R1+R2

The earth wire of a circuit. It links the metal parts of the equipment the circuit feeds back to the main earthing terminal. It is not the normal load-current return path, although equipment can produce protective-conductor current in normal operation. During an earth fault it provides a path for fault current, working with the earthing arrangement and protective device to achieve disconnection. In UK wiring it is green and yellow, or bare inside twin-and-earth cable, with green-and-yellow sleeving wherever it is exposed.

The total opposition to current around the loop a fault current would travel. That loop runs from the supply, along the line conductor to the fault, and back through the protective conductor and the earthing. It is the supply's own part (Ze) plus the circuit's R1 + R2, the resistance of its line and protective conductors. A low Zs lets a big fault current flow, which trips the protective device quickly, so electricians measure it and compare it with the maximum allowed for that device.

Low-resistance measurement proving that conductors which must be complete actually are. That means protective conductors back to the earthing terminal, bonding conductors to services, and the legs of ring final circuits. Performed dead with a low-ohm meter, it is the first step of the test sequence because everything after it assumes the earth path exists.

Resistance describes opposition to current that transfers electrical energy into heat, and is measured in ohms (Ω). For an ohmic component at a fixed temperature, greater resistance means less current at the same voltage. A heating element uses resistance deliberately; cable resistance also causes heating and voltage drop. Resistance is only one part of AC impedance.

Impedance is the combined opposition that resistance and reactance present to alternating current, measured in ohms (Ω). Reactance comes from inductance and capacitance, so impedance is not always the same as a resistance reading. Earth fault loop impedance, Zs, describes the whole fault-current path; R1+R2 describes the resistance of two circuit conductors.