What Is an MCB and What Does It Do?

An MCB (Miniature Circuit Breaker) is a protective device used in electrical installations to automatically disconnect a circuit when excessive current flows.

MCBs are commonly found inside consumer units and distribution boards and protect electrical circuits and their associated wiring against overcurrent caused by overloads and short circuits.

Unlike a traditional fuse, an MCB can normally be reset after it has operated once the cause of the fault has been identified and resolved.

What Does an MCB Protect Against?

An MCB provides protection against two main types of overcurrent:

Overload

An overload occurs when a circuit carries more current than it is designed to handle for a period of time.

For example, excessive load connected to a circuit can cause the current to exceed its intended operating level.

The MCB is designed to disconnect the circuit before sustained excessive current causes dangerous overheating of the wiring.

Short Circuit

A short circuit can produce a very large current in a very short period of time.

An MCB contains a magnetic operating mechanism designed to respond rapidly to high fault currents and disconnect the affected circuit.

Where Are MCBs Used?

MCBs are used throughout domestic, commercial and industrial electrical installations.

In a typical consumer unit, separate MCBs may protect circuits including:

Lighting
Socket circuits
Electric showers
Cookers
Heating equipment
Immersion heaters
Outbuildings
Other fixed electrical equipment

The required MCB depends on the design and characteristics of the individual circuit.

What Do the Numbers on an MCB Mean?

One of the most noticeable markings on an MCB will normally look something like:

B16

B32

C20

The letter identifies the trip characteristic or curve, while the number represents the MCB's rated current in amps.

For example:

B32 = Type B trip characteristic, 32A rated current

C20 = Type C trip characteristic, 20A rated current

The rating should not be changed simply because another size of MCB physically fits the consumer unit.

Common MCB Amp Ratings

Common ratings include:

Other ratings are also available.

A higher amp rating does not mean a better or more powerful MCB. The rating needs to be appropriate for the circuit design, cable and connected load.

What Is the Difference Between an MCB and an RCBO?

Although they can look similar, an MCB and RCBO provide different forms of protection.

MCB: Provides overcurrent protection against overloads and short circuits.

RCBO: Combines overcurrent protection with residual-current protection.

An MCB installation may therefore use a separate RCD to provide residual-current protection to one or more circuits.

An RCBO provides these functions for an individual circuit within one device.

What Is the Difference Between an MCB and a Fuse?

Both are designed to provide protection against excessive current, but they operate differently.

A traditional fuse contains an element that melts when excessive current flows. Once it has operated, the fuse or fuse element needs replacing.

An MCB uses a mechanical switching mechanism and can normally be reset after the fault has been investigated and corrected.

Can I Replace an MCB With Any MCB of the Same Rating?

No.

Two MCBs can both be marked B32, for example, without necessarily being interchangeable.

You also need to consider factors including:

Manufacturer
Consumer unit or distribution board range
Breaking capacity
Physical dimensions
Busbar arrangement
Terminal configuration
Applicable product standard
Manufacturer compatibility information

Even MCBs produced by the same manufacturer may belong to different generations or product ranges.

How Do I Identify an Existing MCB?

Start by looking at the markings on the front and side of the device.

Try to identify:

Manufacturer
Model or part number
Amp rating
Trip characteristic
Breaking capacity
Product range
Number of poles

A clear photograph of the MCB and the complete consumer unit or distribution board can also be useful when trying to identify an older device.

Looking for a Replacement MCB?

ReCircuit Resale stocks a range of new, surplus and discontinued MCBs and electrical distribution components from major manufacturers including Wylex, Hager, Crabtree, Schneider Electric and others.

Older MCB ranges can become difficult to source after a consumer unit has been discontinued.

When searching for a replacement, using the manufacturer and exact model or part number from the existing MCB is usually the best place to start.

Electrical work within consumer units and distribution boards should be carried out by a competent person with the appropriate knowledge and equipment.

Next Guide: MCB Ratings Explained – 6A, 10A, 16A, 20A, 32A, 40A & More