An RCD (residual current device) is a protective device that monitors the balance between the current flowing out on the line conductor and back on the neutral, and disconnects the supply when the difference, the residual current flowing to earth, exceeds its rated value, typically 30 mA for additional protection against electric shock. BS 7671 Regulation 411.3.3 requires 30 mA RCD protection for socket-outlets up to 32 A, and Regulation 411.3.4 extends it to lighting circuits in domestic premises, so almost every circuit in a modern home is behind one.
- An RCD trips on residual current to earth; it does not protect against overload or short circuit.
- 30 mA is the rating for additional protection against shock.
- Type AC, A, F and B differ in the waveforms they can detect; Type A is the minimum for most new work.
- An RCBO is an RCD and MCB in one device, protecting a single circuit.
- A 30 mA RCD must trip within 300 ms at 30 mA and within 40 ms at 150 mA.
What is an RCD in practice?
Inside the device, line and neutral pass through a toroidal transformer. In normal operation the two currents are equal and opposite and their fields cancel. If current leaks to earth, through a damaged cable, a wet appliance or a person, the imbalance induces a current in the sensing coil and releases the trip.
The residual current rating (IΔn) is the leakage at which the device is guaranteed to operate. BS 7671 uses 30 mA for additional protection because that current, interrupted within 40 ms, is unlikely to cause fibrillation. 100 mA and 300 mA devices are used upstream for fault and fire protection but do not provide additional protection against shock.
When is an RCD required?
| Requirement | BS 7671 regulation |
|---|---|
| Socket-outlets rated up to 32 A | 411.3.3 |
| Lighting circuits in domestic premises | 411.3.4 |
| Cables concealed in walls at less than 50 mm | 522.6.203 |
| EV charging points | Section 722 (Type B, or Type A with 6 mA DC detection) |
| Fault protection on TT systems | Chapter 41 |
On an EICR, following Electrical Safety First's Best Practice Guide 4, a socket-outlet circuit without 30 mA RCD protection is generally C3, rising to C2 where the sockets could supply outdoor equipment or serve a bathroom.
What types of RCD are there and how are they tested?
The type of an RCD describes the residual current waveforms it can detect. Modern electronic loads can produce DC components that a basic device cannot see.
| Type | Detects | Typical use |
|---|---|---|
| AC | Sinusoidal AC only | Legacy installations; resistive loads with no electronics |
| A | AC plus pulsating DC, up to 6 mA smooth DC | General domestic and commercial circuits; the usual minimum for new work |
| F | Type A plus composite and high-frequency currents | Single-phase variable-speed drives, such as heat pumps |
| B | Type F plus smooth DC | EV chargers, three-phase drives, PV inverters without separation |
BS 7671 Chapter 53 restricts Type AC to circuits where no DC component is expected, which rules it out for most new domestic work, and Section 722 requires Type B, or Type A with 6 mA DC detection, for EV charging.
RCBOs combine the residual current function with an overcurrent circuit-breaker to BS EN 61009. One RCBO per circuit means an earth fault on the shower does not take out the lights, which is why RCBO boards are now the default for consumer unit replacements.
RCD test sequence
| Test | Current (30 mA device) | Required result |
|---|---|---|
| ½ × IΔn | 15 mA | No trip |
| 1 × IΔn | 30 mA | Trip within 300 ms |
| 5 × IΔn | 150 mA | Trip within 40 ms |
Each test is run at 0° and 180° and the longer time recorded; the 5× result goes on the schedule of test results.
Common mistakes with RCDs
- Recording the 1× time in the 5× column, or vice versa.
- Assuming any RCD gives additional protection. A 100 mA device upstream does not satisfy Regulation 411.3.3.
- Blaming nuisance tripping on the device when the cause is accumulated leakage on one shared RCD; RCBOs usually solve it.
Related terms
- AFDD (arc fault detection device)
- Earth fault loop impedance (Zs)
- Multifunction tester (MFT)
- Schedule of test results
- EICR
- BS 7671
How Certio helps
Certio records this on the BS 7671 model forms without the typing: photograph the board to draft the circuit schedule, point the camera at a Megger MFT-X1 to capture readings, or dictate them. Starter is free for 7 days.