RCD Requirements
When RCDs are required, RCD types, trip times, and AFDD recommendations per BS 7671.
Where RCDs Are Required
• Socket outlets ≤32A: 30mA RCD (Reg 411.3.3)
• Mobile equipment ≤32A outdoors: 30mA RCD
• Cables in walls <50mm deep without earthed metallic covering: 30mA RCD (Reg 522.6.202)
• Bathroom circuits: 30mA RCD (Section 701)
• All final circuits in TT systems: RCD (Reg 411.5.2)
RCD Types
RCD Types
| Type | Detects | Typical Use |
|---|---|---|
| AC | AC sinusoidal only | Basic resistive circuits |
| A | AC + pulsating DC | Electronic equipment, EV chargers (single phase) |
| B | AC + pulsating DC + smooth DC | VFDs, 3-phase EV chargers, PV inverters |
| F | AC + pulsating + composite waveforms | VFDs, inverter drives |
RCD Trip Times
RCD Trip Times
| Rating (mA) | Max at IΔn (ms) | Max at 5×IΔn (ms) |
|---|---|---|
| 30 | 300 | 40 |
| 100 | 300 | 40 |
| 300 | 300 | 40 |
AFDDs (Amendment 2)
AFDD Recommendations (Reg 421.1.7)
Amendment 2 strengthens the recommendation for AFDDs in higher-risk locations:
• HMOs (Houses in Multiple Occupation) • Purpose-built student accommodation • Care homes and sheltered housing • Buildings with sleeping accommodation above commercial premises
Note: AFDDs detect dangerous arc faults that RCDs cannot. While not yet mandatory for all installations, they are increasingly recommended as an additional layer of fire protection.
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