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RCD types: AC, A, F and BType AC, A, F or B?

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There are now lots of different types of RCD. Chapter 53 sets them out in detail, at Regulation 531.3.3.

  • Type AC only detects AC (sine-wave) residual currents.
  • Type A adds pulsating DC, Type F adds mixed-frequency currents, and only Type B detects smooth DC.
  • The limits in 531.3.3: a Type A still works with smooth DC up to 6 mA, a Type F up to 10 mA, and a Type B up to 0.4 × IΔn or 10 mA, whichever is higher.

A Type AC should only be installed where it's known that the load contains no DC components. In modern life, there are very few times you could fit a Type AC.

So the focus is now on Types A, F and B:

  • Type A is the most common. It meets the minimum requirement and is the "one size fits all".
  • Types F and B are for more specific jobs, often seen with things like EV charging.

BS 7671 · Chapter 53 · Reg 531.3.3 · RCD types · Type AC only where the load has no DC components · pp.165–166

residual current waveform Type ACType AType FType B AC (sine wave) pulsating DC mixed frequency smooth DC ✓✓✓✓ –✓✓✓ ––✓✓ –––✓
Our summary of the types. Each type detects everything the one to its left does, plus more. Types A and F cope with small amounts of smooth DC (up to 6 mA and 10 mA), but only Type B is designed to detect smooth DC fully.
Watch out: "A, B and F detect DC"
Only Type B detects smooth DC residual current. Type A detects AC plus pulsating DC. Type F adds mixed-frequency currents (such as from inverter-driven equipment). A large smooth DC current can stop a Type A or F from working properly. That's why EV charge points (Section 722) need a Type B, or a Type A or F with a separate 6 mA DC detection device.

Sparky's closing advice: be clear on when an RCD has to be installed and which job it's doing, and use the regs to your advantage.

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