SP ampere LOW-PEAK® fuses, the 65,000A available fault
will be reduced to 19,000 A (from the let-through charts).
This would allow a standard 22,000 ampere RMS symmetrical (3-cycle) rated
bus.
© 2015 Eaton. All Rights Reserved..
16
Protection of Bus
• U.L. Standard 891 (Switchboards) and U.L. 67
(panelboards) detail short-circuit current ratings for a
durations of 3 cycles, unless the main overcurrent
device clears the short in less than three cycles.
• Motor control centers, per U.L. 845 have short-circuit
current ratings for a duration of 3 cycles, unless the main
overcurrent device clears the short in less than 3 cycles.
• Current Limiting devices can be used for protection of
the bus in these assemblies.
© 2015 Eaton. All Rights Reserved..
17
Protection of Bus
800A Class L Fuses
65,000A RMS Sym. Available
800A MCC with
3 cycle 65kA Bracing
800A Power CB
w/STD = 6 cycles
65,000A RMS Sym. Available
800A MCC with
3 cycle 65kA Bracing
• The top design meets 110.10 of the NEC because a 65kA fault would
be cleared in ½ cycle or less and the MCC is rated for 3 cycles.
• The bottom design violates 110.10 because a 65kA fault could
continue for 6 cycles, but the MCC is only rated for 3 cycles.
© 2015 Eaton. All Rights Reserved..
18
Motor Circuit Protection
this switch,
that is cleared by this protective
device,
With a
Fault Here,
M
these conductors,
and these motor starter contacts and
O.L. relays,
… must all be rated to safely withstand the energy of the fault let-through
current without “extensive damage” to any of the circuit components.
© 2015 Eaton. All Rights Reserved..
19
Motor Controller Short-Circuit Current
Ratings – UL 508
• UL 508 Tests motor
controllers to
minimum short-circuit
current ratings as
indicated in the table.
• Higher ratings are
available
• At standard or higher
ratings, significant,
permanent damage
is allowed per UL
508.
© 2015 Eaton. All Rights Reserved..
20
UL 508 - Industrial Control Equipment
• Maximum Damage Criteria for
Standard & High Fault
Acceptance Criteria
• All Devices
• The door or cover shall not be blown
open.
• No discharge of parts beyond the
enclosure.
• Enclosure cannot become energized.
© 2015 Eaton. All Rights Reserved..
21
UL 508 - Industrial Control
Equipment
Maximum Damage Criteria for Standard & High Fault
Acceptance Criteria (Cont)
Motor Control Devices
The load switch function of the motor control device is able to be inoperative at the
conclusion of the test. The contacts of the motor control device are able to weld
or completely disintegrate.
Overload Relays
Overload relays are allowed
to be burned out.
© 2015 Eaton. All Rights Reserved..
22
Protection Levels –
IEC 60947-1 & UL 508E
• Type 1
• Considerable damage to the contactor
and overload relay is acceptable
• Replacement of components or a
complete new starter may be needed
• There must be no discharge of parts
beyond the enclosure.
• Enclosure cannot become energized.
• Note: Similar to UL508 testing.
© 2015 Eaton. All Rights Reserved..
23
Protection Levels –
IEC 60947-1 & UL 508E
• Type 2
• No damage is allowed to either
the contactor or the overload
relay
• Light contact welding is
allowed, but must be easily
separable
• Testing and documentation is
required to prove Type 2
Protection
© 2015 Eaton. All Rights Reserved..
24
Transfer Switch
Short circuit withstand rating
• Typically defined as 3-cycles at a given magnitude of current
• UL1008 now allows manufacturers to optionally test for longer periods
• UL1008 short circuit withstand testing only ensures that the
contacts do no weld during the test
• This ensures that the transfer switch can transfer to the alternate
source, however, it does not ensure that the normal contacts are
viable and free of damage
• Because of this, the testing does not ensure that the transfer
switch can be transferred back to the original source
Note- Applying a transfer switch within its short circuit withstand
rating does NOT guarantee that the switch survives a
downstream fault without damage.
© 2015 Eaton. All Rights Reserved..
25
Transfer Switch
Short time withstand rating
• UL1008 allows optional testing of transfer switches to achieve
a short time withstand rating
• Short time ratings are also expressed in a number of cycles at
a given magnitude of current.
• The test for short time withstand is treated differently than the
short circuit withstand
• After the short time withstand test, the contacts must be viable.
A temperature rise test is done to verify.
Note- Applying a transfer switch within its short time withstand
rating ensures the transfer switch is still completely functional
after a downstream fault.
© 2015 Eaton. All Rights Reserved..
26
Transfer Switch
Short circuit withstand vs. short time withstand
Short circuit
Short time
Required by UL1008
Optional
Not required by UL1008
3-cycles at rated current
(65 kA, for example)
Typically longer than 3-cycles
Contact damage permitted
Contacts must remain viable
Switch not required to transfer back
to normal power
Ensures that switch can transfer
back to normal power
Manufacturer option to test at
longer times
© 2015 Eaton. All Rights Reserved..
27
Transfer Switch
Contactor withstand/close-on ratings
UL 1008
Ampere
Rating
480 V
600 V
Any Breaker
Specific
Breaker
Any Breaker
Specific
Breaker
Fuse
40, 80, 100
10,000
30,000
10,000
22,000
100,000
150, 200
10,000
30,000
22,000
35,000
100,000
225, 260, 400
30,000
50,000
—
—
200,000
40, 80, 100,
150, 200
30,000
50,000
22,000
35,000
200,000
225, 260, 400
30,000
50,000
50,000
65,000
200,000
600, 800,
1000,1200
50,000
65,000
50,000
65,000
200,000
1600
50,000
65,000
—
—
200,000
© 2015 Eaton. All Rights Reserved..
28
© 2015 Eaton. All Rights Reserved..
29