The short answer
An MCB (miniature circuit breaker) is a resettable protective device fitted in a consumer unit that automatically switches off a circuit when it detects an overload (too much current for the cable) or a short circuit (a direct connection between live and neutral, producing a very large fault current). Once the fault is cleared the MCB can be reset by hand — you simply flip the switch back up. This contrasts with a rewireable fuse, which must have its fuse wire replaced after it operates. Domestic MCBs are typically available in current ratings of 6 A, 10 A, 16 A, 20 A, 32 A, 40 A and 45 A, each matched to the cable size and type of circuit it protects. An MCB does not provide protection against earth-leakage or shock risk — that is the function of an RCD or RCBO.
Every circuit in a modern consumer unit has an MCB — the switch-like device that trips when a circuit is overloaded and that you reset when a trip occurs. Understanding what an MCB does (and, importantly, what it does not do) helps make sense of how a modern board protects your home.
MCB key facts
- Full nameMiniature circuit breaker
- Protects againstOverload and short circuit
- Does NOT protect againstEarth leakage / shock risk (RCD does that)
- Can be resetYes — flip the switch after clearing the fault
- Common ratings6 A, 16 A, 20 A, 32 A, 40 A, 45 A
- Trip curve typesType B (domestic general), Type C (motors/inductive)
How an MCB works: two trip mechanisms
An MCB has two separate mechanisms that can open its contacts:
- Thermal trip (overload): a bimetallic strip inside the MCB carries the circuit current. Under a sustained overload — more current than the circuit is rated for — the strip heats up and bends, mechanically releasing a latch that opens the contacts. This is a deliberately time-delayed response; a brief current surge (such as a motor starting) will not trip it, but a sustained overload will. The hotter the strip becomes, the faster it trips.
- Magnetic trip (short circuit): a solenoid responds to the very large current of a short-circuit fault by magnetically attracting an armature that forces the contacts open. This is essentially instantaneous, disconnecting within milliseconds, because a dead short must be interrupted fast to prevent cable damage.
The combination of both mechanisms in one device means an MCB responds appropriately to both types of overcurrent fault, without needing fuse wire.
MCB versus rewireable fuse
The rewireable fuse does the same basic job — protecting a circuit against overload and short circuit — but with an important practical difference. When a fuse operates, the fuse wire melts and must be replaced; when an MCB operates, it trips to the off position and can be reset once the fault is cleared.
In addition to the convenience of resetting, MCBs offer more consistent protection. Fuse wire can deteriorate over time, and there is a risk (especially in older DIY-maintained boards) that wire of a higher rating than specified has been fitted, or that a fault with a makeshift conductor such as a nail has been used. Either error means the fuse will not operate when it should, leaving the cable behind it without overload protection. An MCB has a factory-set trip characteristic that cannot be altered by the user.
| Feature | MCB | Rewireable fuse |
|---|---|---|
| Overload protection | Yes | Yes |
| Short-circuit protection | Yes | Yes |
| Earth-leakage protection | No | No |
| Reset after fault | Flip switch (seconds) | Replace fuse wire |
| Trip threshold consistency | Factory set, stable | Depends on wire fitted |
| Risk of incorrect repair | None — cannot be modified | Risk of wrong-gauge wire |
MCB versus rewireable fuse comparison. Source: BS 7671:2018+A2:2022; Electrical Safety First guidance.
MCB current ratings and circuit types
Each MCB is rated for the maximum continuous current it will carry without tripping. The rating must be matched to the cable size it protects and the load the circuit supplies. Using an MCB with too high a rating for its cable means the cable could overheat before the MCB trips.
Typical domestic circuit ratings:
- 6 A: lighting circuits (protecting 1.0 mm² or 1.5 mm² cable in most cases)
- 20 A: immersion heaters, garage circuits
- 32 A: ring final circuit (sockets), often protected by 2.5 mm² cable
- 40 A or 45 A: electric cooker circuits
- 40 A or 50 A: electric shower circuits
These are typical values; the correct rating for any circuit depends on the cable size, route, and installation method and should be determined by the designer of the installation, not guessed from the appliance.
Frequently asked questions
Why does my MCB keep tripping?
An MCB trips when it detects more current than the circuit is rated for — from overload (too many appliances) or a short circuit (a fault in a device or wiring). If it trips repeatedly with no obvious fault, the circuit may be overloaded or there may be a wiring fault. Unplug appliances to reduce load; if it still trips with nothing connected, have a registered electrician investigate.
Can I replace an MCB myself?
Working inside a consumer unit is notifiable Part P work in England and Wales and requires a registered electrician, or notification to building control. The incoming tails to the board remain live even with the main switch off, which presents a serious electrocution risk. This is not DIY work.
What is the difference between a Type B and Type C MCB?
The difference is the threshold at which the magnetic (instantaneous) trip operates. A Type B MCB trips instantly at 3–5 times its rated current, which suits resistive loads and normal domestic circuits. A Type C MCB trips at 5–10 times its rated current, allowing higher inrush currents — useful for motor loads, transformers or larger power supplies. Most domestic circuits use Type B.
Sources & further reading
- IET — BS 7671:2018+A2:2022 (18th Edition Wiring Regulations)
- Electrical Safety First — electrical safety in the home
- NAPIT — electrical installation guidance
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property and installation. They are guidance, not a quotation.