Combine valid evidence, compare the right limit and keep fault current separate from RCD sensitivity.
Ze describes the external earth fault loop impedance. For the relevant circuit, Zs can be established from Ze plus the circuit's measured R1 + R2. Use valid supply information and dead-test results; avoid unnecessary live testing.
Worked check
An exercise supplies Ze = 0.23 Ω and R1 + R2 = 0.41 Ω. It also gives an applicable comparison limit of 0.72 Ω, already adjusted for the test conditions.
Zs = 0.23 + 0.41 = 0.64 Ω.
The result is 0.08 Ω below the supplied limit. It passes this numerical comparison, which is one check on the circuit, not all of them. On real work, identify the protective device, the required disconnection time and the temperature allowance before comparing values.
Do not reverse the argument
Subtracting Ze from a live Zs reading does not reliably prove CPC continuity: parallel paths may have lowered the measured loop impedance.
Prospective fault current informs the protective device's required fault-breaking capacity. An RCD's 30 mA sensitivity is a different characteristic and cannot replace that check.
Decision check: a low live loop reading accompanies a failed dead CPC continuity test. Investigate the conductor fault; the low loop reading does not cancel it.
Loop impedance in more detail
- Zs is the total earth fault loop impedance: the external part Ze plus the circuit's own line and protective conductor resistance, R1 + R2. For a given voltage in the usual voltage-source model, a lower Zs permits more fault current. Whether the device meets the required disconnection time depends on its characteristic. Use the applicable Zs limit and account for the actual source behaviour.
- Prefer valid supply information and dead-test evidence where these give the required result safely. A live Ze measurement may require separating the earthing conductor to remove parallel paths. Plan that work and assess diverted-current hazards first: opening the installation’s main switch does not guarantee a current-free earth connection. Restore and verify the protective connection before returning the installation to service.
- For single-phase supplies up to 100 A, IET design guidance uses 0.8 Ω for TN-S and 0.35 Ω for TN-C-S unless the distributor advises otherwise. These are design assumptions, not guaranteed measured maxima. The 16 kA maximum-PFC calculation recommendation also assumes a service cable longer than 2 m; validate the actual supply characteristics by an appropriate method.
- When conductors are cold during testing, allow for their resistance rising in service. A common comparison uses 0.8 × the tabulated maximum. Do not apply this again if the supplied limit already includes a cold-test allowance.
- The maximum can be worked out as Zs(max) = (Cmin × U0) ÷ Ia, where the stated calculation uses Cmin = 0.95 and Ia is the current needed for the required disconnection time. For the usual short-time magnetic-operation check of BS EN 60898 breakers, use 5 × In for Type B and 10 × In for Type C; other conditions require the applicable characteristic.
Worked example. A 32 A Type B breaker has Ia = 5 × 32 = 160 A, so Zs(max) = 0.95 × 230 ÷ 160 = about 1.37 Ω. Against the 0.8 rule, a reading taken cold should not exceed 0.8 × 1.37 = about 1.10 Ω.
- Calculating Zs from Ze + R1 + R2 avoids a live loop test where one would disturb sensitive equipment, and live testing is planned carefully because of its hazards. On an RCD-protected circuit, a suitable low-current loop test reduces the likelihood of unwanted RCD operation, but cannot guarantee that the device will remain closed.
- For TT RCD fault protection, RA × IΔn ≤ 50 V checks the electrode-plus-protective-conductor resistance against the rated residual operating current. A 100 mA device gives 50 ÷ 0.1 = 500 Ω. This does not cap actual fault voltage at 50 V: fault current can be larger until disconnection. Verify the required disconnection time as well. An electrode above 200 Ω may be unstable; a lower value still has to meet the other checks.
What prospective fault current covers
The prospective fault current at a point is the greater of two values. The prospective short-circuit current is assessed between live conductors: line to neutral, or line to line on a three-phase board, where it can be higher. The prospective earth fault current is assessed between line and earth. Find it at the origin and at other relevant points and source operating modes. A breaker needs adequate breaking capacity or a manufacturer-verified backup-protection combination; other switchgear has its own fault-duty requirements. A device with insufficient duty may fail to interrupt safely. On a simple circuit supplied from one end, added conductor impedance generally reduces fault current downstream; additional sources can change that pattern.