An RCD (residual current device) compares the current going out on the line conductors with the current coming back on the neutral, and opens the circuit when the difference, the residual current leaking to earth, reaches its rated value (IΔn). A device rated 30 mA or less gives additional protection against electric shock; 100 mA and 300 mA devices are for fault and fire protection. BS 7671 Regulation 411.3.3 requires 30 mA RCD protection for socket-outlets rated up to 32 A, and Regulation 411.3.4 for lighting circuits in dwellings, so nearly every circuit in a new home sits behind one. Below: how it works, ratings, types, where it is required, testing and EICR codes.
- An RCD trips on residual current to earth. It gives no overload or short-circuit protection unless it is an RCBO.
- 30 mA or less is the rating for additional protection (Regulation 415.1.1). Higher ratings are for fault and fire protection.
- The type (AC, A, F or B) says which waveforms the device can detect. Type AC is limited to fixed equipment with no d.c. components (Regulation 531.3.3).
- The instrument test is one a.c. test at 1 × IΔn: 300 ms maximum for a general device, 130 to 500 ms for an S-type (Regulation 643.8).
- On an EICR, Best Practice Guide 4 gives C2 for no RCD on outdoor mobile equipment or bathroom sockets, and C3 for most other missing RCDs.
How does an RCD work?
Every live conductor of the circuit, the line or lines and the neutral, passes through a toroidal core inside the device. The protective conductor does not. In a healthy circuit the currents sum to zero and their magnetic fields cancel. When current finds another way back, through damaged insulation, a wet accessory or a person, the imbalance induces a voltage in the sensing winding and the trip relay opens the contacts.
An RCD does not limit how much current flows through a person. At 230 V that is set by body impedance and can be well over 100 mA. What the device limits is how long it flows. That is why Regulation 415.1.2 says an RCD is not recognised as a sole means of protection: it is the backup for when basic or fault protection fails or someone is careless.
The test button connects a resistor from line to neutral around the core. It proves the mechanism moves and nothing more.
What does an RCD not protect against?
- Overload and short circuit. A plain RCD (an RCCB) sees a balanced overcurrent as normal. Regulation 536.4.202 covers overload protection of the RCCB itself; rate the upstream device to the manufacturer's instructions.
- Line-to-neutral shock. With no path to earth there is no residual current, so nothing trips.
- Series arcing. A loose termination arcing in series with the load is balanced current. That is the job of an AFDD.
What do the ratings on an RCD mean?
| Marking | What it means | Typical values |
|---|---|---|
| IΔn | Rated residual operating current: the leakage at which the device must trip | 10, 30, 100, 300, 500 mA |
| In | Rated current the contacts can carry. It is not a trip setting | 25, 40, 63, 80, 100 A |
| Type | Residual waveforms it can detect | AC, A, F, B |
| S | Selective (time-delayed) device for use upstream of another RCD | 130 to 500 ms at IΔn |
The IΔn decides the job the device can do.
| IΔn | Job | Regulation |
|---|---|---|
| 30 mA or less | Additional protection against shock | 415.1.1, 411.3.3, 411.3.4 |
| 100 mA, often S-type | Fault protection on TT systems, upstream of 30 mA devices | 411.5 |
| 300 mA or less | Fire protection in locations with a fire risk and on farms | 422.3.9, Section 705 |
On a TT system the RCD usually provides fault protection, and Table 41.5 gives the maximum earth fault loop impedance for each rating: 1,667 Ω for 30 mA, 500 Ω for 100 mA and 167 Ω for 300 mA (50 V divided by IΔn). The table's note warns that an electrode above 200 Ω may not be stable. See our earth electrode testing guide.
For selectivity between two RCDs in series, the upstream device should be an S-type with an IΔn at least three times the downstream one: 100 mA S-type over 30 mA is the usual pairing. An S-type is not suitable for additional protection.
What are RCD Types AC, A, F and B?
The type describes the residual current waveforms the device can detect. Electronic loads put d.c. and high-frequency components into earth leakage, and a device that cannot see them may not trip. The figures below follow BEAMA's technical bulletin on RCD selection.
| Type | Trips on | Typical loads |
|---|---|---|
| AC | Sinusoidal a.c. residual current only | Immersion heaters, resistive heating, older boards |
| A | As AC, plus pulsating d.c.; tolerates up to 6 mA of smooth d.c. | General circuits with electronic loads: IT equipment, LED drivers, induction hobs |
| F | As A, plus composite (mixed-frequency) residual currents; tolerates up to 10 mA of smooth d.c. | Single-phase inverter drives: some washing machines, air conditioning, heat pumps |
| B | As F, plus smooth d.c. and a.c. up to 1 kHz | Three-phase drives, some EV charging points and PV inverters |
Regulation 531.3.3 is short on this: "RCD Type AC shall only be used to serve fixed equipment, where it is known that the load current contains no DC components." That wording arrived with Amendment 2:2022. Anything can be plugged into a socket-outlet, so Type A is the working minimum for new socket and lighting circuits. The IET's FAQ gives the reason: residual d.c. from electronic equipment can blind a Type AC device and stop it operating.
Types matter in series too. Amendment 4:2026 adds a note to Regulation 531.3.3 and a new figure (Fig A53.2): RCDs in series should suit the residual currents expected, and a Type A should not be fitted upstream of a Type F or B. A Type AC, A or F may only sit upstream of a Type B where the RCD manufacturer specifically declares the arrangement suitable. NICEIC and BEAMA guidance has long said a Type AC should not sit upstream of a Type A, F or B.
For EV charging, Section 722 requires each charging point to have its own 30 mA RCD: Type B, or Type A or F with a residual direct current detecting device (RDC-DD). See our EV charger certificate guide.
One more marking to check. Regulation 530.3.201, introduced by Amendment 3:2024 and carried into A4:2026, covers unidirectional and bidirectional devices. An RCD or RCBO marked in/out or supply/load is unidirectional; where solar PV or battery storage can feed back through it, select a bidirectional one.
RCCB, RCBO, SRCD: which device is which?
| Device | Standard | What it does |
|---|---|---|
| RCCB | BS EN 61008 | Residual current only. Needs a fuse or circuit-breaker for overcurrent |
| RCBO | BS EN 61009 | Residual current and overcurrent in one device, usually one per circuit |
| SRCD | BS 7288 | Socket-outlet with a built-in RCD |
"RCD" is the family name. The double-pole device heading a split-load board is an RCCB; the device on each way of a modern board is an RCBO, so a fault on the shower does not take out the lights. Our RCBO vs split-load guide compares the layouts.
Take care with SRCDs. BS 7288:2016 describes them as supplementary protection for whatever is plugged in downstream, not as the circuit's additional protection, so an RCD socket is not a straightforward way to meet Regulation 411.3.3 on new work. Check your scheme's guidance first.
Where does BS 7671 require a 30 mA RCD?
| What needs it | Regulation |
|---|---|
| Socket-outlets rated up to 32 A | 411.3.3 |
| Mobile equipment rated up to 32 A for use outdoors | 411.3.3 |
| a.c. final circuits supplying luminaires in domestic (household) premises | 411.3.4 |
| Cables less than 50 mm deep in a wall or partition, without earthed metallic covering or mechanical protection | 522.6.202 |
| Cables in a partition with metal parts, whatever the depth | 522.6.203 |
| Low voltage circuits serving a location containing a bath or shower | 701.411.3.3 |
| Each EV charging point | 722.531.3.101 |
Regulation 411.3.3 has one exception. Sockets not liable to be used by ordinary persons or children can be left without an RCD if a documented risk assessment involving a skilled person says it is not necessary, and the assessment goes with the certificate. In practice that rules the exception out in every dwelling. The IET set this out in Wiring Matters issue 91 (July 2022).
The requirement has grown by edition. The 17th Edition in 2008 brought in 30 mA protection for general-use sockets up to 20 A and for concealed cables. The 18th Edition in 2018 raised the socket limit to 32 A and added domestic lighting circuits.
RCDs also have to stay closed in normal service. Regulation 531.3.2 says earth leakage in normal operation should be no more than 30% of IΔn, which is 9 mA on a 30 mA device, and that circuits should be subdivided across individual RCDs. That is the regulation behind RCBO boards.
How is an RCD tested and recorded?
Regulation 643.8 asks for one instrument test: alternating current at 1 × IΔn, with a general device disconnecting within 300 ms and an S-type between 130 and 500 ms. The test is the same whatever the type, with the instrument on its a.c. setting. The half-times, five-times and 0°/180° tests stopped being requirements with Amendment 2:2022. Our RCD testing guide goes through the method.
On the schedule of test results, each RCD is recorded by its BS (EN) number, type, IΔn and rating, then the measured disconnection time and a tick for the test button (Regulation 643.10). Regulation 514.12.2 requires a notice telling the user to press the button six-monthly.
How do you code RCD problems on an EICR?
Electrical Safety First's Best Practice Guide 4 (Issue 7.1, October 2023) gives these classifications for domestic and similar installations.
| Finding | Code |
|---|---|
| No RCD for mobile equipment that may reasonably be expected to be used outdoors | C2 |
| No RCD for a circuit supplying a socket-outlet in a location containing a bath or shower | C2 |
| Main RCD on a TT system fails to operate on the instrument or its test button | C2 |
| No RCD for a socket-outlet unlikely to supply outdoor equipment and not serving a bathroom | C3 |
| No RCD for domestic lighting circuits | C3 |
| No RCD for cables less than 50 mm deep with no other protection | C3 |
| No RCD for other bathroom circuits where supplementary bonding is satisfactory | C3 |
| Type AC RCD installed where a Type A is required | C3 |
| Type A or F RCD supplying an EV charging point with no RDC-DD | C3 |
These are starting points; the inspector's judgement of the actual risk decides the code. See our EICR codes guide.
Common mistakes with RCDs
- Reading In as protection. A 63 A RCCB will carry more than 63 A without tripping; only the upstream fuse or circuit-breaker protects it.
- Assuming any RCD gives additional protection. A 100 mA device upstream does not satisfy Regulation 411.3.3.
- Swapping a failed Type AC like for like on a circuit that now feeds electronic loads.
- Blaming the device when the cause is a borrowed neutral between RCDs, or a neutral-to-earth fault on a circuit whose single-pole breaker is off.
- Recording a five-times result in the disconnection time column. The form wants the time at IΔn.
Related terms
- AFDD (arc fault detection device)
- Earth fault loop impedance (Zs)
- MFT
- Schedule of test results
- EICR
- BS 7671
- Earthing arrangements: TN-S, TN-C-S and TT
How Certio helps
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