Transfer Switch Withstand Rating

[CuriousElectron]

Greetings,
I've done some readings on the topic, but one thing wasn't clear to me and I wanted to get a confirmation on this forum.
Let's consider a hypothetical scenario. We've got a Manual Transfer Switch that has a withstand rating of 65kAIC for the standard 3 cycles per UL 1008, protected upstream by a Circuit Breaker having 100kAIC interrupting rating with instantaneous element enabled. If there is a 70kAIC fault downstream of the switch that is cleared within 3 cycles by the Circuit breaker, does that mean that the MTS was improperly selected for this application and could sustain internal damage by the higher fault current than what it is rated for?

I guess my question is, does the 3-cycle kAIC withstand rating of a transfer switch has to be higher than the maximum available short circuit current downstream of it?
Thanks,
EE

 

[dpc]

In the US, yes it needs to be rated higher than the available fault current. Unless the manufacturer will certify that it can withstand higher current for shorter time, there is no way to know, short of doing your own test. Is this often ignored - yes. But it is still an NEC violation since you are applying the device beyond its rating.

Dave

 

[waross]

It depends somewhat on the DC offset of the downstream fault.
There is a 2:1 ratio between the asymmetrical peak current and the symmetrical peak current.
It depends.


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

 

[LionelHutz]

Yes, it does.