IEC 61557 — the standard series for protective-measure test equipment
IEC 61557 — the standard series for protective-measure test equipment
The guide on insulation resistance testing and the guide on loop impedance testing (Zs) each cover one specific measurement performed during a NEN 3140 inspection. This article covers the standard series that often appears on the nameplate of a test instrument — IEC 61557 — and explains why the part number on such a meter indicates what the instrument is, or is not, suited for.
What IEC 61557 is
IEC 61557 is not a single standard but a series of parts, each aimed at one specific type of measurement of a protective measure in low-voltage installations up to 1000 V AC / 1500 V DC. A test instrument built and calibrated to a given part guarantees that the measurement falls within the accuracy limits set by that part.
The main parts
| Part | Subject |
|---|---|
| Part 1 | General requirements (applicable to all other parts) |
| Part 2 | Insulation resistance |
| Part 3 | Loop impedance |
| Part 4 | Resistance of earth connection and equipotential bonding |
| Part 5 | Resistance to earth (earth electrode resistance) |
| Part 6 | RCD testing in TT, TN and IT systems |
| Part 7 | Phase sequence |
| Part 8 | Insulation monitoring devices for IT systems |
| Part 9 | Insulation fault location systems for IT systems |
| Part 10 | Combined measuring equipment combining several of the above functions in one instrument |
Why the part number matters
An instrument certified to Part 4 (resistance of earth connection and equipotential bonding) measures something different from an instrument certified to Part 5 (resistance to earth/earth electrode) — the two measurements look superficially similar (both give a resistance value in ohms), but target different parts of the installation: Part 4 addresses the continuity of protective and bonding conductors within the installation, Part 5 addresses the resistance of the earth electrode itself to the ground. Confusing an instrument's part number leads to a measurement that is technically performed correctly, but verifies the wrong protective aspect.
Practical relevance
When assembling a test-equipment case for NEN 3140 inspections, it is worth checking which IEC 61557 part is listed for each instrument and linking it to the measurements that actually need to be performed according to the inspection protocol: an instrument with only Part 2 (insulation resistance) cannot perform an RCD trip-time test (Part 6), even if its controls look similar to a combined instrument compliant with Part 10.
Common mistakes
- Assuming any instrument marked "IEC 61557" can perform every measurement — the part number specifies a limited function, not full standard compliance for every possible measurement.
- Confusing Part 4 and Part 5 — continuity of protective conductors (Part 4) and earth electrode resistance (Part 5) are two different measurements with different methods and accuracy requirements.
- Using a combined instrument (Part 10) without checking which sub-functions are actually certified — not every combined instrument implements all nine preceding parts; the nameplate or certificate states which functions are covered.
- Ignoring calibration certificates — compliance with IEC 61557 says something about the instrument's design, not about its actual measurement accuracy at the time of use; periodic calibration remains necessary.
Related
Further reading
- IEC 61140Protection class I, II and III — IEC 61140
- InspectieInspecting hand tools — the four test steps and assessment values
- IEC 60364-6 / NEN-EN-IEC 61557-4Protective conductor continuity — the R1+R2 method
- IEC 61439IEC 61439 versus NEN 1010 — manufacturing standard for the panel, installation standard for the installation
- InspectieCalibration of test equipment — traceability and interval
- InspectieTesting the RCD — trip time and residual current (NEN-EN-IEC 61557-6)