Testing the RCD — trip time and residual current (NEN-EN-IEC 61557-6)

Besides the one-off test-button check (which only tests the mechanism, without simulating an actual residual current), a periodic NEN 3140 inspection includes an instrumented test of the residual current device (RCD): a calibrated RCD tester injects a defined residual current and measures the actual trip time. This is a different test from the loop impedance measurement (Zs), which assesses the fault loop of the installation — the trip-time measurement assesses the switching mechanism of the RCD itself.

Two types, two sets of limit values

NEN-EN-IEC 61557-6 describes the test method and the associated trip-time limits. These differ substantially between two RCD types:

An S-type RCD therefore has a minimum trip time (it must not switch too quickly) in addition to a maximum — the goal is that an S-type RCD upstream gives time for a faster, non-delayed type downstream to trip first, so that only the affected part of the installation is de-energized (selectivity between two RCDs connected in series).

Note: the exact test is carried out with an AC test signal at the rated residual operating current I∆n of the RCD — the test methodology and the permitted measurement uncertainty of the tester itself are further elaborated in NEN-EN-IEC 61557-6.

Why this is a separate test

An RCD can still mechanically "work" (the test button trips it) while the trip time at the rated residual operating current has by now drifted outside the limit value — for example due to ageing of the mechanical spring or contamination. Only an instrumented measurement with a calibrated tester (see also the calibration guide) reveals this; the test button alone does not.

Practical relevance

In an industrial hall or horticultural business with many S-type main RCDs (for example upstream of several subordinate distribution boards, to avoid needlessly tripping the entire installation), the trip-time measurement is essential to verify that the intended selectivity between the main RCD and the downstream RCDs is actually present — an S-type RCD that switches too quickly renders the selectivity design worthless in practice.

Common mistakes

1. Using only the test button as a periodic check — this tests the mechanism, not the actual trip time at the rated residual operating current.
2. Assessing an S-type RCD against the limit values of a general type (for example rejecting it because the trip time "seems too long" at 300 ms, while 130–500 ms is in fact correct for an S-type).
3. Confusing the trip-time measurement with the loop impedance measurement — the former assesses the RCD itself, the latter the fault loop/Zs of the installation; both are needed for a complete assessment of the residual current protection.

Related

• Loop impedance measurement (Zs)
• Residual current devices (RCD)
• Calibration of measuring equipment