Helpful video reference. We use eFIXX's CPD module "The Complete Guide To RCDs - Residual Current Devices" as the video reference here. eFIXX (Electrical Training Solutions) is one of the leading UK electrical training providers. This module is accredited CPD and covers the full RCD type family in depth.
1. What an RCD does and why the type matters
A residual current device monitors the difference between the current leaving the consumer unit on the live conductor and the current returning on the neutral. Under normal conditions the two values are equal. If someone receives an electric shock, some of the current takes a path through their body to earth rather than returning via the neutral - the RCD sees that imbalance and disconnects the circuit in under 40 milliseconds.
The 30mA trip threshold matters because at 30 milliamps of body current for more than a few hundred milliseconds, there is a significant risk of ventricular fibrillation. The 40ms disconnection time keeps the exposure short enough to avoid that threshold in most scenarios.
So far, so simple. The complication is the shape of the fault current. Different appliances produce different waveforms when they develop an earth fault, and not every RCD type detects every waveform.
2. Type AC: sinusoidal AC only
Type AC was the original UK standard and you will still find them in many older consumer units. They detect smooth sinusoidal AC fault current at 50Hz - the shape of the mains supply itself.
The problem is that almost no modern domestic appliance produces a smooth sinusoidal fault. A laptop charger, a LED driver, a washing machine motor controller and a dishwasher all use switch-mode power supplies or variable-speed drives that produce pulsed or half-wave-rectified DC when they develop an earth fault. A Type AC RCD sees some of this, but can require a significantly higher actual leakage current before it trips - in the worst cases, it may not trip at all.
BS 7671 Amendment 2 (2022) now restricts the use of Type AC for most domestic circuits. Existing installations with Type AC protection are not automatically non-compliant, but any new installation or significant alteration should use Type A or better.
3. Type A: the current baseline for UK homes
Type A detects sinusoidal AC and pulsed DC fault current. This covers the fault signatures produced by most modern single-phase domestic appliances: washing machines, dishwashers, tumble dryers, ovens with electronic controls, and LED lighting circuits.
BS 7671 specifies Type A as the minimum for new socket outlet circuits, bathroom circuits, and other general domestic circuits. If you have a new consumer unit installed in a UK home today, you would expect to see Type A RCBOs or Type A 2-pole RCDs throughout.
4. Type F: for variable-speed drives
Type F detects sinusoidal AC, pulsed DC and composite waveforms at frequencies up to 1kHz. It was introduced to address fault currents produced by variable-frequency drive (VFD) inverters, which are found in energy-efficient appliances - particularly washing machines with direct-drive motors and some newer heat pump models.
In practice, Type F is not yet commonly specified in domestic installations. Its main application is as a step up from Type A where the appliance manufacturer specifically calls for it. If you are installing a heat pump and the manufacturer's literature says Type F, fit Type F.
5. Type B: EV chargers, three-phase and smooth DC
Type B detects all waveforms including smooth (non-pulsed) DC fault current. It is required by BS 7671 for circuits supplying EV chargers that can produce a smooth DC earth fault, and for three-phase motor circuits in some commercial applications.
In the domestic context, the EV charger requirement is the most relevant. The regulation is Regulation 722.531.3.101 of BS 7671 (18th edition): EV charging circuits must have Type B RCD protection unless the charger incorporates its own DC leakage detection to BS EN 62955.
Most modern chargers marketed at the UK domestic market now include this internal detection, which allows a Type A RCBO on the supply circuit. Always check the charger's installation manual or data sheet before specifying protection - the answer varies by manufacturer and model.
6. RCBOs versus 2-pole RCDs: form factor matters
An RCBO (Residual Current Circuit Breaker with Overcurrent protection) combines an MCB and an RCD in a single module. When one circuit trips, only that circuit is affected. This is now the preferred approach in domestic consumer units because it eliminates the nuisance of losing half the house when one appliance develops a small earth fault.
A 2-pole RCD is a separate module that sits upstream of several MCBs. If it trips, everything on those circuits goes off together. Split-load boards with two RCDs were the standard UK approach before all-RCBO boards became cost-effective, and they are still common. The limitation is that one tripping fault on the RCD group takes out multiple circuits.
7. Where BS 7671 makes RCD protection compulsory
Regulation 411.3.3 and Chapter 7 of BS 7671 require 30mA RCD protection for:
- All socket outlets rated 32A or less (with very limited exceptions for supervised commercial installations)
- Mobile equipment used outdoors
- Cables concealed in walls or partitions at a depth of less than 50mm, unless enclosed in earthed metallic conduit
- Bathroom and shower room circuits
- Circuits in garages and other outbuildings
- Outdoor circuits
- Circuits in locations containing a bath or shower
In a modern all-RCBO consumer unit, all circuits are individually protected and there is no need to work through this list - everything is covered. In an older split-load board, circuits on the unprotected side may lack RCD protection and this would be noted as an EICR observation.
8. How to check which type you have
Open the consumer unit cover (with the main switch off - never touch live conductors). The type code is printed on the face of each device, usually on the lower half of the front panel. Look for "Type AC", "Type A", "Type F" or "Type B". On older boards you may see no type marking at all, which almost certainly means Type AC.
If the board is old enough to have no RCDs at all - typically pre-1990 rewireable fuse boxes or early MCB boards - an EICR will record this as a C2 (potentially dangerous) observation and an upgrade will be recommended.
Need a consumer unit checked or upgraded in Sandwich?
Richard can inspect your existing board, advise on RCD types, and carry out an upgrade to a fully RCBO-protected unit with the correct device types for your circuits.
Contact Richard