The short answer
RCDs are classified by the types of fault current they can detect. Type AC responds only to sinusoidal alternating current earth faults — the standard 50 Hz waveform of the UK mains supply. Type A detects everything Type AC does, plus pulsating DC fault currents — produced by modern electronics such as washing machines, dishwashers, EV chargers and inverter-driven equipment. Type F adds detection of composite high-frequency currents superimposed on mains frequency, present in variable-speed drives and some HVAC equipment. Type B responds to all of the above plus smooth DC fault currents, required where three-phase rectifiers, certain PV inverters or bidirectional EV chargers are used. Under BS 7671 18th Edition, the minimum for most new domestic circuits is Type A, because modern appliances produce pulsating DC components that Type AC will not detect reliably.
The type classification of an RCD is not just a marketing distinction — it determines which real-world fault currents the device can detect. Fitting the wrong type means some faults will simply not trip the protection. With the growth of modern electronics and EV charging, understanding which type you need has become a practical question for many homeowners.
RCD type summary
- Type ACSinusoidal AC faults only
- Type AAC + pulsating DC (minimum for most modern circuits)
- Type FAC + pulsating DC + high-frequency composites
- Type BAll of the above + smooth DC
- BS 7671 18th EditionType A minimum for most new domestic circuits
Type AC — the original domestic RCD
Type AC is the original RCD type and was the standard for domestic installations for decades. It operates correctly on earth-fault currents that are pure sinusoidal alternating current — the 50 Hz waveform of the UK grid. The vast majority of traditional resistive loads (incandescent lamps, electric heaters, simple motors) produce fault currents of this form.
The limitation of Type AC becomes apparent with modern loads. An appliance containing a rectifier or an electronic motor controller does not produce a clean 50 Hz fault current; instead, it produces a pulsating DC component. Type AC may not detect this reliably or at all. In some cases the pulsating DC partially saturates the RCD's toroidal core, reducing its sensitivity to the AC component and making it less effective even for the faults it is supposed to catch.
For this reason, Type AC is no longer considered suitable as the minimum for general domestic use where modern appliances are present, and BS 7671 now points designers towards Type A as the minimum standard.
Type A — the practical minimum for modern homes
Type A detects everything Type AC does, and additionally responds to pulsating (half-wave rectified) DC fault currents. These are produced by modern household appliances with single-phase rectifier circuits — including:
- Washing machines and dishwashers with electronic motor drives
- Most modern EV chargers (Mode 3, single-phase)
- LED drivers and electronic dimmers
- Computer power supplies and chargers
- Heat pumps and inverter-driven appliances
Under BS 7671 18th Edition, Type A is the minimum recommended type for circuits supplying these kinds of loads — which in practice means most modern domestic circuits. A consumer unit being replaced today should use Type A RCDs or RCBOs throughout rather than Type AC, unless the circuits genuinely supply only simple resistive loads.
Type F and Type B — for specialist applications
Type F adds the ability to detect fault currents that include high-frequency components superimposed on the mains waveform, produced by variable-speed drives and some modern HVAC equipment. It is not a requirement for the typical domestic installation but may be specified where that equipment is present.
Type B is the most comprehensive classification. In addition to everything Types AC, A and F detect, Type B responds to smooth (non-pulsating) DC fault currents. These can arise from three-phase rectifier circuits, certain photovoltaic inverter configurations, and bidirectional EV chargers (vehicle-to-grid, V2G systems). BS 7671 and the product standards for V2G chargers specify Type B RCD protection at the charger supply point, because the charger can inject smooth DC into the protective conductor.
| RCD type | Detects sinusoidal AC | Detects pulsating DC | Detects HF composites | Detects smooth DC | Typical domestic use |
|---|---|---|---|---|---|
| Type AC | Yes | No | No | No | Older boards; not recommended for new work |
| Type A | Yes | Yes | No | No | Standard domestic minimum (18th Edition) |
| Type F | Yes | Yes | Yes | No | Variable-speed drives, some HVAC |
| Type B | Yes | Yes | Yes | Yes | V2G chargers, PV inverters, three-phase rectifiers |
RCD type detection capabilities. Source: BS 7671:2018+A2:2022; BS EN 61008 / BS EN 62423 product standards.
Frequently asked questions
What type of RCD do I need for an EV charger?
For a standard single-phase Mode 3 EV charger, a Type A RCD is the minimum required under current standards. Many EV charger manufacturers and BS 7671 guidance specify Type A protection at the consumer unit or within the charger unit itself. For bidirectional (V2G) chargers, Type B protection is required because the charger can produce smooth DC fault currents. Always check the charger manufacturer's installation requirements.
Can I replace a Type AC RCD with a Type A?
Yes, and it is generally an improvement — a Type A is a more capable device that also handles the pulsating DC fault currents that Type AC misses. Check that the replacement device is compatible with your consumer unit's busbar and is the correct current rating. This work should be done by a registered electrician.
Why does BS 7671 now require Type A as the minimum?
Because modern household appliances — washing machines, dishwashers, EV chargers, LED drivers — contain rectifier circuits that produce pulsating DC fault currents. Type AC RCDs may not detect these reliably, or the DC component may reduce their sensitivity to AC faults. Type A was made the minimum to ensure protection remains effective with the appliances now in common use.
Sources & further reading
- IET — BS 7671:2018+A2:2022 (18th Edition Wiring Regulations)
- Electrical Safety First — RCDs and their types
- NAPIT — guidance on RCD types for EV chargers
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.