Residual Current Device (RCD)
Detects imbalance between line and neutral and disconnects.
Where required
30 mA RCD additional protection for socket-outlets rated up to 32 A (Reg 411.3.3), mobile equipment outdoors up to 32 A, AC final circuits supplying luminaires in domestic premises (411.3.4), and cables concealed in walls < 50 mm deep without earthed covering (522.6.202).
Types
Type AC: sinusoidal only (restricted use — Reg 531.3.3). Type A: also pulsating DC. Type F: mixed frequency. Type B: smooth DC (EV, inverters). A4:2026 revised Reg 531.3.3 on selecting RCD type: the type must suit the residual current waveforms expected from the connected equipment (check manufacturer information), and Type AC remains limited to fixed equipment known to produce no DC components.
Testing
Test at 1 × IΔn: a general (non-delay) RCD must trip within 300 ms. Since A2:2022, Reg 643.8 only requires the test at the rated residual operating current; the 5 × IΔn (40 ms) test is no longer required.