Ground fault circuit interruptor test apparatus

Claim 1

A test unit for checking the operation of a current operated core balance protective device connected between the active and neutral conductors of an AC supply circuit through the application to the supply circuit of an AC test leakage current selectable from a predetermined range, said unit comprising: A. terminals for connection to the active and neutral conductors and to earth, B. first circuit means including a variable auto transformer connected between the active and neutral terminals, with the output of the auto transformer providing a test voltage between it and the earth terminal, and a high impedance metering circuit connected between the auto transformer output and the earth terminal to provide a read-out of the test voltage, the high impedance being of such value that the current flowing therethrough and from the supply circuit whilst the test voltage is being selected will be sufficiently small that even when applied for a prolonged period of time the time-current characteristic will not cause any reaction effect in human beings, C. second circuit means connected between the auto transformer output and the earth terminal including an impedance of predetermined value to which the selected test voltage is applied to produce a leakage current therethrough, the AC test leakage current then being the sum of the current through the metering circuit and that through said impedance, D. switching means isolating the second circuit means from the auto transformer output whilst the test voltage is being selected and for applying the selected test voltage to the second circuit means when the operation of the current operated core balance protective device is to be checked, and E. time control means operative to limit the application of the test leakage current to the supply circuit so that the time-current characteristic of the test leakage current is below the threshold at which dangerous patho-physiological effects are to be expected.