I would like to thank everyone for your helpful replies.
The duty cycle could be as short as 6 seconds for a couple of times, but then it would lengthen to 30, 60, or more seconds. Six second is the duty cycle of the test specimen.
No ground reference is needed for this application. These circuits are normally ungrounded with a ground detector built into the battery charger.
I have some 24 volt shunt trip breakers to test, they weigh about 500 lbs each. Our normal variable power supplies struggle at 48 volts. Usually a pair of car batteries show up. I do not like that option, because if somebody screws up they could get a battery acid shower.
Thanks for the great ideas.