I am currently working on a highrise apartment buildingwith 273 units. Each unit has a 100 amp 22k A.I.c rated main breaker located in a central electrical rm on each of the floors. The farthest panel is 125ft the closest 25ft. They are all run in #2 aluminum with 208v(3 phase is main service). We are currently trimming and trouble shooting so the load on the feeders is pretty low. All equipment is series rated. Problem is that a 15 or 20 amp circuit phase to ground will bypass the branch circuit breaker that is rated at 10k aic and knock out the main for the unit. I have tried to research this myself and found that it is common for breakers to trip out "together" but only my main does. All I can think it is the fact that there is no load on the building itself...but that gets a little hazy when I try to run it through in my head. Any takers Im all ears.