An RCBO board gives every circuit its own RCD and breaker, so one fault only switches off that circuit. A dual RCD board shares two RCDs between all the circuits, so one fault can cut power to half the house. Both meet the Wiring Regulations. RCBO boards cost more but suit most homes better, especially families, home workers, rentals and anyone with an EV charger or heat pump.
What do an MCB, an RCD and an RCBO actually do?
They protect against different things. An MCB protects the cable from too much current. An RCD protects people from electric shock. An RCBO does both jobs for one circuit.
Knowing which is which makes the rest of this guide, and any quote you get, much easier to follow.
- MCB (miniature circuit breaker)
- Trips when a circuit is overloaded or shorted. Protects the wiring, not people.
- RCD (residual current device)
- Trips when current leaks to earth, for example through a person. Usually covers several circuits.
- RCBO
- An RCD and an MCB in one device, protecting a single circuit.
- Main switch
- Isolates the whole board. Gives no fault protection on its own.
What is the difference between a dual RCD board and an RCBO board?
On a dual RCD (split-load) board, the circuits are shared between two RCDs, each with a row of MCBs behind it. On an RCBO board, every circuit has its own RCBO and there is no shared RCD.
Both give the 30mA shock protection the current Wiring Regulations (BS 7671, 18th Edition) call for on most home circuits. The difference is what happens when something goes wrong.
- Dual RCD: one fault, half the house off
- A leaking kettle can take out the fridge, lights and Wi-Fi on the same side.
- Dual RCD: leakage adds up
- Small, normal leakage from many appliances shares one RCD and can cause trips.
- Dual RCD: lower cost
- Fewer expensive devices, so the board costs less to buy.
- RCBO: one fault, one circuit off
- Everything else stays on, and the faulty circuit is obvious.
- RCBO: easier fault finding
- No guessing which of six circuits behind an RCD caused the trip.
- RCBO: higher cost
- Each circuit needs its own device, so the board costs more.
Why does nuisance tripping matter so much?
Because the inconvenience of a shared RCD is not just annoying, it can be a safety issue. A trip at night can switch off lights on a staircase, or a freezer while you are away.
Every appliance leaks a tiny amount of current to earth. Computers, TVs, chargers and LED drivers add a little each. On a dual RCD board those small amounts add together on one device, which can push it to trip with no real fault. On an RCBO board each circuit only carries its own leakage, so trips are rarer and point straight to the cause.
People with repeated trips on a dual RCD board sometimes give up and leave a circuit off. An RCBO board removes most of that temptation.
Do I need surge protection (an SPD)?
In most cases, yes. When a consumer unit is replaced, the current Wiring Regulations usually call for a surge protection device unless a risk assessment shows it is not needed.
An SPD sits in the board and diverts voltage spikes, from lightning strikes on the network or switching on the grid, away from your wiring. It will not stop every surge reaching every device, but it reduces the risk of damage to electronics such as TVs, boiler controls, routers and computers. It takes up space in the board, so allow for it when you choose the size.
What is an AFDD and do I need one?
An AFDD (arc fault detection device) detects the electrical arcing that happens when a cable is damaged or a connection is loose, and switches off before it can start a fire.
The Wiring Regulations require AFDDs on socket circuits in some higher-risk buildings, such as certain houses in multiple occupation (HMOs) and high-rise flats, and recommend them elsewhere. For a typical house they are an optional extra rather than a must. They cost noticeably more than RCBOs, and some combined RCBO and AFDD devices are larger, which affects the board size. They are worth thinking about for HMOs, timber-framed homes, or where someone in the home would struggle to get out quickly in a fire. Ask your electrician whether they apply to your property.
Why do new consumer units have to be metal?
Since 2016, new and replacement consumer units in homes have had to be in a non-combustible enclosure, which in practice means metal. The rule came in after fires that started in, or spread from, plastic boards.
This applies to any board you choose, dual RCD or RCBO. An existing plastic board is not illegal and does not have to be removed just because of its case, but its replacement will be metal.
Does it matter whether the RCD is Type A or Type AC?
It can. Type AC devices detect ordinary alternating leakage. Type A devices also detect the pulsing DC leakage that modern electronics can produce, such as induction hobs, washing machines with inverter motors and LED drivers.
The current Wiring Regulations limit where Type AC can be used, so Type A devices are the usual choice in new boards. EV chargers and some heat pumps have their own requirements, and some need extra DC protection. Your electrician should check the manufacturer's instructions for those.
Which type of board suits my home?
For most homes an RCBO board with surge protection is the sensible choice. A dual RCD board is still safe and compliant, and can be fine where budget is tight and the home is simple. Use this as a rough guide:
- If you let a property, choose RCBOs
- Fewer full-house outages, fewer call-outs and quicker fault finding.
- If you run an HMO, ask about AFDDs
- They may be required. Check your licence conditions with the council too.
- If you have a family, choose RCBOs
- A fault in one room won't leave the stairs dark or the freezer off.
- If you work from home, choose RCBOs plus SPD
- Keeps the office circuit running when something else trips.
- If you have an EV charger or heat pump, plan for it
- Dedicated circuit, the right RCD type and space in the board.
- If budget is the deciding factor, a dual RCD board
- Still safe. Ask how circuits will be split between the two RCDs.
Can I upgrade my existing board instead of replacing it?
Sometimes. If your board is a modern metal unit in good condition, it may be possible to swap MCBs for RCBOs, or add an SPD, without replacing the whole thing.
The new devices must be made by the same manufacturer and approved for that board. Mixing brands is not recommended, because the board and its devices are tested to work together. If the board is plastic, very old, has rewireable fuses or has no spare space, a replacement is usually better value.
If you are planning an EV charger, a heat pump or a new shower, think about it before the board is chosen, so there is space for the new circuit. If your board keeps tripping, we can trace the fault and test it. If the board itself needs replacing, Norman will tell you plainly.
Questions
Is a dual RCD consumer unit still allowed?
Yes. A dual RCD board with the right devices and an SPD can meet the current Wiring Regulations. It is a valid choice. The trade-off is that one fault can switch off every circuit on the same RCD.
Is a high-integrity board the same as an RCBO board?
No. A high-integrity board usually has two RCDs plus a few circuits on their own RCBOs or on a non-RCD section. It sits between a dual RCD board and a full RCBO board in cost and convenience.
Will an RCBO board stop all tripping?
No. It will still trip when there is a real fault, which is its job. What changes is that only the faulty circuit goes off, and you can see straight away which one it is.
How often should I press the test button?
Manufacturers usually recommend testing RCDs and RCBOs every few months, often every six months. Press the T or Test button. It should trip immediately, then switch it back on. If it doesn't trip, get it checked.
Do I need building control for a new consumer unit?
Yes. Replacing a consumer unit is notifiable under Part P. A registered electrician notifies building control for you, and you get an Electrical Installation Certificate and a Building Regulations compliance certificate.
Can a new RCBO board trip when my old board didn't?
Yes. An old board with fuses or no RCD may not notice small leaks to earth. A new board can reveal hidden faults, such as a damp outside light. Testing before the change helps find these early.