Abstract: A main circuit loop tester includes a wave form generator for producing a current pulse in the form of a narrow spike. The wave form generator is connected to a power stage for producing a high amplitude output. The loop tester also includes a high frequency generator adapted to produce groups of low current pulses at high frequency. A measuring circuit is adapted to measure the resistive and reactive components of the resulting output signals from the main circuit. The tester is particularly useful for testing electrical installations which include residual current devices.