MCB vs RCBO: What's the Difference?

LIVE ELECTRICAL GUIDE  |  Harbro Electrical

QUICK ANSWER

An MCB (Miniature Circuit Breaker) protects the cable on a circuit from overload and short-circuit current - it does not protect a person from electric shock. An RCBO (Residual Current Breaker with Overcurrent protection) does everything an MCB does, plus it monitors for earth leakage and trips within milliseconds if current is escaping to earth - the fault condition that causes electric shock. Under BS 7671, almost every circuit in a UK domestic installation now needs 30mA earth fault protection, which is why most modern consumer units use one RCBO per circuit rather than a bank of MCBs on a single shared RCD.

MCB vs RCBO is one of the first questions every trainee electrician runs into, and it stays relevant on every board a working electrician specifies for the rest of their career. The two devices look almost identical on the DIN rail, fit the same consumer unit, and both trip - but they protect against entirely different types of fault.

This guide explains what each device actually does, how they differ under the hood, what BS 7671 requires for UK domestic and commercial installations, and the practical decision points a trade electrician uses when specifying a consumer unit.

Harbro Electrical stocks the Live Electrical range of single-pole MCBs (6A-63A, B-curve, 6kA) and the Live Electrical RCBO (Type A, single module, 6A B-curve, 30mA) at ex-VAT trade prices, with free UK mainland delivery on orders over £70.

What Is an MCB?

A Miniature Circuit Breaker (MCB) is an automatically resettable switching device that protects the cable on a circuit against overload current and short-circuit current. It does this using a thermal-magnetic trip mechanism: a bimetallic strip responds to sustained overload by bending and tripping the breaker, while a magnetic coil responds almost instantly to a short-circuit fault.

An MCB does not detect earth leakage. It has no awareness of whether current is flowing safely through the circuit conductors or escaping to earth through a person, an appliance casing or damaged insulation. That distinction matters, because earth leakage - not overload - is the fault condition that causes electric shock.

Key MCB characteristics

• Complies with BS EN 60898-1

• Protects the circuit cable, not the person

• Available in Type B, C and D curves - B-curve is standard for UK domestic lighting and socket circuits

• Rated by current (6A-63A common) and breaking capacity (6kA is standard for domestic)

• Single pole (switches line only) or double pole (switches line and neutral)

• Resets with a simple flip of the switch once the fault is cleared

What Is an RCBO?

A Residual Current Breaker with Overcurrent protection (RCBO) combines an MCB and an RCD in a single module. It provides everything a plain MCB provides - overload and short-circuit protection - plus continuous monitoring for earth leakage current.

Inside an RCBO, a current transformer monitors the current flowing out on the line conductor against the current returning on the neutral. In a healthy circuit these are equal. If current is leaking to earth - through a faulty appliance, damaged cable or a person receiving a shock - the two values become unbalanced. The RCBO detects that imbalance and trips within milliseconds, well before a dangerous shock current can cause injury.

Key RCBO characteristics

• Complies with BS EN 61009-1

• Protects both the cable and the person, in one device

• Available in Type AC, Type A and Type B, based on the type of fault current they detect

• Most common domestic sensitivity: 30mA (for shock protection)

• Same physical width as a standard single-module MCB in modern ranges - fits one way in a consumer unit

• Has a test button (usually marked T) to verify the residual current mechanism functions correctly

MCB vs RCBO: Side-by-Side

Why an MCB Alone Isn't Enough

An MCB protects the wiring. It does not protect a person. If a fault develops where current escapes to earth through a damaged appliance, a person touching that appliance, or damaged cable insulation - none of that current shows up as an overload or a short-circuit as far as the MCB is concerned. The MCB will happily let a lethal leakage current continue indefinitely because its trip mechanism simply isn't looking for it.

This is why UK wiring regulations require earth fault protection in addition to overcurrent protection on almost every circuit. There are two ways to achieve this: fit a shared RCD upstream of a bank of MCBs, or fit an RCBO on each individual circuit.

What BS 7671 Requires

BS 7671:2018 (as amended) is unambiguous about where 30mA additional protection is required. Regulation 411.3.3 requires 30mA RCD protection for socket outlets rated up to 32A, for circuits supplying mobile equipment used outdoors, and for cables concealed in walls or partitions at a depth of less than 50mm (unless they meet specific mechanical protection exemptions). Amendment 3 extended the requirement to lighting circuits in domestic premises.

In practice, this means almost every final circuit in a modern UK domestic consumer unit needs 30mA earth fault protection somewhere in the circuit. The regulations do not mandate an RCBO specifically - a shared RCD protecting several MCBs also satisfies 411.3.3 - but they do require the protection to exist.

A plain MCB with no upstream RCD is only compliant on circuits that fall outside these requirements - for example, a dedicated fixed-wired circuit to a single appliance where the cable is not concealed in a wall and does not supply a socket outlet. In practice these situations are the exception, not the rule, on a typical domestic board.

Shared RCD vs RCBO Per Circuit

There are two ways to give a bank of MCBs the earth fault protection BS 7671 requires.

Split-load board: MCBs on a shared RCD

One or two 30mA RCDs each protect a group of circuits, with a standard MCB fitted per circuit for overload and short-circuit protection. This is a lower-cost way to build a compliant board, but it has a well-known drawback: if any one circuit on that RCD develops an earth fault, every circuit sharing that RCD loses power at once - the lights and the sockets on that side of the board go off together, even though only one circuit is actually faulty.

All-RCBO board: one RCBO per circuit

Each circuit has its own RCBO, combining overload, short-circuit and earth fault protection in a single device dedicated to that circuit alone. If a fault develops, only the affected circuit trips - the rest of the board stays live. This is now the preferred layout for new consumer unit installations and replacements, because it gives far better fault discrimination and a much easier fault-finding job when something does trip.

When to Specify an MCB vs an RCBO

• Specify a plain MCB only where the circuit is exempt from BS 7671's 30mA additional protection requirements - typically a dedicated fixed appliance circuit with surface-run cable, not supplying a socket outlet, and not concealed in a wall. On most modern domestic boards this is the exception rather than the rule.

• Specify an RCBO for any circuit supplying socket outlets, any lighting circuit in a domestic property, any circuit in a bathroom or outdoors, any circuit with cable concealed less than 50mm in a wall, and any circuit where fault discrimination matters - which in practice covers almost every circuit on a domestic consumer unit replacement.

• Specify Type A RCBOs as the current minimum standard for domestic circuits, and Type A or Type B specifically for circuits supplying EV charge points or equipment with electronic power conversion, per Amendment 2. Confirm the specific type requirement against the load before ordering.

Cost Comparison

RCBOs cost significantly more per unit than an equivalent MCB, but the comparison needs to be made at board level, not device level, since a split-load board also needs RCD modules.

The price gap between the two board types has narrowed as RCBOs have become the standard specification, and the additional cost is typically modest relative to the total price of a consumer unit replacement job.

Frequently Asked Questions

What is the main difference between an MCB and an RCBO?

An MCB protects the circuit cable against overload and short-circuit current. An RCBO does the same job as an MCB but adds earth fault (residual current) protection in the same device, so it protects both the cable and the person using the circuit.

Does an RCBO replace the need for an MCB?

Yes. An RCBO is a single device that performs the MCB's overcurrent protection function and the RCD's earth fault protection function together. A circuit protected by an RCBO does not need a separate MCB.

Can I use an MCB instead of an RCBO?

Only if the circuit falls outside BS 7671's additional protection requirements under Regulation 411.3.3 - for example, a fixed appliance circuit that does not supply a socket outlet and is not concealed in a wall. Most circuits on a modern domestic board, including all socket and lighting circuits, require 30mA earth fault protection, which a plain MCB does not provide.

Why does my consumer unit have MCBs and one big RCD instead of RCBOs?

This is a split-load board - an older or budget-specified layout where one or two shared RCDs provide earth fault protection to a group of MCB-protected circuits. It is cheaper to install but means an earth fault on any one circuit trips every circuit sharing that RCD. Many electricians now recommend upgrading to an all-RCBO board on replacement, so only the faulty circuit is affected.

What does RCBO stand for?

Residual Current Breaker with Overcurrent protection. "Residual current" refers to the earth leakage current the device monitors for; "overcurrent" refers to the overload and short-circuit protection it also provides.

What is the difference between an RCBO and an RCD?

An RCD (Residual Current Device) only detects earth leakage current - it provides no overload or short-circuit protection and must be paired with a separate MCB. An RCBO combines both functions in a single device covering one circuit. A shared RCD protects several MCB-protected circuits at once; an RCBO protects only its own circuit.

Why is my RCBO tripping?

Most commonly an earth fault on the circuit - a failing appliance, damp junction box, or damaged cable insulation. It can also trip on genuine overload or short-circuit, the same as a standard MCB would. Isolate the circuit, disconnect appliances one at a time and reset to identify the source before calling an electrician if the fault persists.

Are all RCBOs Type A?

No. RCBOs are available in Type AC, Type A and Type B, depending on the type of residual current fault they are designed to detect. Type A is the current minimum standard for most UK domestic circuits. Type A or Type B is required for circuits supplying EV charge points and equipment with electronic power conversion under BS 7671 Amendment 2.

Does an RCBO cost more to run than an MCB?

No - an RCBO has no ongoing running cost beyond the standard load current it carries. The additional cost is entirely upfront, in the purchase price of the device.

MCBs and RCBOs at Harbro Electrical

Harbro Electrical stocks the full Live Electrical range of single-pole MCBs and the Live Electrical RCBO at ex-VAT trade prices. Free UK mainland delivery on all orders over £70.

Note: the 6A/16A/32A/40A MCB links above all point to the same Live Electrical single-pole MCB product listing, where the specific amperage is selected via product options rather than through separate per-amp pages.

For trade orders and account enquiries, call us on 0191 518 1998 or visit our trade counter at 5 and 6 Whitworth Road, Peterlee, County Durham, SR8 2LY.

All prices shown exclude VAT. Free delivery applies to UK mainland addresses on orders over £70.