Earth loop impedance testing is a process that verifies that, should a fault occur within an electrical installation, sufficient current will operate the fuse or circuit breaker in order to protect the faulty circuit. Earth loop impedance testing is conducted at the furthest point of every circuit, with the circuit breaker being bridged out. RCD & Loop Impedance Testers RCD & Loop Impedance Testers are plugged into a socket or external earth probe to measure the total earth fault loop impedance of the circuit.