Re: CB trips going from normal to emergency power
The location of the breaker with respect to the transfer switch was not defined nor was the type of transfer switch.
However, from my experience with transfer switches and breakers is that the transfer switch (ATS)is either a contactor type TS or the ppopular ASCO or similar and, as such, the transfer is very fast if not basically instantaneous. As you pointed out the trip occurred upon re transfer to the normal source.
A breaker that trips upon transfer has tripped magnetically which I'm probably preaching to the choir. But, what is happening it that the inductive load powered by the emergency source is out of phase with the normal source so that when the load is transferred to the normal source the counter EMF causes a voltage spike which translates to a current spike of a high enough magnitude to trip the breaker.
Usually instantaneous transfer is not a priority because the ATS must sense the loss of power from the normal source, start the generator, then generator comes up to speed and then the ATS transfers. There also may be a time delay normal to emergency TDNE to assure that the loss of power is not just momentary. Of course if the emergency power source we and alternate utility source the transfer could be instantaneous without a TDNE.
But, a Time delay emergent to normal is often not considered and overlooked. A TDEN can do two things. It delays transfer back to normal to assure that the normal source will not fail again shortly after it is restored and, more importantly, that there is a time delay for along enough period of time to allow any counter EMF or out of phase voltage ot decay.
