RCBO Selection Calculator — BS 7671

Type AC only detects sinusoidal AC earth leakage and must NOT be used on general domestic circuits after BS 7671 Amendment 2 (in force 27 September 2022, Reg 531.3.3) — Type A is now the minimum. Type A detects AC and pulsating DC and is the standard for most domestic circuits. Type F additionally detects composite (mixed-frequency) residual currents and may be used on single-phase inverter loads ONLY where the manufacturer confirms the smooth DC residual current is below 10 mA (531.3.3 Note 2). Type B detects all of the above plus smooth DC and is required where the smooth DC residual current is 10 mA or more, or is not specified (531.3.3 Note 3), and for three-phase EV chargers.

EV chargers use power electronics that can produce a smooth DC fault current. A standard Type A RCD can be blinded by smooth DC above 6 mA, so BS 7671 Reg 722.531.3.101 requires protection against DC fault current exceeding 6 mA. This is achieved either by a Type B RCD, or by a Type A (or Type F) RCD combined with a 6 mA RDC-DD (residual DC-detecting device) that disconnects on a DC fault. Most modern Mode 3 EVSE include a built-in 6 mA RDC-DD, so a Type A RCD upstream is acceptable; if the charge point has no DC detection, a Type B RCD must be used. The RCD must protect the charge point individually at a sensitivity not exceeding 30 mA, and it must disconnect ALL live conductors (line AND neutral) — so a two-pole (DP) device is required, not a standard single-pole RCBO. Always check the charger's installation manual.

Type B MCBs trip at 3-5× rated current — suitable for resistive and lightly inductive loads (lighting, sockets, heaters, showers, and single-phase 7 kW Mode 3 EV chargers, which are near-resistive). Type C MCBs trip at 5-10× rated current — needed for moderate inrush loads (motors, transformers, fluorescent lighting, heat pumps, air-con). Type D MCBs trip at 10-20× rated current — for high inrush loads (industrial motors, welding equipment, large transformers). Where the equipment manufacturer specifies a particular curve, follow it. Use our Circuit Breaker Sizing Calculator to determine the correct MCB rating for your circuit.

30mA is the standard for personal protection against electric shock — required for all socket outlet circuits and circuits in bathrooms, outdoors, and any location where there is increased risk. 100mA provides fire protection but NOT personal shock protection. BS 7671 requires 30mA RCD protection for: all socket circuits up to 32A, all circuits in bathrooms, all circuits supplying mobile equipment outdoors. Some fire-sensitive applications use 300mA for fire protection only.

Dual RCD (split-load) consumer units are permitted but have disadvantages: all circuits on one RCD trip together if there is a fault on any one circuit (nuisance tripping loses multiple circuits). BS 7671 Amendment 2 (and reaffirmed by Amendment 3:2024 for domestic lighting, Reg 411.3.4) extends 30 mA RCD protection to virtually all domestic circuits, which makes RCBOs the preferred solution — each circuit has independent protection and a fault trips only that circuit. RCBO boards cost more initially but provide better discrimination and resilience. Most electricians now fit RCBO boards as standard. Note that an EV charge point must be protected by its own dedicated RCD (722.531.3.101), so it cannot share an RCD with other circuits.