I always open the main breaker and any sub panel breakers to prevent current flow/arcing when connecting a meter.Also wear the proper safety stuff(face shield, gloves)in the event there is a fault on the load side ungrounded(hot)conductors that is caused by applied voltage that would not show up with a continuity test from meter output terminals(no meter installed) to ground .
Could the voltage difference of 20-30 volts be caused by a leakage current through your voltmeter to ground?If so it would be interesting to locate the leakage path.
It could be a faulty meter that caused the arc.
Chris
