TradeCalculator.co.uk
RCD Selection Guide — BS 7671 18th Edition
Determine the correct RCD type and trip rating for your circuit based on BS 7671 18th Edition wiring regulations and IET guidance.
How many circuits may share one RCD. Set to 1 to recommend an individual RCBO per circuit (default 6).
On: recommend a per-circuit RCBO (combined RCD + MCB) instead of a shared RCD.
Safety notice
Electrical work in dwellings can be notifiable under Part P of the Building Regulations. Treat these figures as planning guidance only: circuits must be designed, installed and certified to BS 7671 by a competent person, normally a registered electrician.
Next up: related pages
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. Since Amendment 2 (27 Sep 2022) it may only be used for fixed equipment whose load contains no DC component — e.g. simple filament lamps or resistive heaters with no electronic driver. Not suitable for general domestic circuits or LED 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 411.3.4 (added by Amendment 4, BS 7671:2018+A4:2026) requires 30mA RCD protection for AC final circuits supplying luminaires in domestic premises. Regulation 522.6.202 requires 30mA RCD protection for cables concealed in walls at a depth of less than 50mm. Regulation 701.411.3.3 requires a 30mA RCD for all circuits in a location containing a bath or shower. For EV charge points, Regulation 722.531.3.101 requires at least a Type A RCD plus protection against DC fault currents above 6mA. The guide cross-references these regulations based on your circuit type and application.
Frequently Asked Questions
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).
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 quickly. Under BS 7671 (Reg 415.1.1) a 30mA RCD used for additional protection must disconnect within 40 milliseconds when the leakage reaches five times its rating (150mA), and within 300 milliseconds at its rated 30mA. 30mA is around the threshold above which mains current can cause fatal harm to most adults. BS 7671 requires 30mA RCD protection for socket outlets, lighting circuits in dwellings, bathroom circuits, outdoor circuits, and more.
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.
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 Sizing Calculator to determine the correct MCB or RCBO rating for each circuit.
An EV charger requires individual protection by at least a 30mA Type A RCD, plus protection against DC fault currents exceeding 6mA (BS 7671 Reg 722.531.3.101). That 6mA DC protection can be provided by a Type B RCD, by a Type A or Type F RCD combined with an RDC-DD (residual direct-current detecting device), or by an RDC-DD built into the charge point itself. Many modern smart chargers include a built-in RDC-DD, which then allows a standard Type A RCD or RCBO at the consumer unit. Use our EV Charger Cable Sizing Calculator for complete cable and protection sizing.
Was this calculator helpful?
Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.