Abstract: Apparatus is disclosed for detecting a discontinuity or irregularity in a neutral return line of an electrical power distribution network including the neutral return line, an active line and an earth return. The apparatus includes means for measuring a voltage change associated with a deliberate switching of a known impedance in the electrical network wherein the voltage change is due to a discontinuity or impedance irregularity in the neutral return line and means for implementing an algorithm for identifying the discontinuity or impedance irregularity in presence of allowable variations in nominal supply voltage to the electrical network including voltage changes resulting from network operations that mimic or hide a discontinuity or impedance irregularity in the neutral return line. The apparatus also includes means for comparing a result of the measuring with a reference to provide an indication of the discontinuity or impedance irregularity.