An RCD protects a group of circuits against earth leakage, so one fault switches off every circuit in the group. An RCBO combines an RCD and a circuit breaker for one circuit, so only the faulty circuit goes off. RCBOs are the better choice for a family home, and every board we fit has an RCBO on every circuit as standard, at no extra cost.
Three devices, three jobs
- MCB (miniature circuit breaker). Protects the cable from overload and short circuit. It is the modern replacement for a fuse. It does nothing to protect a person from an electric shock.
- RCD (residual current device). Watches for current leaking to earth, the kind of leak that happens when someone touches something live. At 30mA it switches off in a fraction of a second. On older modern boards one RCD covers several circuits.
- RCBO. An MCB and an RCD in one device, protecting one circuit. Overload, short circuit and earth leakage, all on that circuit alone.
Why one fault switches off half the house
Most boards fitted between about 2008 and 2016 are "split load" or "dual RCD" boards. The circuits are divided into two groups, and each group sits behind one RCD. If the washing machine develops a fault, the RCD for that side trips, and the fridge freezer, the lights and the sockets in the front room go off with it.
Shared RCDs also add up the small, normal leakage from every appliance on their side. Enough laptops, chargers and white goods, and the RCD trips with nothing actually wrong. That is the most common cause of "it trips for no reason" calls we get.
After
AfterSide by side
| Dual-RCD board | RCBO board | |
|---|---|---|
| Shock protection | Yes, shared by a group | Yes, per circuit |
| What a fault switches off | Half the house | One circuit |
| Nuisance tripping | More likely, leakage adds up | Rare |
| Finding the fault | Trial and error | The tripped RCBO tells you |
| On our fixed prices | Not offered | Standard, from £449 |
Type A, Type AC and EV chargers
RCDs and RCBOs come in types that describe the kinds of leakage they can detect. Older Type AC devices only see smooth alternating current. Modern appliances, LED drivers, induction hobs and EV chargers can leak current that a Type AC device may not see. BS 7671 now limits Type AC devices to equipment known not to produce that kind of leakage, so Type A is the normal choice for household circuits.
An EV charger needs protection against DC leakage too. Many chargers have it built in, and the installer will confirm what the board needs to provide. If you are planning one, get the board sorted first.
Can you add RCBOs to my existing board?
Sometimes. RCBOs must be the same make and range as the board, because a consumer unit is tested and certified as an assembly. Mixing makes is not allowed. Older boards often have no compatible RCBOs still made, or no room for them, and at that point a new board costs little more than the parts.
