What is the difference between RCD Types AC, A, B, and F?

Type AC detects AC fault currents only (sinusoidal). Type A also detects pulsating DC fault currents — this is important because many modern electronic devices (chargers, inverters) can produce DC fault components that a Type AC would not detect. Type B detects smooth DC faults as well — required for some EV chargers and PV inverters. Type F is designed for circuits with frequency inverters (e.g., variable speed drives).

Why do I need a 30mA RCD?

A 30mA (0.03A) RCD provides personal protection against electric shock. It detects when current is leaking to earth (e.g., through a person touching a live part) and disconnects the supply within 40 milliseconds. 30mA is the maximum current that most adults can withstand without serious injury. BS 7671 requires 30mA RCD protection for socket outlets, bathroom circuits, outdoor circuits, and more.

What is the difference between an RCD and an RCBO?

An RCD (Residual Current Device) provides earth leakage protection only and protects a group of circuits. An RCBO (Residual Current Breaker with Overcurrent protection) combines RCD protection with MCB overload/short-circuit protection in a single device for one circuit. RCBOs prevent a fault on one circuit from tripping other circuits — recommended for critical circuits like smoke alarms and freezers.

Do all circuits need RCD protection in a domestic installation?

Under BS 7671 18th Edition, most domestic circuits require 30mA RCD protection. This includes all socket outlets up to 32A, cables concealed in walls at less than 50mm depth (most domestic wiring), bathroom circuits, outdoor circuits, and EV charger circuits. Very few domestic circuits can justify exemption from RCD protection. Use our Circuit Breaker Calculator to determine the correct MCB or RCBO rating for each circuit.

What RCD do I need for an EV charger?

An EV charger requires minimum 30mA Type A RCD protection. If the charger does not have built-in DC protection (6mA DC detection), then a Type B RCD is required per BS 7671 Section 722. Many modern smart chargers include built-in Type A DC protection, which allows the use of a standard Type A RCD or RCBO at the consumer unit. Use our EV Charger Cable Sizing Calculator for complete cable and protection sizing.

RCD Selection Guide — What Type of RCD Do I Need? BS 7671

How We Calculate This

This guide recommends the appropriate RCD (Residual Current Device) type and trip rating based on BS 7671 18th Edition wiring regulations. RCDs are life-saving devices that detect earth fault currents and disconnect the supply to prevent electric shock and fire.

RCD types explained

Type AC: Detects sinusoidal AC fault currents only. Suitable for simple resistive loads (heaters, basic lighting).
Type A: Detects AC and pulsating DC fault currents. Required for circuits with electronic equipment (computers, chargers, LEDs with drivers). Now the standard for most domestic circuits.
Type B: Detects AC, pulsating DC, and smooth DC fault currents. Required for some EV chargers, PV inverters, and variable speed drives.
Type F: For circuits feeding equipment with frequency inverters (VFDs). Rarely needed in domestic installations.

Trip ratings

30mA: Personal protection against electric shock. Required for most domestic circuits per BS 7671.
100mA: Fire protection. Used where personal protection is not the primary concern.
300mA: Fire protection for larger installations. Common in commercial switchgear as upstream protection.

Key BS 7671 regulations

Regulation 411.3.3 requires 30mA RCD protection for socket outlets up to 32A. Regulation 701.411.3.3 requires 30mA RCD for all bathroom circuits. Section 722 covers EV charging installations. The guide cross-references these regulations based on your circuit type and application.

Frequently Asked Questions

Related Calculators

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.

RCD Selection Guide — BS 7671 18th Edition