Loop Impedance Measurement (Zs) — Technique, No-Trip and Limit Values

The loop impedance measurement determines the actual impedance of the fault current path (Zs) in practice, using an installation tester. This is a different measurement from the RCD function test (§531 — tests the reaction time/threshold of the residual current device itself) and from the table values for maximum Zs (§413/AUV — the theoretical limit values per protection type and disconnection time). This guide deals specifically with the practical measurement technique using a loop impedance tester.

No-trip versus standard mode

Loop impedance testers generally offer two measurement modes:

• No-trip mode: a low test current, intended to not trip the RCD on the measured circuit during the measurement. The test current generally stays below approximately 50% of the rated RCD current (for example < 15 mA for a 30 mA RCD).
• Standard/high-current mode: a higher test current that is more realistic for the actual fault situation, but that can trip the RCD on the circuit.

No-trip is not an absolute guarantee against unwanted tripping — always check beforehand whether an accidental disconnection during the measurement could cause a dangerous situation (for example with equipment that must not simply lose power).

The 0.8 correction factor

The table values for maximum Zs (from the disconnection-time tables, see the §413 guide) apply on the assumption of approximately 10°C ambient temperature. To account for the increase in conductor resistance under fault-current heating, the rule of thumb used in practice is that the measured Zs must be ≤ 0.8 × the table value.

• This is a simplified rule of thumb, not an exact calculation — at extreme temperatures (for example an attic space in summer heat or an unheated space in winter) a more precise temperature correction is needed instead of the fixed 0.8 factor.
• Some sources instead use a temperature-dependent cT correction factor from a table (for example ~0.96 at 35°C) instead of the fixed 0.8 rule of thumb — if in doubt, check which method the measuring equipment used and the applicable normative framework prescribe.

Prospective fault current (PFC)

The loop impedance measurement does not only provide Zs, but also the prospective fault current (PFC) — the fault current that would occur in the event of an actual fault at that point, calculated according to Ohm's law:

$$I_{PFC} = \frac{U_0}{Z_s}$$

Of the two possible fault paths (phase-earth within the installation versus only the external loop up to the connection), the highest value is recorded as the relevant PFC for assessing the breaking capacity of the protective device at that point.

Common mistakes

1. Using standard mode on an RCD-protected circuit without considering the consequences of an unintended disconnection — no-trip mode is intended for this, but does require that the tester and settings are chosen correctly.
2. Applying the 0.8 factor at extreme temperatures without checking whether a more precise temperature correction is needed — the rule of thumb is a simplification, not a universal truth.
3. Confusing the loop impedance measurement with the RCD function test — one measures the impedance of the fault current path, the other tests the reaction time of the residual current device itself; these are two different measurements with different purposes.
4. Measuring only the internal loop (Zs) and ignoring the external loop (Ze) when assessing the PFC — the highest of the two values is decisive for the required breaking capacity of the protective device.

Related

• §413 — Automatic Power Off (AUV)
• §531 — Residual Current Devices (RCD)
• Measuring Instruments & CAT Classification