Arc Flash Question 1

[richanton]

I'm doing an arc Flash evaluation of my bldg distribution system, and I'm trying to understand why the calories vary so much under certain circumstances.

My example is I have 2 identical 480V power circuit breakers with 600 amp trips, one breaker feeds a 400kva 480-208/120 transformer, the other a 500kva. The breaker settings, cable type and length are identical.

Yet when I evaluate the panelboards on the secondary side for arc flash, the 500kva units are only 1 calorie, while the 400kva transformer secondary panelboard is 36 calories. The problem appears to be that the breaker time delay is excessive for the 400kva unit, but fine for the 500kva.

Are these results possible?

 

[dpc]

Yes. Relying on primary side protection for faults on the secondary of a transformer commonly results in extremely high incident energy levels. Fault current will be lower for the 400 kVA transformer and this is causing much slower clearing times. A breaker that size should have adjustable instantaneous trip settings. Try lowering the settings. With molded case breaker, the fault must be cleared on instantaneous trip to get reasonable incident energy. If it's tripping on thermal element, it will be bad. Also, set maximum arc time of 2 seconds.

Lower instantaneous pickup setting of primary protection can, of course, cause coordination problems and also nuisance tripping on inrush.

 

[richanton]

Thanks. I wasn't sure if I had entered incorrect data somehow.