RCD Types: AC, A, F and B Explained
Not every RCD detects every fault: Type AC sees only clean AC, Type A adds pulsating DC, Type F handles mixed frequencies, and Type B catches smooth DC — get the type wrong and modern electronic loads can blind the device to a real fault.
An RCD is only as good as the fault it can see, and the electronics in modern loads — variable-speed drives, EV chargers, solar inverters, LED drivers — can produce residual currents that a basic RCD is simply not built to detect. The RCD type letters (AC, A, F, B) describe exactly which kinds of residual current each device will respond to.
This guide explains the four types, what residual current waveforms each detects, why Type AC is no longer the default choice, and where Type A, F and B are needed — including the EV and solar work where Type B or specific DC-fault detection is essential.
Key takeaways
- Type AC detects only sinusoidal AC residual currents — no longer suitable as a default under the 18th Edition.
- Type A detects AC plus pulsating DC residual currents — the general-purpose choice for most circuits with electronic loads.
- Type F detects Type A currents plus mixed frequencies — for circuits with frequency inverters, like some appliances and drives.
- Type B detects all of the above plus smooth (pure) DC residual current — required for EV charging and many solar/three-phase installations.
- Choosing too low a type can leave the RCD blind to the very fault current the equipment can produce.
Why RCD type matters
An RCD trips on residual current — the imbalance between line and neutral that means current is leaking to earth. But the shape of that leakage current depends on the load. A simple resistive fault produces a clean AC residual current; electronic equipment with rectifiers and switching can produce pulsating DC, mixed frequencies, or even smooth DC. Each RCD type is built to detect a defined set of these waveforms and no more.
The danger is a type mismatch: a smooth DC residual current can saturate the sensing core of a lower-type RCD and stop it detecting even ordinary AC faults — a phenomenon called blinding. So fitting the wrong type isn't just a device that misses exotic faults; it can be an RCD that fails to protect against the everyday ones too. That's why the 18th Edition pushed the default up from Type AC.
The blinding problem
A DC residual current from an EV or inverter can magnetically blind a Type AC or Type A RCD, stopping it seeing an AC fault as well. Match the type to the load, or the RCD may protect nothing.
Type AC and Type A
Type AC detects only sinusoidal AC residual currents. It was the workhorse for decades, but the 18th Edition no longer regards it as suitable for general use, because so many modern loads produce DC components it cannot see. It survives only where the load genuinely produces nothing but clean AC leakage — an increasingly rare case in a house full of electronics.
Type A detects sinusoidal AC and pulsating DC residual currents. Pulsating DC is what you get downstream of the single-phase rectifiers in LED drivers, chargers, washing machines and the like, so Type A covers the vast majority of ordinary domestic and commercial circuits. Under the 18th Edition, Type A is effectively the sensible minimum general-purpose RCD for most final circuits.
Type F
Type F detects everything Type A does plus residual currents at mixed frequencies — the composite waveforms produced by single-phase frequency inverters. It's aimed at circuits feeding equipment with variable-speed drives on a single-phase supply, such as some modern washing machines, tumble dryers, air conditioning units and industrial single-phase drives.
Type F sits between A and B: it handles the frequency-varying leakage that A can't, but not the smooth DC that B is designed for. Where a manufacturer specifies Type F for a particular appliance or drive, that recommendation exists because the equipment produces the mixed-frequency residual current that lower types would miss.
Type B: smooth DC and where it's essential
Type B detects all the residual currents the lower types do plus smooth (pure) DC residual current. Smooth DC leakage comes from three-phase rectifiers, many solar PV inverters, some EV charging arrangements and other power-electronic equipment. Only a Type B RCD can detect it — and because smooth DC can blind the lower types, using Type B where smooth DC is possible protects the AC detection as well.
This is where the rules bite in real work. EV charge points must have protection against DC fault current: that means either a Type B RCD, or a Type A RCD combined with a device providing 6mA DC fault detection (RDC-DD) built into the charger — the two accepted routes. Solar PV and three-phase drive installations similarly often call for Type B where smooth DC residual current can occur. Fitting a Type A alone on such a circuit leaves a real, undetectable fault path.
EV: Type B or A + 6mA DC detection
An EV charge point needs DC fault protection. Either a Type B RCD, or a Type A RCD plus built-in 6mA DC fault detection (RDC-DD) in the unit. A plain Type A on its own isn't enough.
Choosing the right type
Work from the load. If the circuit only ever sees clean AC leakage, Type AC is technically adequate but rarely the right call today. For general domestic and commercial circuits with ordinary electronics, Type A is the sensible minimum. For single-phase frequency-inverter loads, follow the equipment maker and use Type F. For anything that can produce smooth DC — EV charging, solar PV, three-phase rectified loads — use Type B (or the accepted Type A plus RDC-DD arrangement for EV).
The types nest: a higher type detects everything the lower ones do, so when in doubt going up a type is safe, only more expensive. What's never safe is going down — fitting a Type AC or Type A where smooth DC can occur risks blinding the device and losing protection entirely. Check appliance and inverter documentation; the required RCD type is usually stated for exactly this reason.
Frequently asked questions
What's the difference between Type AC, A, F and B RCDs?
Type AC detects sinusoidal AC only; Type A adds pulsating DC; Type F adds mixed frequencies from single-phase inverters; Type B adds smooth (pure) DC. Each higher type detects everything the lower ones do plus more.
Why isn't Type AC recommended any more?
Modern loads produce DC components in their leakage current that a Type AC can't see, and smooth DC can blind it to AC faults too. The 18th Edition no longer treats Type AC as suitable for general use — Type A is the sensible minimum for most circuits.
What RCD type does an EV charger need?
It needs protection against DC fault current: either a Type B RCD, or a Type A RCD combined with a 6mA DC fault detection device (RDC-DD) built into the charge point. A plain Type A on its own doesn't provide the required DC fault protection.
Do solar PV installations need Type B RCDs?
Often, yes. Many solar PV inverters and three-phase rectified loads can produce smooth DC residual current that only a Type B RCD can detect. Follow the inverter manufacturer's documentation, which states the required RCD type for the equipment.
From guidance to action
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