ECN Forum Forums General Topics of the Trade General Discussion Area distribution breaker trips branch circuit doesn't

I had an emergency call at a large office building, dark and stormy night, The main was tripped. I turned off all the breakers and reset the 2000 Amp (I think) main. Then turned on each breaker one at a time till it tripped again. I got everything on except the one 200amp panel. Turned out an A/C unit had a ground fault. Check the heater. Good luck Rod