RCBO vs RCD: the difference, which to fit, and what the regs require

Published 3 September 2026 · updated 3 September 2026 · SparkCerts guides for UK electricians

Customers ask the question when they see the price difference between two consumer unit quotes. Apprentices ask it when the wholesaler offers both. The short answer is that an RCBO is an RCD and an MCB in one device, and the longer answer decides how you build boards, what you code on an EICR, and how you explain nuisance tripping to a landlord.

What an RCD does

A residual current device compares the current going out on the line conductor with the current coming back on the neutral. In a healthy circuit they are equal. If some current is leaking to earth, through a damaged cable or through a person, the two are no longer equal and the device trips when the difference exceeds its rated residual current. For additional protection of people that rating is 30 mA. Larger ratings, 100 mA and 300 mA, are used for fire protection and for discrimination on upstream devices. An RCD on its own gives no protection against overload or short circuit; those still need a fuse or MCB.

What an RCBO adds

A residual current breaker with overcurrent protection puts the RCD function and an MCB in one module. It protects a single circuit against overload, short circuit and earth leakage, and it isolates only that circuit when any of them happens. That is the practical difference on a board: with a split-load consumer unit one 30 mA RCD covers half the circuits, so a faulty kettle in the kitchen puts the freezer, the lights and the alarm on that side into darkness. With an RCBO per circuit the kettle trips the kitchen sockets and nothing else.

Types: AC, A, F and B

On an EICR a Type AC device protecting a circuit with electronic loads is a common observation. It is usually coded C3, improvement recommended, unless there is evidence the device is failing to operate, and the recommendation is a Type A replacement.

Where BS 7671 requires 30 mA protection

• Socket outlets up to 32 A (411.3.3), which in a house means every socket circuit.
• Mobile equipment used outdoors up to 32 A (411.3.3).
• Lighting circuits in domestic premises (411.3.4, added in the 18th Edition).
• Cables concealed in walls at less than 50 mm depth without earthed mechanical protection (522.6.202 and 522.6.203).
• Special locations: bathrooms (701.411.3.3), swimming pools, saunas, agricultural buildings, caravans and more.
• TT installations, where fault protection normally depends on an RCD because the electrode resistance is far too high for an MCB to disconnect in time. See UK earthing systems.

Put those together and a modern domestic board has RCD protection on essentially every final circuit, which is why the split-load versus RCBO question comes up on every board change.

Split-load board or an RCBO per circuit?

A split-load board uses two 30 mA RCDs, each protecting a bank of MCBs, with the circuits divided so that a trip on one side leaves some lighting and some sockets working on the other. It is the cheaper build: two RCDs and a row of MCBs at a few pounds each. An all-RCBO board costs more per way, typically two to four times the price of an MCB, but every circuit is independent, nuisance tripping is confined to the circuit causing it, and fault finding is faster because the tripped device tells you where to look. For a rental, where a tripped RCD means a call-out, the RCBO board tends to pay for itself. A hybrid is common too: RCBOs for the freezer, the alarm and the lighting, shared RCDs for the rest.

Testing: what the trip times must be

An RCD or RCBO is tested at its rated residual current and, where it provides additional protection, at five times that current. The limits from BS 7671 and Guidance Note 3 are:

The device should also not trip at half its rated current. Record the times on the schedule of test results; the RCD trip-time tool lists the limits for each test, and SparkCerts flags a time over 40 ms as you enter it. A healthy 30 mA device usually trips in 15 to 30 ms at five times rated current; one that is creeping toward the limit is worth a note even though it passes.

Nuisance tripping

Most nuisance tripping is not a faulty RCD but accumulated leakage. Every appliance with a filter or an electronic supply leaks a little to earth, and on a shared RCD those leakages add up until a kettle element pushes the total past 30 mA. The cure is to split the load across more devices, which is the RCBO argument again, or to find the one appliance carrying most of the leakage with a clamp meter. A genuinely faulty RCD is found with the trip-time test above.

Common questions

Is an RCBO better than an RCD?

For most installations, yes. An RCBO protects a single circuit against overcurrent and earth leakage together, so a fault on the shower does not drop the lights and freezer as well. A shared RCD on a split-load board is cheaper but one fault takes out everything on its side.

What is the difference between Type AC and Type A RCDs?

Type AC responds only to sinusoidal AC residual currents. Type A also responds to pulsating DC, which modern electronics, LED drivers and chargers produce. BS 7671 now limits Type AC to fixed equipment known to have no DC component, so Type A is the default for domestic circuits.

Which circuits must have a 30 mA RCD?

Socket outlets up to 32 A, mobile equipment used outdoors up to 32 A, lighting circuits in domestic premises, cables buried less than 50 mm in a wall without earthed mechanical protection, and circuits in bathrooms and other special locations. Most domestic final circuits therefore need one.

How fast must an RCD trip?

At its rated residual current a general Type RCD must trip within 300 ms. Where it provides additional protection, at five times its rated current, 150 mA for a 30 mA device, it must trip within 40 ms.

Do I need a Type B RCD for an EV charger?

It depends on the charger. Charging equipment can produce smooth DC residual current, so BS 7671 Section 722 requires either a Type B RCD or a Type A RCD with a separate 6 mA DC detection device. Most current chargers include the DC detection, so a Type A RCBO is common.