What is a circuit breaker? How do breakers work?

A circuit breaker is a type of overcurrent protective device that shuts off the supply to an electrical circuit when there is a large flow of electrical current.

In all electrical systems worldwide, we have to have some means of shutting off the electrical supply when there is a fault. Each electrical circuit will need its own form of fault protection, overload protection and even electrical shock protection. The circuit breaker is one of the devices that electricians choose to achieve this circuit disconnection.

1) How does a Circuit Breaker Work?

We will be discussing the Miniature Circuit Breaker or MCB in this article. A circuit breaker is installed at the origin (start, beginning) of the electrical circuit and monitors the level of electrical current that is flowing through the circuit. When the level of electrical current exceeds the rating of the circuit breaker, it will trip, disconnecting the circuit and stopping the flow of electricity. Circuit breakers work very well at this task and can be more beneficial than traditional fuses.

2) When does a Circuit Breaker Trip?

A circuit breaker will trip when there is a short circuit and when there is a gradual and prolonged overload. 

On prolonged gradual overload, the circuit breaker uses a bimetallic strip which heats up, bends, and disconnects the flow of electricity as it pulls away from the contacts inside the breaker thus disconnecting the circuit. 

Under Short Circuit conditions, the circuit breaker will operate using its solenoid tripping system which operates much more quickly than under gradual overload conditions.

(Diagram: Short Circuit that will cause an MCB to trip)

3) What happens when circuit breakers trip?

Where electrical current levels exceed the circuit breakers rating and the device trips, there is often nowhere for the flow of electric current to go and an electrical arc is produced upon disconnection. This electrical arc needs to be safely dissipated to prevent danger which is why circuit breakers have an arc chamber to safely dissipate the arc. 

If the circuit breaker has tripped due to an overloaded circuit the bi-metallic strip will need to cool down and return to its original shape and position in order to make safe contact within the circuit breakers terminals once again. 

If the circuit breaker has operated due to electrical faults such as short circuit or earth fault current then it can be reset almost immediately. This assumes, of course, that the fault conditions have been corrected such as damage to cabling, faulty electrical appliances unplugged and so forth.

4) What are the different types of circuit breakers?

There are many different types of circuit breaker available for use in electrical systems. These devices protect against short circuits, ground faults and fires. There are some differences between these faults and we need devices able to detect the differences. The main types of circuit breakers are:

4.1) MCB

The Miniature Circuit Breaker or MCB for short. Operates when excess current above the rating of the device is present in the electric circuit. This can happen when a line to ground fault or line to earth fault is present. These are the most common devices found in your home's electrical system but are gradually being replaced in favour of the RCBO. In the UK in 2025, the most popular device being installed is now the RCBO.

(PHOTO: Inside a Circuit Breaker)

4.2) MCCB

Molded Case Circuit Breaker or MCCB for short. These devices operate in much the same way as MCBs but are usually much larger devices and intended for use in commercial and industrial wiring systems.

4.3) RCD

The Residual Current Device or RCD is a device that monitors electrical circuits to ensure that the current flowing in the Line Conductor and Neutral Conductor is substantially the same. Where there is a discrepancy, the device will trip in accordance with its rating. They are commonly used as safety devices electric shock protection in our homes and businesses where their rating is 30mA (30 milli amps) or lower. The term RCD is often used to refer to a broad range of devices that incorporate residual current protection. These devices are similar to the American / Canadian GFCI devices.

4.4) RCBO

Residual Current Breaker with Overload. The RCBO has all the benefits of an RCD but with an MCB effectively built in. The device will trip for earth faults in the same way an RCD does but will also trip for gradual overload and short circuit problems like standard circuit breakers do.

4.5) RCB

The residual current breaker or RCB is a Residual Current Device that will only trip for earth faults and NOT for overload faults. It is designed to break the circuit when there is an imbalance in the electrical current flowing through the line and neutral of the circuit.

4.6) AFDD

An Arc Fault Detection Device or AFDD contains an RCD, overload protection, short circuit protection AND protection against electrical arcing. They are effectively an RCBO with arc fault protection built in. These devices are similar to the American / Canadian AFCI devices. These devices are used to prevent fire hazards and reduce the risk of electrical fires. AFDD circuit breakers are likely to reduce in price during 2025 and will be more affordable than ever this year.

4.7) GFCI

The ground fault circuit interrupter is a device that monitors for ground faults in electrical circuits. They are available as breakers or outlets and operate in much the same way as any RCD device does. The GFCI is used in America and Canada whereas the RCD is used in the UK and Europe.

4.8) AFCI

The Arc Fault Circuit Interrupter is a device that monitors for arc faults. These devices are similar to the AFDD devices in Europe but are used in the United States and Canada.

(PHOTO: An RCD (RIGHT) plus several circuit breakers)

5) What types of MCB are there?

There are a number of types of Miniature Circuit Breaker. These devices are generally classified based upon their tripping characteristics. There are several types available including type A,B,C,D,K,Z. The most common types in use tend to be B,C and D. Each circuit breaker type has different applications but each offers overcurrent protection and limits electrical current reaching unsafe levels to protect the cables on the electrical circuit.

5.1) B type MCB

This circuit breaker will typically trip instantaneously when subject to a fault current of between 3 times and 5 times its rated current. The B type is good for general use and is often found in most domestic premises.

5.2) C type MCB

This circuit breaker will typically trip instantaneously when subject to a fault current of between 5 times and 10 times its rated current. The C type breaker is use for high start-up current equipment. Such equipment could include motors and large LED banks of lighting for example.

5.3) D type MCB

This circuit breaker will typically trip instantaneously when subject to a fault current of between 10 times and 20 times its rated current. These devices are used where very large machinery is in use creating very high start-up currents such as electrical installations containing industrial machinery, welding equipment, large motors, and other industrial applications.

5.4) Type A MCB

This circuit breaker will typically trip instantaneously when subject to a fault current of between 1 and 2 times its rated current. Type A MCBs are used where sensitive equipment is being protected.

5.5) Type K MCB

This circuit breaker will typically trip instantaneously when subject to a fault current of between 8 times and 12 times its rated current. These devices are used where there are inductive loads that have a risk of inrush currents upon startup such as motors.

5.6) Type Z MCC

This circuit breaker similarly to a type A MCB, will typically trip instantaneously when subject to a fault current of between 2 times and 3 times its rated current. It does however trip much faster than an A type MCB. These devices are used to protect very delicate electronic equipment and will typically operate in less than 0.1 seconds.

6) How do I know if my circuit breaker is faulty?

A faulty circuit breaker can manifest in a few ways.

6.1) Buzzing Noise

There can be a buzzing noise coming from the circuit breaker panel, fuse box, consumer unit or place where the device is installed. This can indicate that the circuit breaker may be failing.

6.2) Circuit Breaker Trips

If the circuit breaker trips often without obvious cause then this is a sign the device could be on its way to failure. 

6.3) Burnt out connections

If there is evidence of thermal damage or burnt out connections on your circuit breaker then it should be replaced immediately and the cause of the problem investigated.

7) What do the numbers mean on an MCB?

​There are some numbers printed on each MCB which tell us how they are suitable to protect our electrical wiring, equipment, and installation. A qualified electrician will carefully consider the selection of an MCB dependent upon the type of equipment, earthing system and risk of electrical overloads. A basic circuit breaker such as a Type B MCB will cover most domestic needs but what do they numbers mean on each device?

7.1) ​Type Of Circuit Breaker

​Type B,C,D. Whatever the type of circuit breaker, there should be a marking to indicate which it is. 

7.2) Product Code

​Each Manufacturer will have a product coder for their equipment to enable easy identification. 

7.3) Breaking Capacity

​The maximum level of fault current in amps that the circuit breaker can take before being destroyed or unable to contain an electrical arc. 

7.4) Rating

​The maximum acceptable level of current that can flow through the circuit breaker safely for a short period. A circuit breaker is not designed to run at its rated capacity continuously or for a prolonged period. Around 80% of the rated value is recommended for continuous loading. 

7.5) Energy Limiting Class

​The energy absorption capability of the circuit breaker. Class 3 has the highest energy limiting capabilities. 

7.6) Voltage and Frequency

​The voltage and frequency that the circuit breaker is designed to handle.

8) How long do Circuit Breakers last?

Circuit breakers tend to last, on average, around 30 years. Many manufacturers will guarantee their switchgear (circuit breakers, trip switches, RCD devices) for a period of up to 10 years. The lifespan of circuit breakers can be affected by numerous factors such as installation conditions, number of trip operations, external influences and loading.

9) Why are Circuit Breakers Important?

Circuit breakers are an important part of any electrical installation. These tiny electrical devices protect cables from overload, fire, and damage. From an electrical safety point of view, circuit breakers (MCB, RCD, RCBO etc) are a vital part of the home's electrical service panel or consumer unit. An older home may still have fuse wire type overcurrent protective devices which are not able to resist fault currents as high as circuit breakers can.

10) Conclusion

Whilst appearing to be a simple switch, the inner workings of a Miniature Circuit Breaker or indeed any form of trip switch, are rather complex. If you do not have circuit breakers protecting your electrical installation or if your circuit breakers are over 30 years old then it may be time to consider an upgrade. A professional electrician will be able to advise on the best products on the market.