Earth fault loop impedance

Earth fault loop impedance is the impedance of the path available to earth-fault current from a circuit or point in an installation. It is often recorded as Zs for a circuit, while IET (Institution of Engineering and Technology) model forms record external earth fault loop impedance as Ze at the supply origin. The value matters because a higher impedance can limit fault current and affect whether a circuit-breaker, fuse or RCD (residual current device) can disconnect within the required time. In home energy work it can appear on Electrical Installation Certificates, Electrical Installation Condition Reports, circuit schedules, EV chargepoint paperwork, inverter documents and heat-pump electrical checks.