Zs

Earth Fault Loop Impedance

The total impedance of the path a fault current takes: out from the source, along the line conductor to the fault, and back through the protective conductor and earthing arrangement. A low Zs lets a large fault current flow, which is what makes the protective device trip inside its required disconnection time — so measured values are compared against the maximum the installed device allows.

Where to look it up: BS 7671 Chapter 41.

Taught and practised in the Testing & Inspection section.