Understanding RCD Types: Essential Guide for EV Charging Safety

Residual Current Devices (RCDs) are essential safety components in electrical installations. Designed to prevent electric shocks and reduce the risk of fire, they operate by detecting imbalances in the electrical current and disconnecting the circuit within milliseconds if a fault occurs. RCDs are mandated under UK Wiring Regulations (BS 7671), and play a vital role in both domestic and commercial installations.

What is an RCD?

An RCD constantly monitors the current flowing through the live and neutral conductors in a circuit. Under normal conditions, the current in both should be equal. If a fault occurs, such as current leaking to earth (e.g. through a person receiving a shock), the RCD detects this imbalance and disconnects the power—typically within 30 milliseconds, reducing the risk of serious harm.

In line with the 18th Edition Wiring Regulations (BS 7671:2018+A2:2022), RCDs are required on most circuits in domestic premises.

Main Types of RCDs

Detailed Breakdown

Type AC RCDs are the most basic and detect only standard alternating currents. They were widely used in older installations but are no longer recommended due to their inability to detect DC leakage from modern electronics.

Type A RCDs detect both AC and pulsating DC currents, which are common in appliances with electronic components. They are now the standard choice for domestic installations and must be used with devices such as washing machines, dishwashers and EV chargers that include built-in DC fault protection.

Type F RCDs are suitable for appliances that use variable-speed drives, such as heat pumps and air conditioning units. These devices can produce high-frequency fault currents that Type A models may not detect.

Type B RCDs provide full protection against AC, pulsating DC, smooth DC and high-frequency currents. They are used where equipment does not have built-in protection against DC leakage, such as solar PV systems and some types of EV chargers.

S Type RCDs feature a time delay and are used to ensure selectivity in installations where multiple RCDs are connected in series. They prevent upstream devices from tripping unnecessarily, allowing only the faulty circuit to be isolated.

RCD Requirements for Electric Vehicle (EV) Charging

Regulation 722.531.3.101 of BS 7671:2018+A2:2022 states that each EV charging point must be protected by its own RCD (Type A, F, or B) rated at no more than 30mA. It must disconnect all live conductors, including the neutral.

There are two main approaches for EV installations:

• Type B RCD: Offers complete protection against all fault current types including smooth DC. Ideal for chargers without internal protection, but more expensive.

• Type A RCD: More affordable and suitable if the charger itself provides integrated DC fault protection up to 6mA, which most modern UK models do.

For installations with multiple EV charge points, a Type B RCD may be required upstream to account for cumulative DC leakage. Always follow manufacturer guidance and consult a qualified electrician when selecting RCDs for EV systems.

Understanding PEN Fault Detection

Using the Seren EV charger by Aurora Lighting as an example, this unit integrates 30mA AC and 6mA DC RCD protection along with PEN fault detection. In the UK’s TN-C-S (PME) earthing systems, PEN refers to a conductor that combines the functions of neutral and earth. A PEN fault, where the neutral becomes disconnected, can result in dangerous voltages appearing on exposed conductive parts.

PEN fault detection prevents this by monitoring the supply voltage. If abnormal conditions are detected (indicating a broken PEN conductor), the device isolates all live conductors to prevent electric shock. This safety feature eliminates the need for additional earth rods in many EV charging installations and ensures compliance with BS 7671.

Recommended Products for EV Charging Installations

Most home EV chargers are installed outdoors, which calls for weatherproof protection. The FuseBox IP65-rated EV boards are a popular choice, featuring:

• 6-module capacity in a robust aluminium enclosure

• Pre-fitted with various circuit protection options (e.g. 63A RCDs, surge protection devices, or 32A RCBOs)

• Available in grey or white finishes

FuseBox also offers indoor-rated EV boards with similar flexibility in circuit protection options.

Choosing the Right RCD

Selecting the correct RCD depends on the type of circuit and equipment being used. General guidance includes:

• Type A: Standard for most modern domestic applications

• Type F: Best for circuits with frequency-controlled appliances (e.g. air conditioning, heat pumps)

• Type B: Required for systems with potential for smooth DC leakage (e.g. solar PV or EV chargers without built-in DC protection)

• S Type: Use where coordination is needed between multiple RCDs

Always consult a qualified electrician to ensure compliance with the latest UK wiring regulations and to determine the appropriate level of protection for each installation.

Note: All electrical work must be carried out by competent professionals in accordance with BS 7671 and applicable UK regulations.

For more information on Understanding RCD Types: Essential Guide for EV Charging Safety talk to Expert Electrical Supplies Ltd