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RCD type selector. AC, A, F or B?
See what each RCD type detects, then pick what the circuit supplies. Get the type to fit, the reason, and the BS 7671 regulation behind it.
RCD type selector
The four types
Type AC
Trips on alternating residual current that follows a smooth sine wave. Nothing else.
Type A
Everything Type AC detects, plus pulsating DC, the half-wave shape many electronics produce.
Type F
Everything Type A detects, plus mixed-frequency residual current and pulsating DC on top of smooth DC up to 10 mA.
Type B
Everything Type F detects, plus smooth DC residual current.
Each type detects everything the one before it does: B covers F, F covers A, and A covers AC.
Choose what the circuit supplies to see the RCD type.
Type is not the same as rating
- The type says what shape of residual current the RCD can detect. The rating, such as 30 mA, says how much current makes it trip.
- Additional protection by a 30 mA RCD is a separate requirement. Pick the right type and the right rating.
Common questions
Can I still fit Type AC RCDs?
Only in limited cases. BS 7671 allows Type AC for fixed equipment where the load current is known to contain no DC components. For socket circuits and general use, fit Type A or better.
Why can DC stop an RCD working?
Smooth DC residual current can magnetise the core inside an RCD that isn’t designed for it. The RCD may then fail to trip, or trip too late, when a real fault happens.
Is a higher type always better?
A higher type detects more kinds of residual current, but it costs more and may be larger. Choose the type the equipment needs, and follow the manufacturer’s instructions.
What is an RDC-DD?
A residual direct current detecting device. It is often built into EV chargers and cuts the supply if DC residual current goes over 6 mA. It is made to BS IEC 62955 and lets a Type A or Type F RCD be used upstream.
Which edition of BS 7671 applies?
BS 7671:2018+A4:2026 was published on 15 April 2026. The previous version, A2:2022 with A3:2024, was withdrawn on 15 October 2026, so new work should now be designed to Amendment 4.
Sources
- IET Wiring Matters: which RCD type? (September 2019).
- IET Wiring Matters: protective, isolation and switching devices explained (July 2026).
- Heat Pump Association: RCD guidance (March 2025).
- SELECT: BS 7671 Amendment 4, what’s new (PDF).
A guide for learners and electricians, based on BS 7671:2018+A4:2026. Always follow the equipment manufacturer’s instructions; the designer makes the final choice. Related: EICR code helper and electrical calculators.