You have two 600 amp emergency generators ones almost maxed out at 570 amp and the other is at 200amp.It's decided that the load be balanced between the two and the job is done under a P.E stamp. Shortly there after occasionally and only during a load test the upstream 800 amp breaker before the t-switch will trip just as it transfers back to standard power. I know an inphase monitor will resolve the issue by delaying the transfer. I was thinking we have motors free wheeling that are generating power back into the buss work. Also we have 12 elevators some of which are emergency power and likely grouped to one MCC per wing. I wonder if these MCC’s are all fed from the same generator? I’ve seen this before on a single 320a stand-by system where either the 200a main circuit breaker for the emergency elevator MCC would trip or just like now the 800 amp circuit breaker before the t- switch. During the load test the t-switch transfers to emergency so fast that you can’t see it pick up the load but if you pay attention you can see I a flicker when it switches back. Eventually both breakers got weak .I think this is the same problem but would like to hear as many ideas as to why it could happening as possible