The facility does have 100% backup power. But this occurs on utility only.
Look at the OP.
What things would you look for?
Your original post did not say anything about a generator, but if it was happening on transfer from utility to generator, and you did not have 100% of the loads on generator, I would say one or more generator feed circuits were parralleled somewhere, back feeding the utility until they tripped. But since 100% of the loads are generator powered, this should not be the problem. I had this problem on a blue box store, the lighting maintenance contractor had tied both generator and utility circuits together in the fixtures while changing ballast. They did this to approx. 50 fixtures. Since both circuits were the same phase, but out of different panels, no breakers were tripping until they lost utility, and it backfeed the POCO until the all of the affected breakers tripped due to overload.
