"Earth fault loop impedance" is a measure of the impedance, or electrical resistance, on the earth fault loop of an AC electrical circuit, explains Alert Electrical. The earth fault loop is a built-in safety measure within electrical systems to prevent electric shock.
Coming into contact with a live current in an AC circuit could potentially be fatal. This can occur by accident when a non-live portion of an electrical system becomes live due to a system fault or short. An upstream protective device, such as a residual current device, must quickly detect such a short has occurred and intervene by shunting the current through the earth fault loop rather than the individual in contact with the live current.
Alert Electrical further explains that the greater the impedance value on the earth fault loop, the longer it takes the RCD to make the current path switch. If the earth fault loop impedance value is too great, this delays the switch to the point a person may still receive a harmful, even fatal electrical shock before the switch can take place. For that reason, the earth fault loop impedance of every AC circuit must be low enough for the RCD to function properly.