Earth fault loop impedance testing method
The value of the earth fault loop impedance is the sum of transformer coil winding resistance, phase conductor (L1) resistance and the protective conductor (PE) resistance as well as the source earth resistance and installation resistance.
The Ze earth fault loop impedance measurement is made on the supply side of the distribution board and the main means of earthing, with the main switch open and all circuits isolated. The means of earthing will be isolated from the installation’s earthing system (earth rods) bonding during the test. The Ze measurement will confirm the earth fault loop impedance as the sum of the resistances of the transformer coil winding, phase conductor or supply side and protective conductor resistance, but not the installation earth resistance.
The Zs earth fault loop impedance should be tested at the furthest point of each circuit. In most cases the circuit breaker needs to be bridged out. The total earth fault loop impedance can be measured by plugging a loop tester into a socket outlet, or in some cases with an external earth probe. The value of the earth fault loop impedance is the sum of the resistances of the transformer coil winding , phase conductor (L1) and protective conductor (PE) as well as source earth and installation earth resistance.
When using an external earth probe, the earth fault loop impedance can be measured by touching an external probe directly to an earth bar, collector and connection point of an earth bar. The same measurement can be done by touching the earth probe to exposed, conductive parts of equipment in the circuits and exposed metal parts.
Anyone on site must avoid shock hazard while establishing contact and while doing the test. When buying a loop tester ask for distribution board test leads so that Ze and Zs measurements can be done.