He is a question I've been pondering over for a while. Consider connecting a 3PH 200V 400Hz 60KVA aircraft power unit to a load bank for testing. The inputs to the load bank is 3ph +N (no earth wire, Inputs to aircraft do not have an earth)
The generator is not connected to earth, however the tow bar and wheels will touch ground.
Load bank neutral is not connected to chassis
The star point of the generator is however is connected to the chassis.
The Generator has no RCD protection, only a thermal overload, set to200Amps approx.
The load bank manufacturer recommend connecting the load bank chassis to a good earth point, we have been using a solid steel building iron girder which is earthed to the incoming mains earth.
So, what happens if the load bank develops a line to chassis earth fault? Will the GPU trip out. What potential will be seen on the load bank chassis, will the building framework become live?
Someone in our works recons that because the building is earthed, the Earth will drag the supply down to Zero, i dont agree.
I think that current must flow back to the generator with sufficient magnitude to trip it out, this I believe cannot happen because the earth fault path is limited by the high earth loop impedance (mA will flow) therefore the generator will not trip out. So, will the building frame be a 200V potential. I also think that if the load bank chassis is connected to the incoming neutral then high fault current will flow and trips will operate.
Well long question, and thoughts or views?