Through Fault Current

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OCPD

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Hello All,
I have a question on short circuit test for panelboards. I read online that the panelboards have a 3 cycle withstand rating (0.05s) based on UL standard. My question is most of the breakers which are installed as the Main typically may take more time (worst case) than 0.05seconds to open and clear the fault.

As per my interpretation the short circuit test is based on the through fault (branch breakers) rather than the main. So the 3 cycle rating is for the through fault not for internal fault. Is my thinking correct on it. And what is considered to be protecting for the internal bus faults?
 
OCPD said:
Hello All,
I have a question on short circuit test for panelboards. I read online that the panelboards have a 3 cycle withstand rating (0.05s) based on UL standard. My question is most of the breakers which are installed as the Main typically may take more time (worst case) than 0.05seconds to open and clear the fault.

As per my interpretation the short circuit test is based on the through fault (branch breakers) rather than the main. So the 3 cycle rating is for the through fault not for internal fault. Is my thinking correct on it. And what is considered to be protecting for the internal bus faults?
Click to expand...

You need to consider the '3-cycle clearing time' at the panel interior's rated fault current, which is probably in excess of 65kA.

If the fault is internal to the panel bussing, what difference does its fault withstand rating make?
 
jim dungar said:
You need to consider the '3-cycle clearing time' at the panel interior's rated fault current, which is probably in excess of 65kA.

If the fault is internal to the panel bussing, what difference does its fault withstand rating make?
Click to expand...

Hi Jim,

Attached is a TCC of a Square D P-Frame breaker which is remote main for a I-Line panelboard. This is were the question came now if there is a fault of 8.209kA at the panel the upstream breaker takes over 3 cycles (0.579s) to clear. Does it meet UL standard test of 3 cycle clearing time? How to interpret the UL withstand rating requirement in this case? It will be same if the fault is very less in such a case it will take more time.
 

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In my understanding, when you say 3 cycle rating there has to be a current rating. Example, 65kA at 3 cycle. For lower currents, use I squared t as constant and calculate the new time on new current levels. You can plot withstand current in tcc to see it clearly.


Sent from Mars
 
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