the present invention is directed to a ground - current isolating circuit for providing a low voltage barrier between a service neutral conductor and an associated grounding system . the voltage barrier circuit comprises a first current unidirectional element and a second current unidirectional element connected in antiparallel . the unidirectional elements are preferably diodes having a characteristic threshold voltage . the current in a forward biased direction is blocked by the circuit until an applied forward biased voltage exceeds the threshold voltage . the present invention also is directed to a method of isolating a neutral conductor temporarily from an associated grounding system during testing of a structure grounding system . the method includes connecting a voltage barrier circuit in parallel with a conductor bonding the neutral conductor to the associated grounding system , then disconnecting the bonding conductor ; conducting a test of the structure grounding system associated with the structure , and reconnecting the bonding conductor between the neutral conductor and grounding system .