Why does disconnecting the AC power protect the terminal from fault currents emanating from the ground system or telco lines ?
Fault current flows from high voltage to low voltage references over a closed circuit. Opening the AC power circuit assures that elevated voltage on the grounding system can not reference (seek) the lower voltage grounds on the commercial power service. Similarly, fault current induced on the telco lines can not reference the lower voltage power grounds across the protected equipment. The only remaining lower voltage reference is the grounding system. The open circuit prevents the flow of fault current through the protected electronics. Instead all fault current is forced to the grounding system.
