RCD vs RCBO: Why One Fault Takes Out Half Your House

The question I get asked more than any other about consumer units is some version of this: the problem is one appliance in the kitchen, so why does half the house go off? The answer is short. The device protecting that kitchen circuit is not protecting only the kitchen circuit. It is shared with five or six others, it has no way of knowing which of them leaked, so it disconnects the lot. That is not a fault in your board. It is what a shared RCD is built to do.

This piece is about the two devices at the centre of that, what separates them, and the principle underneath the whole thing. If you want the wider picture including MCBs and AFDDs, I have covered all four devices in MCBs, RCDs, RCBOs and AFDDs explained. This one stays on the two that get compared most.

There is a companion piece on RCD types at RCD types explained: AC, A, F and B, which covers what each type can detect and why a Type AC struggles with modern loads.

RCD and RCBO, side by side

An RCD works by comparing what goes out on the live with what comes back on the neutral. If the two do not match, the missing current has found another way to earth, and the device cuts the supply to everything behind it. An RCBO does exactly the same watching, plus the overload job of an MCB, but it sits on one circuit and cuts that circuit alone. Both detect the same thing, and a 30 mA device of either kind gives you the same protection against a shock. The difference is how much of your house is standing behind the device when it operates.

Both are 30 mA devices in a domestic setting, and that number matters more than people realise. A 30 mA RCD is not a device that sits perfectly still until it sees exactly 30 mA. The product standard it is built to says it must not operate below 15 mA, and by the time it sees 30 mA it must have operated, within 300 milliseconds ^[2]. Anywhere in between is fair game, and in practice most of them go somewhere around 18 to 23 mA.

Earth leakage is normal, and it is not a fault

Every appliance in your house lets a tiny amount of current escape to earth. Filters, capacitors, motor windings, LED drivers, the insulation inside a heating element. None of it is broken. The standards appliances are built to allow it, and how much they allow depends on what the appliance is. An earthed, fixed, motor-driven appliance is permitted up to 3.5 mA on its own ^[3], a small double-insulated item is held to a quarter of a milliamp, and a fixed heating appliance is allowed an amount that scales with how powerful it is, up to a ceiling of 5 mA.

Add up a kitchen full of them, plus the electronics, the chargers and the lighting drivers scattered round the rest of the house, and you can be carrying a genuine handful of milliamps before anything unusual has been switched on. That is standing leakage, and it is there all day, every day, in a perfectly healthy installation.

Why pooling it causes trips

BS 7671 puts a ceiling on how much of that background leakage should sit behind a single RCD: no more than 30 per cent of what the device is rated at. On the 30 mA devices used in homes, that works out at about 9 mA ^[2].

Now put six circuits behind one device. All six of them are drawing down the same 9 mA allowance. Get the house up to 12 or 14 mA of ordinary background leakage on a cold damp morning with the heating running, and you are only a couple of milliamps short of the point where the device is allowed to operate. Switch on one more appliance and the whole bank goes off, with nothing wrong anywhere.

That is the mechanism, and it is only one of the six things that trip an RCD. If your board is tripping right now and you want the full diagnosis, including the five causes that are genuine faults, read why your RCD keeps tripping. It walks you through isolating the cause yourself before you ring anyone.

Discrimination, which is the heart of it

Most electricians, me included, still call it discrimination. It is the word the trade grew up with. The standard itself now calls it selectivity, and it means the same thing: when something goes wrong, the protective device closest to the fault is the one that operates, and everything else stays on.

Put like that it sounds like an engineering nicety. It is not. It is the whole difference between a small annoyance and a bad evening:

• The freezer. A shared RCD that drops the kitchen bank while you are out takes the freezer with it. You find out when you get home, and by then it has been off for hours.
• The alarm, the router, the doorbell. All the quiet things that only announce themselves once they have been off for a while.
• The stairs at night. If the upstairs and downstairs lighting sit behind the same device, one leaking fitting puts you on the landing in the dark hunting for a phone torch.
• Working from home. A tripped kitchen circuit that also kills the office socket costs you the call you were on, not just the kettle.

An RCBO delivers discrimination because there is nothing else behind it. One circuit, one device, one thing that goes off. A shared RCD cannot deliver it however good the device is, because by the time the leakage reaches it, it has already been added together with everything else in the group. The information about which circuit caused it is gone.

There is a second half to discrimination worth knowing, because it is the one honest limit on the whole argument. Where one RCD sits behind another, the standard sets out what has to be true for the right one to operate first: the upstream device has to be a delayed type, and it has to be rated at least three times higher than the one below it. Put two 30 mA devices one behind the other and neither of those is true, so there is no telling which one goes ^[4].

That matters if your supply already has an RCD in it before the consumer unit, which happens on some earthing arrangements and on some feeds out to a garage or an outbuilding. In that situation, changing the board on its own will not stop the upstream device operating. I check for it before I price anything, and I will tell you if it applies to you.

Which of the three boards have you got?

Almost every domestic consumer unit falls into one of three layouts. You can work out which one you have without touching anything.

The fastest check of all is one you already have. Think about what happened last time something tripped. If it was a single circuit, you have RCBOs. If it was a group, you do not.

Whichever layout you have, press the test button on each RCD or RCBO every three months. It should switch the power off straight away. If it does not, ring me ^[6].

If you want to go further into what the enclosure, the main switch and the labelling should look like, that is all in the consumer unit guide.

Why I never fit split-load boards

I should be straight with you about what is a rule and what is my choice, because the two get blurred a lot in this trade.

The standard tells me the outcome it wants. It says to divide an installation into enough circuits to avoid danger, to keep the inconvenience down when something faults, and to reduce unwanted RCD tripping caused by ordinary leakage rather than by a fault. It does not tell me which device to buy, and a split-load board can be made to meet it ^[1].

I never fit split load boards. If an existing split-load board needs a device replacing, I will do that. But where I am putting a new consumer unit in, it goes in with an RCBO on every circuit, because that is what gives you discrimination and because it takes the shared leakage problem out of the board.

That is my standard, not a legal requirement, and I would rather you heard it that way round. A split-load board in your house today is not something I would call unsafe on sight, and two RCDs is still the usual arrangement in domestic boards ^[5]. It is a layout that hands you a worse day every time something goes wrong, and since the difference in materials on a new board is small next to the labour, I do not see a good reason to fit one.

If the property is rented, the layout matters more, not less. With a split-load board, an ordinary appliance fault takes half the property off, and the first you hear about it is a tenant ringing you about something that was never dangerous in the first place. Where every circuit has its own RCBO the tenant loses one circuit and can usually tell you which one. If you let property in York, the rest of what I do for landlords is on the landlord hub.

What an RCBO does not fix

Three things, and I would rather say them here than have you find them out afterwards.

• It does not reduce your leakage, it stops it being pooled. The total standing leakage in the property is identical the day after. What changes is that the 9 mA allowance is no longer shared. Each circuit has its own, which is usually a long way from being used up.
• A circuit carrying a lot of leakage on its own can still trip its own RCBO. A kitchen full of appliances is the usual candidate. If that happens it is a real finding worth chasing, not a nuisance to be lived with.
• A genuine fault still trips the device, and it should. Degraded cable insulation, a failing shower element, water in an outdoor socket: none of that is cured by changing a breaker. The RCBO limits how much of the house goes off with it. That is all it claims to do, and it is worth having.

Anyone telling you an RCBO board means the electrics never trip again is selling you something. What it means is that when they do trip, you lose one circuit and you know instantly which one, unless an RCD already sits upstream of your consumer unit, which is the one case I check for before I price anything.

What it costs to move from split load to RCBO

Every board is scoped on a site visit, because the things that move the figure are things I need to see: the circuit count, the state of the meter tails and the main bonding, whether the earthing arrangement needs attention, and whether the existing enclosure, busbar and neutral bars can take individual devices or whether the unit needs replacing outright.

You get a written estimate with the figure shown plus VAT before I start, and there is nothing extra at the end. If changing a couple of devices inside your existing board solves the problem you have, I will tell you that rather than sell you a full board. Book a no-obligation consumer unit survey in York and I will scope it properly.