Type AC, A and B RCDs for home EV chargers: what you need to know

Helpful video reference. We use Artisan Electrics' tutorial "Don't Make This Mistake When Installing an EV Charger — RCD and RCBO selection" as the video reference here. Artisan Electrics is a Cambridge-based electrical company with over 369,000 YouTube subscribers, specialising in EV chargers, solar and battery storage. Their explanation of RCD selection under UK wiring regulations is accurate and practical.

1. Why RCD type matters for EV chargers

A residual current device (RCD) monitors the difference between the current flowing out on the line conductor and back on the neutral. If more than 30mA is flowing in the wrong direction, the RCD trips in under 40 milliseconds, cutting the supply before a lethal shock can develop.

The problem is that EV chargers produce fault currents that a simple Type AC RCD cannot detect. The electronics inside a Mode 3 charge point (the type used for dedicated home chargers) can produce a smooth DC component in any residual current. A Type AC RCD is designed for pure 50 Hz alternating current faults only. It will not trip on a smooth DC fault, which means it is not sufficient for EV charger circuits.

2. What a Type AC RCD detects (and its limits)

Type AC RCDs are the most common type in older UK consumer units. They respond reliably to sinusoidal alternating fault current at 50 Hz. You will find them protecting socket circuits, lighting circuits and most general circuits in homes built before 2010.

They are not suitable for EV charger circuits because the power electronics in modern chargers can create pulsating DC or smooth DC fault currents. A smooth DC fault will actually saturate the Type AC RCD's core and make it less sensitive to the AC fault component as well, so you can end up worse off than with no RCD at all.

3. What a Type A RCD adds

A Type A RCD detects everything a Type AC does, plus pulsating DC residual current and smooth DC residual current up to 6mA. This makes it suitable for the majority of single-phase Mode 3 EV chargers sold for home use, including units from Ohme, Myenergi, Andersen and most other popular brands.

In practice, most dedicated home chargers either include a Type A RCBO internally (in which case the installer may fit a standard MCB in the consumer unit) or require an external Type A RCBO on the circuit. Either way, the key is that Type A protection must be present somewhere in the circuit.

4. When a Type B RCD is required

A Type B RCD detects all alternating and DC fault current types up to 1000 Hz. It is physically larger and significantly more expensive than a Type A, but some charger manufacturers specify it.

Three-phase chargers (for homes with three-phase supplies) typically require Type B protection. Some single-phase chargers with active power factor correction circuitry also specify Type B in their installation manuals. Always follow the manufacturer's stated requirement, not just minimum BS 7671 guidance.

5. The PME earthing complication

Most UK homes are supplied with PME (protective multiple earthing), also known as TN-C-S. In this system, the earth and neutral conductors are combined in the supply network and separated at your meter. It is generally safe, but there is a specific risk for EV chargers.

If the combined neutral-earth conductor on the supply network develops an open circuit (a fault that does happen, particularly on older overhead supplies in rural areas), your PME earth terminal could rise to a dangerous voltage relative to true earth. Anyone touching the car body while standing on the ground could receive a shock.

BS 7671 regulation 722.411.4.1 addresses this. For a Mode 3 charger on a PME supply, the installer must either: fit a TT earth electrode (an earth rod in the ground) for the charger circuit, or use a device with RDC-DD (residual DC detection device) protection, or specify a Type B RCBO. The specific solution depends on the charger model and the installer's assessment of the site.

6. Confirming your installer's specification

Before accepting a quote, ask the installer two things: what RCD type they are fitting, and what the charger manufacturer specifies in the installation manual. These answers should match. If an installer proposes fitting a standard Type AC RCBO to save cost, that is non-compliant with both BS 7671 and the manufacturer's instructions.

At the end of the installation, you should receive an Electrical Installation Certificate (EIC) or Minor Works Certificate confirming that the circuit has been installed and tested to BS 7671. Keep this document. If you sell the property or make an insurance claim, it will be needed.

When to call us

Richard installs OZEV-approved 7.4 kW home chargers across Sandwich, Deal, Dover, Ramsgate and Canterbury. Every installation is carried out to BS 7671 with the correct RCD type for your charger and earthing system, and a full Electrical Installation Certificate on completion.