Razzap
Member
I recently wired up a three-phase, 51 kVA isolation transformer to a fusable disconnect switch. When it was first energized, everything seemed to be fine. From then on, nearly every time the disconnect was turned off and the back on, a fuse (or fuses) would blow. Completely random, sometimes phase A, next time maybe phases B and C together, once in a while it would be OK, and so on. We swapped out transformers (three other identical units), cabling from the disconnect to the transformers, everything. We have gone through three boxes of fuses, still no fix. What is odd is that the secondary is not even hooked up! When only XO on the secondary was grounded to the enclosure, it blew fuses EVERY time. Here are the specs: Primary 480 V Delta with taps, Secondary is 480 V Wye with a XO connection. 51 kVA, 60 Hz, 3 Phase, 4.4% impedance. Fusable disconnect is 60 amp, with 45 amp LPJ time delay fuses (sometimes even 50 or 60 amp fuses!) Three #6 AWG wires with a #6 EGC in a cable feeds the transformer. This acts like a short circuit, too fast for an AMprobe reading. Fuses blow immediately upon energizing the transformer. Cleaning the factory primary tap connections seemed to help a little. I have run out of ideas, anyone out there have a similar expirience?