Earth loop impedance testing is essential since if a live conductor is accidentally connected to an earth conductor in a faulty appliance or circuit, the resulting short-circuit current to earth can easily be high enough to cause electric shock or generate enough heat to start a fire. Normally, the fuse will blow or another circuit protection device will trip, but a situation may arise where the actual short-circuit current in a faulty installation is of insufficient level and the protection device would thus take too long to activate. The delay can be disastrous for life and property. It is therefore necessary to know if the impedance of the path that any fault current would take is low enough to allow sufficient current to flow in the event of a fault and that any installed protective device will operate within a safe time limit.

 

As measurement of circuit loop impedance is made with the supply normally on, precautions must be taken to avoid the possibility of electric shock and danger to personnel working in the vicinity of the circuit under test.

 

Verifying protection by automatic supply disconnection