Calibration of Test Equipment — Traceability and Interval

A NEN 3140 inspection is only as reliable as the measuring instrument used to carry it out. An insulation resistance tester, loop impedance tester or RCD tester that has itself drifted out of tolerance can wrongly pass an unsafe installation — or, conversely, wrongly fail a safe one. Calibration is therefore not an administrative formality, but a precondition for the validity of the inspection result itself.

Accuracy requirements: NEN-EN-IEC 61557

The accuracy and tolerance requirements that measuring instruments for electrical safety tests (insulation resistance, loop impedance, RCD trip time, earth resistance) must meet are laid down in NEN-EN-IEC 61557. This is the standard against which manufacturers have their instruments certified, and against which calibration is checked.

Calibrating versus adjusting

In practice these terms are often used interchangeably, but they are not the same:

• Calibrating: determining the deviation of an instrument relative to a traceable reference standard — without adjusting the instrument.
• Adjusting (verifying): calibrating plus correcting/adjusting the instrument so that it measures within tolerance again.

A calibration report without correction therefore only states how far an instrument has drifted — not that the problem has been resolved.

Where to have it calibrated

For a result that is genuinely traceable to the national/international measurement standard, calibration must take place at a laboratory that is accredited to ISO/IEC 17025 — in the Netherlands recognised by the Raad voor Accreditatie (RvA). Calibration by a non-accredited party (or an internal "comparison against another instrument") does not produce a traceable, legally defensible result in the event of a dispute over an inspection outcome.

Calibration interval

• Common practice is a calibration interval of 1 year.
• With intensive or heavy use (daily field use, construction-site conditions, frequent drops/knocks), the interval is in practice often shortened to 6–9 months — comparable to the RI&E-based shortening of the inspection interval for work equipment that is used heavily (see the RI&E guide).
• In addition to periodic calibration, an instrument should undergo a functional check before each use (for example measuring a test resistor or a known reference value) — this does not replace calibration, but catches gross instrument faults in between.

Common mistakes

1. Treating calibration as a one-off purchase requirement — an instrument that was certified at the time of purchase can drift out of tolerance due to drops, knocks or ageing; periodic recalibration remains necessary.
2. Calibrating at a non-accredited party — without RvA/ISO 17025 accreditation, the result is not traceable and is therefore legally weaker in the event of a dispute.
3. Confusing calibration with the daily functional check — a quick test-resistor check before use does not replace an annual laboratory calibration.
4. Not applying a shorter interval under heavy field use — just as with work equipment itself, intensive use increases the likelihood of drift beyond tolerance.

Related

• Inspection
• Loop impedance measurement (Zs)
• Test instruments & CAT classification