Type A vs Type B RCDs, and Which One an EV Charger Needs

A Type A RCD trips on ordinary AC earth leakage and on pulsating DC, and still works with a small amount of smooth DC present (up to 6 mA). A Type B RCD does all of that and also trips on smooth DC fault current. That one difference is why an EV charger needs either a Type B RCD, or a Type A RCD together with a charger that has its own 6 mA DC detection built in.

What an RCD does, in two sentences

A residual current device compares the current going out on the line conductor with the current coming back on the neutral. If some is missing, it is leaking somewhere it shouldn't, possibly through a person, and a 30 mA RCD disconnects the circuit fast enough to reduce the risk of a fatal shock.

The catch is that not all leakage looks the same. Modern electronics can leak current that isn't a clean AC wave, and an RCD designed only for AC may not see it, or may be "blinded" by it so it fails to trip on a fault it would otherwise catch.

The four RCD types: AC, A, F and B

BS 7671 describes four types. The IET summarises them like this:

• Type AC trips on AC residual current only. It suits loads with no electronics, such as a simple immersion heater, electric shower or tungsten lighting.

• Type A trips on AC and on pulsating DC residual current, and tolerates smooth DC up to 6 mA. It suits most modern household loads, including LED drivers, induction hobs and IT equipment.

• Type F does everything a Type A does, handles smooth DC up to 10 mA, and is designed for loads with variable speed motors, such as some washing machines, dishwashers and air conditioning units.

• Type B trips on AC residual current up to 1 kHz, on pulsating DC and on smooth DC. It suits inverters, some solar PV systems, lifts, welding equipment and EV charging where smooth DC fault current could exceed 6 mA.

Each type up the list covers everything the one before it does, plus more.

Why is Type AC now restricted?

Because homes are full of electronics that can produce DC leakage. BS 7671 only allows a Type AC RCD in limited cases, where it is known that there will be no DC fault current on the circuit. In a modern house that is hard to promise for a socket circuit, so new work normally uses Type A as the minimum.

If your consumer unit has Type AC RCDs, it is not automatically dangerous or illegal. BS 7671 isn't retrospective. But if you're adding a charger, a heat pump or other electronic loads, it is worth knowing, because DC leakage from those loads can affect Type AC devices upstream.

Why do EV chargers need DC fault protection?

A car's on-board charger converts AC to DC, and some fault conditions can put smooth DC current onto the earth path. Smooth DC above 6 mA can stop a Type A RCD from tripping. So an ordinary RCD on its own is not enough, and choosing the right protection is part of every EV charger installation we carry out.

BS 7671's section on EV charging, as the IET sets it out, requires each charging point with a socket-outlet or vehicle connector to be protected individually by an RCD of Type A, Type F or Type B, rated at no more than 30 mA and disconnecting all live conductors. Where the charger doesn't provide DC protection itself, a Type B RCD is needed. Where the charger does, a Type A or Type F RCD is enough.

That DC protection is usually a residual direct current detecting device (RDC-DD) built into the charger, made to BS IEC 62955, which cuts the supply when smooth DC leakage reaches 6 mA. Many home chargers include one, but confirm it on the data sheet before you buy, because it decides which RCD your installer fits.

Type A plus 6 mA detection, or a Type B RCD?

Both are compliant. The differences are practical:

• Type A with a charger's built-in DC detection is the usual home arrangement. Type A devices are cheaper, easy to source, and fit a standard consumer unit or a small enclosure.

• A Type B RCD is needed when the charger has no DC detection. Type B devices cost considerably more and are larger.

For most households, choosing a charger with built-in 6 mA DC detection is the simplest way to keep the installation standard. It is one of the extras we list in our breakdown of EV charger installation cost.

Watch the RCDs upstream of the charger

DC fault current doesn't only affect the RCD protecting the charger. It can pass back and affect RCDs further up the installation. The IET warns that the hierarchy of RCDs matters here, and specifically that a Type AC RCD should not be installed upstream of a Type A RCD.

In a typical house, that means looking at the whole board, not just the new circuit. If the charger's circuit comes from a split-load board with Type AC RCDs, or the house has an old RCD at the origin, the installer may recommend changing the charger's supply arrangement or the board itself. Sometimes a consumer unit upgrade is the cleanest fix, particularly when the board is old or full.

How to tell which RCD type you have

Each RCD or RCBO is marked with its type on the front of the device, as a letter, a symbol or both. If you can't read the marking, or you'd need to remove a cover to see it, leave it for an electrician. An EICR or a survey for a charger will record what's there, along with the age and condition of the board.

Heavier electronic loads, from chargers to heat pumps, are a big reason RCD choice matters more than it used to, as we cover in our look at UK home electrical trends. If you want your board checked before a charger goes in, anywhere across Wolverhampton, Telford or Shropshire, we can look at it on a survey.

RCD questions, answered

Do all sockets need RCD protection?

Most do. BS 7671 requires 30 mA RCD protection for socket-outlets rated up to 32 A, and for mobile equipment up to 32 A used outdoors. There is a narrow exception backed by a documented risk assessment, but it can't be used where the sockets are used by ordinary people, children or vulnerable people, so in practice it doesn't apply in homes.

Is an RCBO the same as an RCD?

An RCBO is an RCD and a circuit breaker in one device, so each circuit gets its own residual current protection. RCBOs come in the same types, so a Type A RCBO behaves like a Type A RCD.

Does a heat pump or solar inverter need a special RCD?

It depends on the equipment. The IET lists inverters among the loads that may need Type B, while many other electronic loads are fine on Type A. The manufacturer's installation instructions state what the equipment needs, and the installer should follow them.