RCD protection for EV chargers

How AC and DC residual-current protection is selected for a home EV chargepoint.

An EV chargepoint needs residual-current protection selected for the currents its power electronics and the connected vehicle can produce. A general-purpose RCD label is not enough.

For the normal Mode 3 home-charging arrangement, each charging point using a BS EN 62196 socket or connector is individually protected by an RCD rated at no more than 30 mA. The RCD must disconnect all live conductors. The final circuit may also need RCD protection for other reasons, including its cable route, fault-protection design or a TT earthing arrangement.

The two parts of the problem

The design has to deal with both:

AC and pulsating-DC residual current, using a suitable 30 mA RCD
smooth DC residual current that could impair an upstream RCD

EV charging equipment can produce smooth DC leakage. If enough of it passes through an RCD that is not designed for smooth DC, the device’s sensing core can saturate and fail to respond correctly to another fault.

BS 7671 therefore provides two common routes:

a Type B RCD, which is designed to respond to smooth DC as well as the residual currents covered by Types A and F
a Type A or Type F RCD combined with suitable residual DC detection in the charging equipment, normally an RDC-DD that causes disconnection if smooth DC reaches 6 mA

A Type AC RCD is not suitable for an EV charging point.

What the device names mean

Type A RCD. Detects sinusoidal AC and pulsating DC residual currents. It can be used for a charging point when the required smooth-DC protection is also provided.

Type F RCD. Adds defined response to certain composite and higher-frequency residual currents, but it is not a substitute for the required smooth-DC measure.

Type B RCD. Detects smooth DC and a wider range of residual-current waveforms. It can provide the required route where the chargepoint does not include suitable DC detection.

RDC-DD. A residual DC detecting device to BS IEC 62955 monitors the DC component and acts through switching equipment. It does not automatically provide the complete 30 mA RCD function.

RDC-PD. A protective form covered by the product standard can combine defined residual-current detection and switching. Its declared conformity still has to match the role assigned to it.

“6 mA DC detection” and “30 mA RCD” are not interchangeable descriptions. One protects the operation of the other unless the product is specifically declared and certified to perform both functions.

Can the RCD be inside the charger?

BS 7671 does not require the charging-point RCD to be at the consumer unit. It can be within the EV charging equipment if it complies with one of the RCD product standards recognised for that duty.

This is where product descriptions can be misleading. A circuit-board function said to behave “like an RCD” is not necessarily an RCD complying with the required standard. The installer should obtain the declaration of conformity and identify separately:

the 30 mA RCD function and its product standard
the 6 mA smooth-DC function and its product standard
the switching arrangement that disconnects the live conductors
the manufacturer’s required upstream protection

If the built-in function does not meet the recognised RCD requirement, a suitable RCD is still needed outside the charger. The decision and supporting documents belong with the installation certificate.

Upstream RCDs matter too

Residual and DC leakage does not stop at the final protective device. Every RCD upstream of the charging point sees the current that passes through it.

The designer should consider:

the type and rating of each upstream RCD
cumulative leakage from the charger and other circuits
whether DC could impair an upstream device
discrimination so one charging fault does not unnecessarily disconnect unrelated circuits
the effect of more than one chargepoint sharing an upstream path
the consumer-unit manufacturer’s limits for device combinations and thermal loading

Adding an EV circuit to a spare way on a shared split-load board can therefore be a poor design even when it physically fits. Individual charging-point protection does not make the upstream arrangement irrelevant.

A long-duration load

Home charging can hold a high current for hours. The cable, terminals, protective device, enclosure and consumer-unit assembly must all be suitable for that duty and installed to their manufacturers’ instructions.

Repeated tripping should be investigated rather than dismissed as a sensitive RCD. Causes can include genuine insulation or vehicle faults, accumulated leakage, a loose or overheated termination, the wrong RCD type, unsuitable device hierarchy, supply-voltage problems or a charger fault. The time of night is not a diagnosis.

Testing and records

Initial verification should prove the fixed circuit and the RCD function using suitable equipment and the current BS 7671 procedure. EV-specific test adaptors can simulate vehicle states and make the chargepoint accessible to the test instrument.

The smooth-DC function should be checked in the way required by the chargepoint and test-equipment manufacturers. It should not be confused with the ordinary RCD test-button check. Where a built-in device is relied upon, its prescribed functional test, declared standard and switching action should be recorded.

The handover record should state:

the RCD type, rating, location and product standard
whether 6 mA DC detection is built in or separate
every relevant upstream RCD
the measured RCD and circuit test results
the charger model, firmware and protection settings
any manufacturer limitation affecting testing or maintenance

RCD and PEN protection are different

RCD protection addresses residual current and electric-shock fault protection. Open-PEN protection addresses a failure of the combined neutral and protective conductor on a PME network. A single enclosure may contain both functions, but one does not replace the other.

Their positions follow from the parking arrangement, supply arrangement and the protection built into the chargepoint.