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

Type AC only detects sinusoidal AC earth leakage — not suitable for modern electronics. Type A detects AC and pulsating DC — standard for most domestic circuits. Type F (previously called Type A + super-immunised) detects AC, pulsating DC, and high-frequency earth faults generated by inverter-driven loads like heat pumps and EV chargers. Type B detects all types including smooth DC — required for three-phase EV chargers and some industrial equipment.

Why do EV chargers need Type F or Type B RCDs?

EV chargers use power electronics (rectifiers) that convert AC to DC for battery charging. If a DC earth fault occurs inside the charger, a standard Type A RCD may not detect it — the DC component can saturate the RCD core and blind it to faults. BS 7671 requires Type F (for single-phase chargers with onboard Type B protection) or Type B (for three-phase chargers). Most Mode 3 chargers (dedicated EVSE) include onboard DC leakage detection, requiring only a Type A upstream, but always check the manufacturer's installation guide.

When should I use Type B vs Type C MCB characteristic?

Type B MCBs trip at 3-5× rated current — suitable for resistive and lightly inductive loads (lighting, sockets, heaters, showers). Type C MCBs trip at 5-10× rated current — needed for moderate inrush loads (motors, transformers, fluorescent lighting, EV chargers, heat pumps). Type D MCBs trip at 10-20× rated current — for high inrush loads (industrial motors, welding equipment, large transformers).

What RCBO sensitivity do I need — 30mA or 100mA?

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.

Can I use a dual RCD consumer unit instead of individual RCBOs?

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). AMD3 of BS 7671 requires RCD protection for virtually all circuits, making RCBOs the preferred solution — each circuit has independent protection. RCBO boards cost more initially but provide better discrimination and resilience. Most electricians now fit RCBO boards as standard.

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RCBO Selection Calculator — BS 7671

Select the correct RCBO type (AC, A, F, or B) based on load type, with MCB characteristic (B, C, or D) and rating. Covers BS EN 61009-1 RCD type requirements for modern installations.

Load TypeDetermines RCD type and MCB characteristic

Design Current

Maximum expected load current

How We Calculate This

This calculator selects the appropriate RCBO type based on your load characteristics, following BS EN 61009-1 and BS 7671 18th Edition Amendment 2 requirements.