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Low voltage- Short Circuit Specification 1

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NickParker

Electrical
Sep 1, 2017
421
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.
 
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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
 
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
 
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
 
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)
 
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
 
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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