We have a Large park that is lit at night by a underground 3 phase power. That lights come on via a relay that energizes at night. When I take a voltage reading of power coming into the relay, each phase to ground is 277 and phase to phase is 480Vac. But when I energize the relay turning on the lights, two of the phases to ground read 480Vac and the other only reads 3.8Vac. What's causing this? Short to ground?