Abstract: A double-clamp earth tester device, having a voltage-inducing transformer (VT) (30), a current-sensing transformer (CT) (32), a voltage source adapted to supply a voltage across a primary winding (31) on the VT core (44) and a signal processing unit (35) adapted to measure a resultant signal output from a secondary winding (33) on the CT core (46), is susceptible to errors induced by electromagnetic interference between the cores (44, 46) of the two transformers. Previously, the effects of this interference have been mitigated by surrounding one or both cores (44, 46) with electromagnetic shielding (42). This had lead to devices having a bulky head portion and attendant difficulties in accessing sites having restricted access (52). These problems have been overcome by providing means to modify the signal output form the secondary winding (33).

Abstract: A method and apparatus for testing the loop impedance in residual current circuit breaker (RCCB) protected circuits is provided. An alternating test current is applied across the phase earth connections of a supply and a plurality of voltage samples are then taken across the connections. The voltage samples are subsequently transformed from time space to frequency space to allow the component due to the test current to be isolated. The loop impedance is then calculated.