Today had a call that parking lot lights were not illuminating. Here is the set-up: Primary V 208/120 3P feeding a 75KVA transformer. Steps up to 480/277v.
From transformer, circuit goes to a photo cell controlled contactor, which than energizes a panel with lighting circuits.
Photo control working correctly. Contactor opens and closes correctly. However...with contactor CLOSED I have these voltages from phase to ground on HV side. A:210 B:340 C:340. When contactor is OPEN I have A:177 B 178 C:160 Phase to phase whether A-B or A-C or B-C is 479-481V no matter if contactor is open or closed. I have never seen this before. Any theories.
