Phase Nullify in the Electric Bus

IS the neutral for the system grounded.If not then it may be a case of arcing ground as voltage has risen from phase voltage to line voltage between phase and ground.

Maybe it's the way your voltmeter is connected into the circuit. If you're connecting one lead to the R phase instead of Neutral or Ground, you will read what you are reading.

Uppalsn,

In some industrial settings the circuits are intentionally not referenced to ground (or earth). These are called "ungrounded delta" in the USA. The purpose is so that any accidental ground in the equipment does not cause a fault trip of the protective device and loss of power to the equipment or process. In these cases, however, there should be a ground detector that would warn the maintenance workers when one phase became grounded. They could then locate and fix the problem without any loss of production. I have seen and worked with ungrounded delta systems operating at 480 and 2300VAC.

I suspect this is the type of circuit you have observed.

--JMM