RCD types AC/A/F/B — classification and selection by load type

The 531-rcd guide describes when an RCD is required and at what tripping current. That requirement alone is not enough: an RCD must also be the correct type for the fault-current waveform that the connected load can actually produce. IEC 60755 (general requirements for residual current protection) and IEC 62423 (the product standard that specifically formalises type F and type B) describe this classification.

The four types

The types are nested: a type B automatically also meets the requirements of type F, A and AC; a type F meets those of A and AC, and so on. The reverse is not true — a type AC does not detect any DC component at all and can fail completely on a fault with a DC leakage current, without this being visible from the outside.

Why this is more than a margin issue

A smooth DC leakage current (such as can occur with an insulation fault on the DC side of a frequency converter, EV charger or PV inverter) can magnetically saturate the toroidal core of a type AC or type A RCD. A saturated core can no longer detect the AC component of the same or another leakage current — the RCD still appears to function correctly (the test button often still works, because it simulates a pure AC leakage current), while the actual protective function has already failed.

Note: this article gives the classification framework. The exact application requirement (for example type B mandatory for a charge point without internal DC detection, see the 722-ev-charging guide) follows from the specific NEN 1010 chapter for that installation, not from IEC 60755 itself.

Practical relevance

When replacing an existing distribution board — for example when adding a charge point, a frequency converter on a pump or fan, or a PV inverter — it is not enough to check that "a 30 mA RCD" is present. The question is always: which fault-current type can this specific load produce, and is the installed RCD classified for that? See also the safe-torque-off frequency converters guide for the context of three-phase frequency converters.

Common mistakes

1. Reusing an existing type AC or type A RCD when adding a frequency converter, charge point or PV inverter without checking whether the required type (F or B) is actually present.
2. Assuming the RCD test button demonstrates the full protective function — the test button usually only simulates a pure AC leakage current and therefore does not prove that a type AC/A RCD would also correctly detect a DC leakage current.
3. Applying type B everywhere as a default "to be safe" — costly and often unnecessary; the correct type follows from the specific load, not from a generic precautionary rule.

Related

• §531 — Residual current devices (RCD)
• §722 — EV charge points
• Safe torque off (STO) for frequency converters