16/02/2026
How many electricians do you know that, when carrying out an EICR, only carry out Zs testing?
Zs testing is a live earth fault loop impedance test. It measures the total opposition to fault current around the entire loop — line conductor, CPC, connections, the supply transformer, distributor’s earth path and any reactance present in the circuit. It confirms that, under fault conditions, enough current will flow to achieve automatic disconnection within the required time. What it does not do is prove the integrity of each conductor, joint or ring configuration. It tells you the loop works under load — not necessarily that the copper is healthy.
Resistance testing (R1, R2, R1+R2 and ring continuity) is a dead test. With the supply isolated, you are measuring the true resistance of the conductors and terminations only. This exposes high-resistance joints, loose connections, broken rings and poor CPC continuity. Here’s where it gets interesting: you may record a slightly higher resistance during dead testing, yet when you carry out a live Zs test the reading appears lower and comfortably within limits. That’s because impedance includes supply characteristics and AC effects. Inductive and capacitive elements within the system, along with the contribution of the network, can influence the measured loop value. The supply effectively becomes part of the test result, and that can mask marginal conductor issues.
So a “good” Zs does not automatically mean a sound circuit.
Zs proves disconnection time.
Resistance proves conductor integrity.
If you’re only doing live loop tests on an EICR, you’re checking performance under fault — not verifying the condition of the wiring itself. And that’s a big difference. ⚡
