Loop Impedance (Zs) & RCD Disconnection-Time Verification
Two measurements answer one question: will the protective device actually disconnect a faulted circuit fast enough? Loop impedance (Zs) checks the overcurrent path against the device's U0/Ia characteristic; RCD trip-time and trip-current testing checks the separate, faster earth-leakage path. Both belong in the same test sequence — neither replaces the other.
How the verification fits the test sequence
Continuity first
Protective conductor and bonding continuity is confirmed before any loop or RCD measurement — without a sound earth path, Zs and RCD readings mean nothing.
Loop impedance (Zs)
Phase-to-earth fault loop impedance is measured per circuit and compared against the maximum value permitted by the protective device's rated current and characteristic (the U0/Ia principle) — not a single fixed number.
RCD trip time and current
For circuits with residual-current protection, trip time and trip current are tested against the applicable pass criteria for that device — covered in depth on our separate RCD-type reference.
Protocol and sign-off
Every reading is logged in the measurement protocol against the standard's criteria — pass/fail per circuit, ready for handover, insurance, or the authority having jurisdiction.
Why loop impedance and RCD testing are two separate checks
The governing principle for Zs
The maximum permissible loop impedance isn't a fixed figure — it's derived from the protective device's rated current and disconnection characteristic against the nominal voltage to earth. Change the breaker, change the limit.
Overcurrent devices act on Zs
A fuse or breaker only clears a fault fast enough if the loop impedance stays under the value its own characteristic allows — that's what the Zs measurement confirms circuit by circuit.
RCDs catch what breakers miss
Earth-leakage faults too small to trip an overcurrent device can still be dangerous. RCD trip-time and trip-current testing verifies that separate, faster protection path independently.
Neither test substitutes for the other
A correctly rated breaker with too-high Zs may not disconnect fast enough. An RCD alone doesn't confirm the overcurrent path is sound. Both belong in the record.
Thresholds live in the protocol, not in our copy
Exact maximum Zs and RCD pass criteria depend on the specific device, circuit, and applicable standard — we report the measured value against the correct threshold for that circuit, not a generic number.
Part of the full IEC 60364-6 sequence
Zs and RCD testing sit alongside continuity, insulation resistance (measured in MΩ at the appropriate test voltage), earth electrode resistance, and polarity checks in one coherent inspection.
What the verification covers
A focused engineering check you can hand to your client, insurer, or inspection body.
- Zs loop impedance per circuit
- RCD trip-time and trip-current testing
- Protective conductor continuity check
- Measurement protocol with pass/fail results
- Engineering review against the applicable standard
Frequently asked questions
What's the actual difference between loop impedance and RCD testing?
Loop impedance (Zs) confirms the overcurrent protective device — fuse or breaker — will disconnect a phase-to-earth fault fast enough, based on its own rated current and characteristic. RCD testing confirms a separate, typically much faster disconnection path for earth-leakage currents that an overcurrent device may never see. They test different failure modes on the same circuit.
What is the maximum permissible Zs value for my circuit?
There is no single number — the limit is derived from the protective device's characteristic and rated current against the nominal voltage (the U0/Ia principle), and it changes per device and circuit. The correct threshold for your installation is stated in the measurement protocol, not assumed in advance.
Does every circuit need RCD trip-time testing?
Every circuit that is actually protected by an RCD or RCBO needs its trip time and current verified as part of the sequence. Whether a given circuit requires RCD protection at all is set by the applicable wiring regulation for that installation and use case.
Do RCD types (AC/A/F/B) affect how the test is interpreted?
Yes — different RCD types respond to different fault current waveforms, which matters for both device selection and test interpretation. We cover the type differences in detail on our separate RCD-types reference rather than repeating it here.
How often should loop impedance and RCD testing be repeated?
There's no universal interval. Periodic re-testing frequency depends on the installation type, how intensively it's used, and the applicable standard or insurer requirement — it's set per project, not assumed.
Verify your installation's disconnection times
Send us the panel drawings or an existing protocol and we'll review the loop impedance and RCD test coverage with you — as an independent engineering check, not a sales pitch.