Short Circuit Withstand Rating

[polarseltzer]

I understand that switchboards are built to have a withstand time rating of 3 cycles. Now that is at the rating of the switchboard, for example 100kA. I am struggling with a coordination issue where I can turn off the instantaneous setting of the main breaker (SQD Masterpact NW40H3, Micrologic 6.0A 4000A plug) which would improve the coordination with feeder breakers. My concern is that this defeats the instantaneous protection for the switchboard. The breaker does have an instantaneous override of 85kA. The maximum fault current available is slightly less than 25kA. So is there a damage type curve for switchboards that would show for example that the switchboard can withstand 25kA for 2 seconds? It would be similar to a conductor damage curve. With the instantaneous turned off the breaker would trip around 0.3 secs(18 cycles, does this provide adequate protection for the switchboard?

 

[dpc]

If this breaker was provided with the switchboard, I doubt that turning off the adjustable instantaneous is going to cause the bus withstand rating to be exceeded. If you have a 100 kA withstand and breakers with 100 kA ratings, there will be plenty of 25 kA faults or even much higher that might not clear on instantaneous even with instantaneous turned on. Because this is a switchboard and not switchgear, the instantaneous override will always be active and should protect the switchboard.

I suggest putting the question to Square D if you are concerned.

 

[veritas]

polarseltzer

I thought Masterpact and Micrologic are Schneider? or is it SQD in the USA? Also, most boards I've seen have a 1s or 3s rating (not 3 cycles) - in the IEC world at least.

When sizing a switchboard for fault current ratings the two most important criteria (again IEC) are the short-circuit withstand, Icw, and the peak current, Ip. According to IEC 3439.1:2002, Table 4, if your Icw = 100kA then the peak is 2.2*100 = 220kA. If your fault current is 25kA (I assume this to be the symmetrical r.m.s. value) then your theoretical worst case peak is 2.828*25 = 70.7kA - so no problem with the peak.

Icw is associated with a time value as it represents the thermal withstand capability of the board. You can thus use I1^2*t1 = I2^2*t2 to determine the withstand time of your board. Let's say its 100kA for 1s, then for 25kA the withstand time is a whopping 16s!

Hope this helps.

 

[dpc]

Square D is owned by Schneider, at least as of today. Breakers sold in the US carry UL and ANSI ratings (and probably IEC as well).