RCD Types: AC, A, F, and B Explained

RCDs (residual current devices) are classified by the type of earth fault current they can detect. The four main types — AC, A, F, and B — each detect progressively more waveforms. Using the wrong type means the RCD may not trip during a fault, leaving people unprotected.

Modern homes are full of electronic loads that produce non-sinusoidal fault currents — LED drivers, EV chargers, heat pumps, washing machines with inverter motors, and IT equipment. Understanding which RCD type is needed for each circuit is now a core competency for UK electricians.

This guide explains what each type detects, when it is required under BS 7671:2018+A4:2026, the cost differences, and practical guidance for specification.

Type AC is the simplest and cheapest RCD type. It detects sinusoidal (pure sine wave) AC earth fault currents only. This was adequate when domestic loads were predominantly resistive (heaters, incandescent lighting, kettles) or simple inductive (motors with direct mains connection).

Type AC is no longer recommended for new domestic installations in the UK. BS 7671 Regulation 531.3.3 effectively requires Type A as the minimum for most circuits. Existing Type AC devices remain acceptable where the loads are purely sinusoidal, but this is increasingly rare.

Type A detects sinusoidal AC earth fault currents and pulsating DC earth fault currents. It is designed to handle the fault characteristics of single-phase rectifier loads — which covers the vast majority of modern domestic appliances.

Type A is now the standard choice for most domestic circuits. It provides reliable protection for modern electronic loads at a modest cost premium over Type AC. For a consumer unit upgrade or new installation, Type A RCBOs should be the baseline specification.

Type F detects everything Type A detects plus composite fault currents — complex waveforms produced by single-phase frequency converters (variable speed drives). These generate fault currents that combine multiple frequencies, which Type A may not reliably detect.

Not all appliances in these categories require Type F — many are compatible with Type A. The equipment manufacturer instructions are the definitive reference. If the instructions specify Type F, you must use it. If they specify "Type A minimum", Type A is sufficient but Type F would also work.

Type B is the most comprehensive RCD type. It detects sinusoidal AC, pulsating DC, composite, and smooth DC earth fault currents. Smooth DC fault currents are produced by three-phase rectifier circuits and DC power systems.

Type B devices are significantly more expensive (£80 to £200 for an RCBO) and physically larger than other types. They should only be specified where the load genuinely produces smooth DC fault currents. For most domestic single-phase installations, Type B is not required. Some EV charger manufacturers include built-in DC fault detection (6mA DC detection), which allows the use of a Type A RCD upstream — check the manufacturer documentation.

The cost difference between RCD types is significant, particularly for Type B devices:

For a typical 12-way domestic RCBO board, using Type A across all circuits costs approximately £180 to £360 for the RCBOs. Replacing one or two circuits with Type F for an EV charger and heat pump adds £20 to £40 to the total. Type B should only be specified where genuinely required — the cost difference is not justified on a precautionary basis.

The simplest approach for domestic installations: specify Type A RCBOs as the default for all circuits, then upgrade individual circuits to Type F or Type B where the connected equipment requires it. Always check the appliance manufacturer instructions.

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