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Low voltage- Short Circuit Specification

Low voltage- Short Circuit Specification

Low voltage- Short Circuit Specification

(OP)
My low voltage switchgear specifications says "Rated Short-Circuit Withstand Current: 25 kA rms for 3 cycles". Does this mean do I have to specify all the breakers (Incoming & Outgoing), bus bars with the same short circuit rating, that is 25kA rms,3 cycles.

RE: Low voltage- Short Circuit Specification

All components must be rated for the Available Short Circuit Current available from the source.
For example, if your ASCC is 8 kA, all components must be rated for 8 kA or greater.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Low voltage- Short Circuit Specification

That specification is born out of the UL spec for "Switchboards" (UL-891) vs "Switchgear (UL-1551). UL-1551 Switchgear must withstand 30 cycles of fault current, UL-891 listed switchboards and panelboards only have to withstand 3 cycles. So in a nutshell, that is a minimal spec, not a difficult one to attain with common equipment.


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

RE: Low voltage- Short Circuit Specification

A simple example. Larger systems may need more complicated calculations.

50 KVA, 240 Volt transformer. impedance Volts = 3.6%
FLC = 50000VA/240Vs = 208.3A
208.3A / .036imp = 5.787 kA
All devices must be rated in excess of 5.787 kA
A second calculation may be done to consider the impedance of the cable from the transformer to the switch gear. This will often drop the ASCC enough that 5 kA rated equipment may be used in this example.

For larger systems it may be necessary to consider also the grid Available Short Circuit Current.
Using the transformer impedance alone yields a worst case result.
Considering the grid capability may reduce the ASCC.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Low voltage- Short Circuit Specification

(OP)
But in most cases, why the bus bar has to be specified for 1s or 3s even if the Incoming breaker could clear the fault within 3 cycles (50ms)

RE: Low voltage- Short Circuit Specification

3 Cycles is the best case circuit breaker opening, rather than a realistic timeframe if things don't work out the way they should. I'll admit I don't know anything about the UL ratings, but its not uncommon to see higher ratings for a shorter duration (20kA for 0.2sec is a common one I've seen). Often the withstand rating will be higher due to other factors such as current carrying capacity, than what the minimum size for 3 cycles would be. Whilst the withstand rating may need to be at least 3 cycles, it doesn't necessary mean that the circuit breaker will open that fast, as a result, you'll find ratings like 1 second instead.

EDMS Australia

RE: Low voltage- Short Circuit Specification

If every fault at the bus will be cleared in 3 cycles, you'll find it very difficult to coordinate with downstream overcurrent devices. If you are OK with sacrificing coordination and accepting loss of power to the entire bus for a lot of fault downstream of the bus, then 3 cycles is OK.

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