Testing type B RCDs — why an ordinary RCD tester can give a false 'pass'

The guide on RCD trip-time testing covers the general trip-time and trip-current limits for a periodic test per NEN-EN-IEC 61557-6. Those limits apply to every RCD type, but the way a tester generates the residual current to measure that trip time must match the type of RCD being tested. For a type B RCD that is a fundamentally different story than for the more common type A or AC.

Why type B is different

As the guide on RCD types (AC/A/F/B) describes, a type B RCD (per IEC 62423) must trip on three fundamentally different kinds of residual current:

Sinusoidal AC up to around 1000 Hz (same as type A/AC).
Pulsating DC (same as type A).
Smooth (pure) DC — this is the distinguishing feature of type B, and precisely the kind of residual current that often arises from an earth fault on the DC side of a variable-frequency drive or a PV inverter with certain topologies.

Why a standard RCD tester does not always expose this

An instrumental RCD test injects a defined test current and measures the actual trip time — but not every RCD tester can generate all three required test-current waveforms. A simple, cheaper tester that only produces a sinusoidal or pulsating test waveform in fact only tests the type A/AC behaviour of the RCD. If that tester returns a "pass" on a type B RCD, that says nothing about the RCD's behaviour under a smooth DC residual current — precisely the scenario for which type B is specifically certified. A type B RCD with a fault affecting only its DC detection can therefore pass a full trip-time test without the underlying problem coming to light.

What is actually needed

A proper test of a type B RCD requires a test instrument that, per NEN-EN-IEC 61557-6, can generate the multi-frequency and DC test currents appropriate to the RCD's type — in practice a tester that explicitly states it is suitable for type B (or F), not only type AC/A. When purchasing or deploying RCD test equipment, it is therefore essential to check the tester's stated type range against the RCD type actually present in the installation, rather than assuming "an RCD tester is an RCD tester."

Practical relevance

Installations with variable-frequency drives, EV charge points (see the guide on IEC 62955 RDC-DD detection at charge points) or certain PV inverter topologies often use a type B RCD precisely to detect smooth DC residual currents. During the periodic NEN 3140 test of such an RCD, it is essential that the tester used actually tests that specific behaviour — otherwise the test gives a false sense of assurance that does not cover what the RCD is meant to catch in practice.

Common mistakes

Using an AC/A-only RCD tester on a type B RCD and interpreting the resulting "pass" as full confirmation of the DC detection function — that function was in fact never tested.
Assuming any RCD tester with a "universal" label supports every test waveform without checking the specifications — not every tester that can test multiple RCD types also supports the smooth-DC test waveform specific to type B.
Confusing the test waveform with the RCD type — the type A test waveform (pulsating DC) does not test the same fault as the type B-specific smooth-DC waveform; a passing type A waveform test says nothing about the type B-specific function.
Not periodically calibrating the test equipment itself (see the guide on calibrating measuring equipment) — a tester that is itself out of calibration can give a deviating trip time regardless of whether the correct test waveform is used.