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Slide1
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)Slide2
Component ProtectionSlide3
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.Slide4
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.Slide5
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-neutralSlide6
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.Slide7
Component Protection
Component
short-circuit current ratings
depends
upon:
Amount of fault current (RMS sym. short-circuit current, and in some cases, peak let-through current) and length of time, or
I
2
t Withstand Ratings
These values have been established through short-circuit testing of
the components.
If both the let-through currents (I
RMS
and I
p
) and the time it takes to clear the fault, or the let-through I
2
t, of the current-limiting overcurrent protective device are less than the withstand rating of the electrical component, then that component will be protected from short-circuit damage.
Slide8
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 SwitchesSlide9
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). Slide10
Conductor Protection
Withstand ratings for
conductors (5 sec & I
2
t):
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)Slide11
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 formulaSlide12
Conductor Protection
ICEA insulation damage
tables
Maximum Short Circuit Withstand Current (A)Slide13
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).
Slide14
Conductor Protection
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 short-circuits by a 60A Class RK1 fuse.
Short-Circuit
60A Class RK1 Fuse -
LPS-RK60SP
LOW-PEAK
® Dual-Element Fuse
40,000A
RMS Sym.
Available
460V, 3
Ø
20HP
10 AWG THW CopperSlide15
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.Slide16
Busway Protection
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.
Short-Circuit
800 Amp Plug-in Bus
KRP-C800SP 800 Amp LOW-PEAK
Time-Delay Fuses
Bracing Required?
65,000A
RMS Sym.
Available
800A SwitchSlide17
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.Slide18
Protection of Bus
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.
800A
Power
CB
w/STD = 6 cycles
65,000A RMS Sym. Available
800A MCC with
3 cycle 65kA Bracing
800A Class L Fuses
800A MCC with
3 cycle 65kA Bracing
65,000A RMS Sym. AvailableSlide19
Motor Circuit Protection
M
and these motor starter contacts and O.L. relays,
With a
Fault Here,
that is cleared by this protective device,
these conductors,
… must all be rated to safely withstand the energy of the fault let-through current without “extensive damage” to any of the circuit components.
this switch
,Slide20
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.Slide21
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.Slide22
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
.Slide23
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.Slide24
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 Slide25
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.Slide26
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.Slide27
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 timesSlide28
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——
200,000Transfer SwitchContactor withstand/close
-on ratings Slide29