Loop resistance is measured in a live-earth conductor loop comprising a residual current device powered by an alternating current mains supply. A measurement circuit having a low-pass filter characteristic arranged to substantially remove signal components at the frequency of the alternating current mains supply is used to measure, in a first period, a voltage across live and earth conductors of the live-earth conductor loop to determine a first voltage. A current application circuit is used to apply a unipolar test current having a magnitude below a trip current value of the residual current device for a second period and the measurement circuit is used to measure a second voltage between the live and earth conductors. Based on a difference in test current between the first and second periods and a difference in voltage between the first voltage and the second voltage, the resistance of the live-earth conductor loop is calculated.