Went on a service call where the 100amp breaker feeding a 75KVA dry type was tripping two or three times about 10 to 15 minutes apart. The breaker was hot but could be reset. I found one of the load lugs was loose and I don't mean a little, as it was backed out 3/8 to 1/2 inch further than the others. The insulation on this conductor was toast for about 18 inches. I taped this section of the conductor, torqued the lug on the breaker and set up a return trip at night to replace the conductor. As the transformer feeds two 100 amp panels that supply the kitchen of a retirement/nursing home. Problem solved right?? Wrong!! They called back the next day with breaker tripping again two or three times in thirty minutes again. Returned and put a amprobe recorder on the transformer feeder, at which time it was running at about 40 to 50 amps and was fine. About 17 hours later it tripped again twice about 10 to 15 minutes apart. The recorder showed maximun amps as 632 on A phase, 305 on B phase and 674 on C phase. At no time have the 100 amp mains or any branch breakers in the two panels that are feed by the transformer tripped. How do I meg this transformer?? Can I just turn off the feeder breaker to the transformer and the mains on the two panels that are fed by the transformer or do I need to unhook the transformer at the taps to isolate it for megging? Also what readings (ballpark) should I expect to find on a good or bad transformer when it is megged ? Help please!
