Impedance

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.

The portion of the earth fault loop that lies outside the installation — the supply transformer winding, the distributor's cables and the earth return path, measured at the origin with the installation's own parallel paths disconnected. Ze depends on the earthing system: a TT installation relying on an electrode in soil sees a far higher value than a TN supply with a metallic return.

The combined resistance of a circuit's line conductor (R1) and its circuit protective conductor (R2), measured end-to-end during dead testing. It proves the protective conductor is continuous, and added to Ze it predicts the circuit's earth fault loop impedance before the supply is ever switched on.

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.

Alternating current reverses direction repeatedly, unlike direct current. An AC voltage also changes polarity over its cycle. Frequency, measured in hertz (Hz), tells you how many complete cycles occur each second. In a three-phase system, the three AC waveforms are offset in time; three-phase does not mean three times the frequency.