Is this a 480Y277 volt system?
If so trivial ground faults on ANY 277V lighting circuit will give you trip level currents.
BTW that GFCI trip at the MAIN will not normally trip the branch circuit -- beware.
Derived circuits -- ie those run off of an isolating transformer -- are not going to be at issue; typically 480 delta to 208Y120.
The reference to a 200A GF interrupt rating should NOT be taken to mean that 200 amps has to be shorting to ground before it trips. Instead, that rating means that the equipment can tolerate such shorts and trip correctly. GF protection that only kicked in over 200 instantaneous amps wouldn't be any kind of life-saver for anyone or anything.
Pinched hot conductors, especially in site-lighting, have been the normal culprits for me. In these winter months photo-cell triggered contactors might have your short appearing with a passing cloud.
Concentrate on 277V to ground circuits.
