RCD vs MCB vs RCBO: Understanding Your Consumer Unit
If you've ever looked inside your consumer unit and wondered what all the switches do, you're not alone. Understanding the difference between RCDs, MCBs, and RCBOs helps you know what's protecting you — and what to do when something trips. Here's a plain-English guide for homeowners in {location}.
MCB — Miniature Circuit Breaker
An MCB protects an individual circuit (e.g., your kitchen sockets, upstairs lights, or cooker) against overload and short circuit. If too much current flows through the circuit — because of a fault or because you've plugged in too many appliances — the MCB trips and cuts the power.
- Each MCB protects one circuit
- They come in different ratings (6A for lighting, 16A for immersion heaters, 32A for sockets, 40A for cookers, etc.)
- They replace the old rewirable fuses and are far more reliable
- MCBs do not protect against earth faults — that's the RCD's job
RCD — Residual Current Device
An RCD monitors the balance of electrical current flowing out on the live wire and returning on the neutral wire. If it detects a difference (meaning current is leaking to earth — possibly through a person), it trips within 30 milliseconds. This is fast enough to prevent a fatal electric shock in most cases.
- RCDs protect against earth faults and reduce the risk of electric shock and electrical fires
- In a standard split-load consumer unit, one RCD typically protects a bank of circuits (e.g., all your downstairs circuits)
- The downside: when the RCD trips, all circuits on that bank lose power, not just the faulty one
- RCDs have a "test" button — you should press it every three months to check it works
RCBO — Residual Current Breaker with Overload
An RCBO combines the functions of an MCB and an RCD in a single device. It protects an individual circuit against overload, short circuit, and earth faults.
- Each circuit gets its own independent protection
- If one circuit faults, only that circuit trips — everything else stays on
- This is the gold standard for consumer unit protection
- More expensive than a split-load RCD setup, but far more convenient
Which Setup Is in Your Home?
Most consumer units installed in {location} in the last 10–15 years use one of two configurations:
- Dual RCD (split-load): Two RCDs each protecting a bank of MCBs. Cost-effective and meets regulations, but a fault on one circuit knocks out all circuits on that bank
- Full RCBO board: Every circuit has its own RCBO. More expensive to install but means a single fault only affects one circuit. Increasingly popular and preferred by many electricians
What to Do When Something Trips
- MCB trips: A specific circuit has overloaded or short-circuited. Unplug the last thing you connected and reset the MCB. If it trips again, there's a fault on that circuit — call an electrician
- RCD trips: An earth fault has been detected on one of the circuits it protects. Unplug appliances one by one and reset. If you can't identify the cause, call an electrician in {location}
- RCBO trips: A fault on that specific circuit. Same approach — unplug, reset, and call an electrician if it keeps tripping
Understanding your consumer unit isn't just technical knowledge — it's practical safety awareness. If you're unsure what protection your home has, a qualified NICEIC or NAPIT registered electrician can assess your setup and advise on any upgrades needed.