Ground power is supplied to aircraft through the external power plug. This plug is designed with four large pins and two smaller pins. The small pins (E & F) are shorter in length than the other four (A, B, C, & N).
Every aircraft has a Ground Power Control Unit of some type installed. This unit verifies the incoming voltage and phasing. If the supplied power is correct, the unit will output a 28 VDC control voltage. This output goes through a jumper on E & F of the GPU plug and back to the external power contactor. If the GPU plug is pulled out with power on, the two short pins will loose contact first. This will interrupt the control voltage to the contactor and it will go open. No arcing will occur from the large power pins.
I have seen safety wire wrapped around pins E & F on numerous aircraft. This is done to compensate for a bad GPU plug. The use of safety wire on these pins defeats the disconnect feature. Serious pin to socket arcing will occur when the plug is removed. The aircraft will not like it and the person pulling the plug might not like it either. I remove any wire found on these pins. If a GPU has a bad plug, DON'T USE IT.
Modern GPU's must sense the 28 VDC control voltage. If it is not present, the GPU will stop supplying power.