Where should the earth fault loop impedance of a radial circuit be measured?

Measuring the earth fault loop impedance at the furthest point of a radial circuit is essential for accurately assessing the circuit’s performance and safety. This measurement location provides a clear indication of the impedance that the circuit conductor presents to earth fault conditions, which is critical for ensuring that protective devices operate correctly in the event of a fault.

When the impedance is measured at the furthest point in the circuit, it accurately reflects the worst-case scenario in terms of voltage drop that could occur during a fault. This is vital for confirming that the disconnection times of protective devices are within the recommended limits, as set out in relevant standards and regulations. It also helps identify any potential issues, such as inadequate earthing or excessive resistance, that might not be detected if measurements were taken at other points along the circuit.

Measuring at alternative locations, such as the distribution board or halfway point, may yield different values that do not represent the actual conditions experienced at the furthest point, making those measurements less reliable for assessing the overall safety and functionality of the circuit. Therefore, the correct approach is to always measure at the furthest point of the radial circuit.