I was recently contacted as a third party testing agency to check a school ground system because of nuisance GFCI breaker tripping (several different breakers in different panels and some even during the evening hours. The design load and tested circuit amperage is less than 6 amperes per 20 amp circuit breaker. Everyone wants to assume that the obvious reasons are not the fault. Items like not having crossed neutrals or grounds on the 3Ø circuits i.e. 1,3,5, & 2,4,6 have been checked out.
So here are the basics. The school is new... 1200 ampere 480/277v 3Ø 4W service with the following grounds: water main, ground rod triode, bldg. steel, perimeter 3/0 ground ring, rebar.... the 480 volt gear feeds a 300KVA 50% K-rated 480-208/120volt transformer bonded to the bldg steel with a 3/0 bare copper GEC as well as bonded with bonding jumpers at the ground bushings at the flex fittings. The only grounding of the 1200 ampere 208/120 3Ø 4W switchboard is a 3/0 CU GEC through each of three secondary flex conduits and mechanically bonding to the transformer chassis (thus mechanically bonding to the building steel). Bldg limitations precluded me from doing a fall of potential test with test points in clean soil. So this left me doing a 2 point earth resistance test of all 5 of the GEC's and a 2 point test between extraneous conductive parts. This test was done on both the 480/277 and 208/120v systems. The test results yielded nothing greater than .01 ohms on each grounded electrode system. The odd thing is a large amount of noise as indicated by a megger model DET 20C. I couldn't get a reading on the ground voltage yet I assume it had to be more than 5 volts to trigger the noise alarm on the meter. Any suggestions would be appreciated.... Thank you