RCBO's vs MCB's: Explained a few different ways...

An MCB protects the cable. An RCBO protects the cable and the person. That is the whole difference in one line, and everything else follows from it. An MCB trips on overload and short circuit. An RCBO does the same job and adds residual current detection, so it also disconnects when current is leaking to earth, which is what happens when someone touches something live.

Regulation numbers refer to BS 7671:2018+A2:2022, the IET Wiring Regulations. Sources are listed in full at the end of this article.

What each device actually does

Both devices sit on a DIN rail in your consumer unit and both will disconnect a circuit. They are watching for different things.

Why RCBOs win in practice: Regulation 314.1

The strongest argument for RCBOs is not shock protection, because a split load board with RCDs provides that too. It is what happens to the rest of the house when something goes wrong.

BS 7671 Regulation 314.1 requires an installation to be divided into circuits so that a single fault causes the minimum inconvenience, and so that danger and inconvenience during inspection and testing are avoided. That sounds abstract until you look at a board.

On the right hand arrangement, a faulty kettle can take out the lighting, the freezer and the alarm at the same time, in the dark, at three in the morning. On the left, only the socket circuit the kettle is on disconnects. That is the practical meaning of minimum inconvenience, and it is why RCBO boards have become the default specification for new domestic work rather than a premium option.

What BS 7671 actually requires

Two requirements do most of the work in a domestic installation. The first is additional protection for socket outlets.

There is a documented exception, and it is narrower than people assume. RCD protection can be omitted only for socket outlets in the second category, and only “where a suitably documented risk assessment undertaken with the involvement of a skilled person (electrically) determines that RCD protection is not necessary”. The IET is explicit that “the documented risk assessment shall be provided with the appropriate electrical installation certificate”. In a normal house that exception effectively does not apply.

The requirement also has a ceiling. It “does not apply to socket-outlets rated at more than 32 A. Neither is there a distinction between single and three-phase supplies.”

The second requirement is Regulation 411.3.4, added by Amendment 2, which brought AC final circuits supplying luminaires in domestic premises into scope for 30 mA additional protection. Between sockets and lighting, that is most of a house.

The Type A change most people missed

Buying an RCBO is no longer just a question of rating and curve. You also have to buy the right RCD type, and the old default is no longer acceptable for general domestic circuits.

Modern loads contain switch mode power supplies, LED drivers, inverters and EV chargers, all of which can put a small DC component into the residual current. A Type AC device cannot see that reliably. It can be desensitised by it and may operate late or not at all.

Choosing the right device: curve and breaking capacity

Two more specifications decide which product you actually order, and both are on the front of the device.

Curve type: how fast it trips on a surge

The letter before the current rating, such as the B in B32, is the magnetic trip curve. It describes how many times the rated current has to flow before the device trips instantly, and it is defined in BS EN 60898-1.

Breaking capacity: 6 kA or 10 kA

Breaking capacity is the highest fault current the device can interrupt safely. It has nothing to do with the load and everything to do with how close you are to the supply transformer.

The prospective fault current is measured, not guessed. It appears on the Electrical Installation Certificate, and it is the number that decides whether 6 kA devices are acceptable.

When an MCB is still the right choice

RCBOs are not automatically correct for everything, and specifying them everywhere is a way of spending money without buying safety.

• Circuits that must not be RCD protected. A fire alarm panel or certain security systems may be specified without RCD protection so that an unrelated earth fault cannot disable them.
• Where an upstream RCD already provides protection. If the circuit sits behind a suitable RCD, adding a second residual current device in series can create discrimination problems rather than solving them.
• Commercial and industrial distribution. Where circuits feed fixed equipment rather than socket outlets, and the risk assessment supports it.
• Board space and budget on a limited alteration. Adding one circuit to an existing split load board is a different decision from specifying a new installation.

Choosing between products

1. Confirm the RCD type. Type A as a minimum for general domestic circuits. Type B where EV charging or solar is involved.
2. Confirm the breaking capacity. 6 kA for most domestic work, 10 kA where the measured prospective fault current or the specification demands it.
3. Pick the curve. B for standard domestic lighting and power. C only where inrush genuinely justifies it.
4. Match the manufacturer to the board. Devices must be the type verified by the consumer unit manufacturer for that enclosure. Mixing brands in a board is not a like for like swap.
5. Check the module width. Full width and compact or mini RCBOs differ. On a full board, a compact device can be the difference between fitting and not fitting.

What we stock

Our range is organised by brand and breaking capacity, which are the two things you need to match when ordering.

Frequently asked questions