Circuit breakers explained: MCBs, RCDs and RCBOs compared

The three devices in a modern consumer unit or control panel divide the protection job neatly: an MCB protects the circuit against overcurrent, an RCD protects people against earth leakage, and an RCBO folds both into one module. None of them protects against everything — which is the single most useful fact in the subject, because most protection mistakes come from assuming one device covers a risk that actually belongs to the other.

This guide explains what each device in the circuit breaker family actually detects, what the B, C and D on an MCB and the AC, A and B on an RCD mean, when per-circuit RCBOs earn their cost, why RCDs nuisance-trip and how to stop it — and where fuses still fit in a resettable world.

What does an MCB actually do?

A miniature circuit breaker watches current with two separate mechanisms. A thermal element — a bimetal strip heated by the load current — trips on sustained overload, deliberately slowly, so that brief surges pass but a circuit running persistently beyond its rating doesn't. A magnetic element trips instantly on the massive current of a short circuit. Between them they replace the fuse in fixed wiring with something resettable, precise and incapable of being "fixed" with the wrong size of replacement.

What an MCB cannot see is leakage. A faulty appliance passing 100 mA through a person to earth is a lethal event and a rounding error to a 32 A breaker — the current is far too small to register as overload. That's not a design flaw; it's the boundary of the job, and it's why the MCB has a partner.

What do MCB types B, C and D mean?

The letter is the instant-trip threshold — how many multiples of the rated current the magnetic mechanism needs before it fires:

The letter exists because of inrush: motors, transformers and switch-mode supplies gulp many times their running current at switch-on, and a Type B breaker feeding a compressor will trip on the start-up surge of a perfectly healthy machine. Panel builders live on Type C for exactly this reason — and pair the breaker with dedicated motor overload protection, which guards the motor windings while the MCB guards the cable. Going up a letter is not a free upgrade, though: the higher the instant-trip threshold, the more fault current the circuit must be able to deliver for a short to trip it promptly, which is why curve selection on real installations is a calculation, not a preference.

What does an RCD do that an MCB can't?

A residual current device ignores how much current flows and watches instead for whether it all comes back. Live and neutral pass through a sensing core together; in a healthy circuit their currents are equal and opposite and the core sees nothing. The moment some current finds another path — through damaged insulation, through water, through a person — the imbalance registers, and at the 30 mA sensitivity used for personal protection the device disconnects within milliseconds, fast enough to keep an electric shock survivable.

The mirror-image limitation applies: an RCD is blind to overload. A circuit drawing three times its rating with every electron dutifully returning down the neutral looks perfectly balanced to the sensing core while the cable cooks. MCB and RCD aren't alternatives at different price points; they're two halves of one protection scheme — and higher-sensitivity ratings (100 mA, 300 mA) serve a different job again, protecting against fire from long-term leakage rather than against shock.

What are RCD types AC, A and B?

The second letter code on modern RCDs describes what shape of leakage current the device can detect, and it has quietly become important. Type AC detects sinusoidal AC leakage only — adequate in a world of kettles and filament bulbs. Type A adds pulsating DC leakage, which is what the rectifier front-end of practically every modern electronic device produces under fault, making it the sensible default in installations full of LED drivers, inverter appliances and IT equipment. Type B adds smooth DC detection for the loads that can produce it — EV charging and solar inverters chief among them, which is why EV distribution equipment specifies it.

The trap in the codes: smooth DC leakage doesn't just evade a Type AC device — it can magnetically saturate its sensing core and blind it to the AC leakage it would otherwise catch. An outdated RCD upstream of modern loads can be less protective than its test button suggests, which is a genuinely good reason the type letters are worth reading.

What is an RCBO, and when is it worth it?

An RCBO combines the MCB's overcurrent protection and the RCD's leakage detection in one DIN-rail module, giving each circuit its own complete protection. The payoff is discrimination: in a traditional split-load consumer unit, one RCD guards a group of circuits, so a faulty appliance in the kitchen can black out half the house — including the freezer and the home office. With RCBOs, the faulty circuit alone disconnects, and fault-finding starts with a labelled module instead of a process of elimination. The cost per circuit is higher; the cost per incident is dramatically lower, which is why RCBO-per-circuit boards have become the quality default in new installations and refits, and why the same logic applies in machine panels where one leaky heater shouldn't drop an entire production cell.

Why does an RCD nuisance-trip?

Almost never because it's faulty — usually because it's doing its job against an accumulation nobody planned. Every piece of electronic equipment leaks a little to earth by design through its filter components; a dozen devices on one RCD can sit close enough to the 30 mA threshold that a kettle's switch-on transient tips it over. The fixes are structural rather than heroic: spread loads across RCBOs, and the standing leakage divides with them. Damp is the other repeat offender — outdoor sockets, garden circuits and condensation-prone garages leak when it rains, and a trip that correlates with weather is water until proven otherwise. And where a Type AC device guards modern electronics, the blinding effect above can produce behaviour that looks random. The one piece of user maintenance that matters: press the test button periodically — quarterly is the usual advice — because an RCD that hasn't moved in years can stick, and the button is the only rehearsal it gets.

Where do fuses still fit?

Everywhere the resettable devices don't reach. The BS 1362 cartridge in every UK plug protects the appliance flex; equipment-level fuses protect individual boards and instruments with a precision and speed MCBs don't attempt; and high-rupturing-capacity fuses still back up entire installations where fault currents exceed what breakers can safely interrupt. The two families are colleagues rather than generations — our guide to fuses covers that half of the story, and surge protection handles the third job neither does: clamping voltage spikes rather than interrupting current. One honest caveat on the resettable side: breakers are mechanical devices with a rated number of fault operations, and after clearing a major short an MCB deserves inspection rather than automatic trust — reset is not the same as renewed.

A final boundary worth restating from our wiring colours guide: understanding a consumer unit is reader-level; working inside one is notifiable work for a registered installer in England and Wales. The knowledge here is for specifying, diagnosing and asking better questions — the screwdriver part has rules.

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Frequently asked questions

What's the difference between an MCB and an RCD?

An MCB trips on overcurrent — overload and short circuit — protecting cables and equipment. An RCD trips on current leaking to earth, protecting people from shock. Each is blind to the other's fault type, which is why circuits need both, or an RCBO combining them.

What does 30 mA mean on an RCD?

The leakage current at which it disconnects — 30 mA is the sensitivity specified for protecting people, tripping within milliseconds under a heavy fault. Higher ratings such as 100 mA or 300 mA protect against fire from sustained leakage rather than against electric shock.

Should I use a Type B or Type C breaker?

Type B (tripping instantly at 3–5 times rated current) for general and domestic circuits; Type C (5–10 times) where inrush is normal — motors, contactors, transformer and PSU loads. If a healthy machine trips a Type B at switch-on, the curve is wrong, not the machine.

What is an RCBO?

A single DIN-rail device combining MCB and RCD protection for one circuit. Its advantage is discrimination: a fault disconnects only its own circuit rather than a whole RCD group, and diagnosis starts at a labelled module instead of half the building.

Why does my RCD trip when it rains?

Water in an outdoor socket, junction or garden circuit leaking to earth — the correlation with weather is the diagnosis. The RCD is working correctly; the fix is finding and sealing the wet joint, not fitting a less sensitive device.

Do circuit breakers wear out?

Yes — they're mechanical, with a rated number of operations and a finite capacity for clearing major faults. A breaker that has interrupted a severe short circuit merits inspection, and one that trips increasingly easily may be failing rather than oversensitive.

Protect the circuit and the person

From single MCBs and RCBOs to consumer units and motor protection, the resettable half of circuit protection lives here — alongside the fuses that still do the rest.