MCBs (Miniature Circuit Breakers): Ratings, Types, and Why Brand Matters

Your extension is nearly finished. The electrician has wired every circuit, and the consumer unit is going in. He asks whether you want to supply the MCBs yourself to save money. You say yes, search Screwfix, find what looks right, order six. When he arrives, he tells you they're the wrong brand for your board. They physically clip onto the DIN rail but they haven't been type-tested with your consumer unit. He can't use them. You've wasted £15 and a day of waiting for replacements. This is the most common mistake homeowners make with MCBs, and it's entirely avoidable if you understand one rule: the brand must match your consumer unit.

What it is and what it's for

An MCB is an automatic switch that cuts power to a circuit when the current flowing through it exceeds a safe level. Every circuit in your house has one: the lighting circuit, the socket ring main, the cooker, the shower. They sit inside your consumer unit (the metal box near your electricity meter that most people still call the fuse box) clipped onto a metal strip called a DIN rail.

MCBs replaced the old rewireable fuses that older properties had. The principle is the same: if a circuit draws too much current, the protective device disconnects it before the cable overheats and starts a fire. The difference is that a blown rewireable fuse needs a new piece of fuse wire threaded through a ceramic holder. An MCB just needs its switch flipped back up. No fumbling with fuse wire in the dark.

Inside the MCB, two separate mechanisms work together. A bimetallic strip (two metals bonded together that bend when heated) handles sustained overloads. If you plug too many appliances into one circuit, the current heats the strip, it bends, and trips the switch. This is a slow process: a circuit running at 50% over its rating might take minutes or even hours to trip. The second mechanism is a solenoid (an electromagnet) that reacts to short circuits. A dead short across live and neutral produces a massive current spike, the solenoid fires instantly, and the MCB disconnects in milliseconds. Between them, the thermal and magnetic trips cover both types of electrical fault that could damage your wiring.

Types and trip curves

The letter printed on the front of every MCB (B, C, or D) tells you its trip curve. This defines how much current the magnetic trip needs before it fires instantly. The thermal trip works the same way on all types.

A naming trap catches people out. "Type B" and "B curve" mean the same thing. When you see a label that reads "B32", it means Type B trip curve, 32 amp rating. Some retailers list products as "B Curve MCB" and others as "Type B MCB". Identical devices, different labelling conventions.

Why does the trip curve matter? A Type B MCB rated at 32A will trip magnetically (instantly) when the current hits somewhere between 96A and 160A (3-5 times 32). A Type C at the same 32A rating won't trip magnetically until the current reaches 160A to 320A. The higher threshold on Type C means it tolerates the brief surge of current that motors draw when starting up. But that tolerance comes at a cost: Type C needs a lower earth fault loop impedance (a measure of how easily fault current can flow back to the source) to guarantee it disconnects fast enough to be safe. Your electrician measures this with test equipment. Swapping from Type B to Type C without testing is not a DIY job. It requires verification that the circuit's earth path can deliver enough fault current to trip the higher-threshold device within the time limits set by BS 7671 (the UK wiring regulations).

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Circuit ratings: which MCB goes where

Every circuit in a domestic installation has a standard MCB rating determined by the cable size feeding it. The MCB protects the cable, not the appliance. You can't just fit a bigger MCB to stop nuisance tripping without checking the cable can handle the higher current. Oversize the MCB and you remove the protection: the cable overheats before the MCB trips, and that's how fires start.

For a kitchen extension, your electrician will typically specify 3-6 MCBs (or RCBOs, which combine MCB and RCD protection in one device). A basic setup: one 6A for lighting, one 32A for the socket ring, one 32A for the oven, and one 20A for a dedicated dishwasher radial. Add a 32A for an induction hob and a 16A for a fridge/freezer on its own circuit (so a fault elsewhere doesn't defrost your food), and you're at six.

How they work in practice

When an MCB trips, the toggle switch drops to the middle or down position. Before resetting it, unplug everything on that circuit and try again. If it holds, plug appliances back in one at a time until you find the one causing the problem. If it trips immediately with nothing plugged in, you have a wiring fault. Call an electrician.

A persistent trip is a symptom, not a diagnosis. The MCB is doing exactly what it's supposed to do: disconnecting a circuit that's drawing too much current. The three most common causes are an overloaded circuit (too many high-draw appliances on one ring main), a faulty appliance (a washing machine with a failing motor will draw progressively more current), and a short circuit in the wiring itself (a nail through a cable in the wall, rodent damage, degraded insulation in older properties).

The old approach of jamming a nail into a rewireable fuse holder to stop it blowing is the same mistake in a different era. The MCB is there for a reason.

Brand compatibility: the rule you can't ignore

MCBs from different manufacturers are not interchangeable. A Hager MCB will not safely work in a Wylex consumer unit, even though both clip onto a DIN rail and both are rated to BS EN 60898. The physical dimensions of the busbar connection, the terminal heights, and the clip mechanism all differ between manufacturers. Consumer units are type-tested as complete assemblies: the enclosure plus the specific protective devices it was designed for. Fitting a different brand invalidates that type testing.

This isn't theoretical. In every electricians' forum thread on the subject (and there are dozens), the message is the same: match the brand of MCB to the brand of consumer unit.

Some limited cross-compatibility exists. Hager boards accept Steeple and Volex MCBs. Crabtree Loadstar and Wylex units share a similar form factor. MK breakers physically fit Merlin Gerin boards (though at a lower breaking capacity). But the Crabtree Starbreaker range accepts nothing except Crabtree devices. These exceptions are known to experienced electricians. They are not an invitation for homeowners to mix and match.

The standard: BS EN 60898

Every MCB sold for domestic use in the UK must comply with BS EN 60898-1:2019. This European standard (retained in UK law after Brexit) sets the performance requirements for devices operated by "uninstructed persons" (that's you). It defines current ratings from 6A to 125A, requires a minimum breaking capacity of 6kA (meaning the MCB can safely interrupt a fault current of 6,000 amps), and specifies the trip curve tolerances for Type B, C, and D.

For domestic installations, 6kA breaking capacity is the standard. Some retrofit MCBs designed for older boards only manage 3kA. If your electrician specifies a 6kA device, don't substitute a 3kA one because it was cheaper. The lower breaking capacity means the MCB might not be able to safely interrupt the maximum fault current your supply can deliver. Your energy supplier's service fuse limits incoming fault current, and in most domestic situations 6kA is adequate, but it's not a specification to downgrade.

There's a separate industrial standard, BS EN 60947-2, for MCBs used in commercial and industrial installations. These devices look similar but are built for higher fault currents and harsher environments. You don't need them in a house, and they shouldn't be fitted in a domestic consumer unit.

Cost and where to buy

MCBs are cheap. Remarkably cheap for something that protects your house from burning down.

For a typical kitchen extension needing 4-6 MCBs, you're looking at £6 – £18 total in parts. The MCBs themselves are a negligible fraction of your electrical costs. Labour, cable, the consumer unit itself, testing, and certification dwarf the cost of the individual breakers. This is why most electricians supply them as part of the job. Fighting over who supplies six components that cost less than a takeaway pizza isn't worth the hassle of getting the wrong brand.

MCBs vs RCBOs: should you care?

If your electrician is installing a new consumer unit for your extension, the chances are it will be populated with RCBOs rather than MCBs. An RCBO (residual current breaker with overcurrent protection) does everything an MCB does, plus it detects earth leakage faults that could cause an electric shock. It combines the MCB and RCD into a single device.

The practical advantage: when an RCBO trips, only that one circuit goes off. With MCBs behind a shared RCD (the older split-load configuration), a fault on any circuit trips the RCD and kills every circuit on that half of the board. Your kitchen lights go off because the garage had a fault.

RCBOs cost more. An individual RCBO runs £25 – £35 compared to £1 – £3 for an MCB. But the consumer unit itself is simpler (no separate RCDs needed), and the fault isolation is far superior. For new work in 2026, all-RCBO boards have become the standard recommendation. Your consumer unit page covers this choice in detail.

Alternatives

RCBOs are the direct upgrade from MCBs. They cost 10-15 times as much per device but provide earth fault protection that MCBs can't. For new consumer unit installations, RCBOs are the better choice and increasingly the default. Your electrician will advise.

Rewireable fuses are the predecessor technology. If your property still has a rewireable fuse board, it predates the current wiring regulations by decades. Adding extension circuits to a property with rewireable fuses will almost certainly trigger a consumer unit replacement. The rewireable fuse board won't meet the RCD protection, SPD, and metal enclosure requirements that BS 7671 demands for new work.

Where you'll need this

• Second fix electrics - MCBs (or RCBOs) installed in the consumer unit during second fix, one per circuit
• Electrical layout planning - circuit planning determines how many MCBs you need and what ratings

MCBs appear in every project that involves electrical work. Whether you're adding a single socket or wiring an entire extension, the protective devices in the consumer unit are what stand between your cables and a fire.

Common mistakes

Buying MCBs without checking your consumer unit brand. The single most frequent mistake in every electricians' forum discussion. Physical fit does not equal compatibility. The devices must be the same brand as the consumer unit they're going into. No exceptions worth taking.

Upsizing an MCB to stop nuisance tripping. The MCB rating protects the cable, not just the appliance. A 32A MCB on a 2.5mm cable is there because that's the maximum safe current for that cable. Fitting a 40A MCB doesn't fix the overload. It removes the protection.

Confusing "Type B" with "B curve". They mean the same thing. If someone tells you that you need a "B curve 32A MCB" and you search for "Type B 32A MCB", you'll find the right product. Don't buy a "Type C" thinking it's a different thing from a "C curve". Same device.

Assuming MCBs protect against electric shock. They don't. An MCB will happily watch you get electrocuted because the fault current flowing through your body (potentially as low as 30 milliamps) is nowhere near the MCB's trip threshold. RCDs and RCBOs provide shock protection. MCBs protect wiring.

Switching from Type B to Type C without testing. Forum posts are full of homeowners who swapped to Type C to stop a nuisance trip, not realising that Type C requires a lower earth fault loop impedance to comply with disconnection time requirements. The swap requires specialist test equipment and a qualified electrician. It's not a like-for-like change.