Circuit tester

摘要

A method of making a measurement of line-earth impedance of a three-wire mains circuit which includes a residual current device such as an earth-leakage circuit breaker, which method comprises the steps of: (a) measuring the voltage across the line-neutral circuit and the neutral-earth circuit in an unloaded state; (b) connecting a suitable known load resistance across the line-neutral circuit, so as to cause a relatively high test current to flow, repeating both measurements for the loaded state and calculating the value of the test current from the value of the load resistances and the measured voltages; (c) dividing the difference voltages by the test current to give the resistances of the line-neutral loop (RLN) and the neutral lead (RN); (d) injecting a low DC current into the neutral-earth circuit and measuring the voltage change to determine the neutral-earth loop resistance (RNE); and (e) determining the line-earth loop resistance RLE from the sum of the live lead resistance (RL) and the earth lead resistance (RE) by calculating: <DF>RL = RLN - RN </DF> <DF>RE= RNE - RN; </DF> and <DF>RLE = RL + RE </DF> <IMAGE>