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Miniture Circuit Breakers in campervan electrical systems

Miniature Circuit Breakers (MCBs) are indispensable devices in electrical systems, offering protection against overcurrents and short circuits. These compact devices are particularly vital for campervan electrical systems, where limited space and safety considerations play crucial roles. 

In this article, we will explore what MCBs are, how they work, and how they can be implemented effectively in campervan electrical systems. Additionally, we’ll discuss two different types of MCBs and decipher the codes associated with them.

Understanding Miniature Circuit Breakers

Miniature Circuit Breakers are automatic electrical switches designed to protect electrical circuits from damage caused by excessive currents. They serve as vital safeguards, preventing overloads and short circuits that could potentially lead to fires or damage to sensitive equipment.

Functioning of MCBs

The primary function of an MCB is to interrupt the flow of current when it exceeds a predetermined threshold. They consist of a switch mechanism and a bimetallic strip or an electromagnetic coil. When the current exceeds the rated limit, the bimetallic strip heats up and bends, causing the switch to trip and disconnect the circuit. In the case of an electromagnetic coil, a magnetic field is generated when the current exceeds the set limit, tripping the switch. This quick response time ensures the protection of the electrical system.

Selecting MCB’s for Campervan Electrical Systems

Campervans have limited space, making MCBs an ideal choice due to their compact size. Here are some considerations for implementing MCBs in campervan electrical systems:

Load Calculation: Begin by calculating the anticipated electrical load of each circuit. This includes appliances, lighting, heating, and any other A/C electrical devices you plan to install. This calculation will help determine the appropriate ampere rating for the MCBs.

Circuit Design: Designate circuits based on the electrical load calculations, grouping similar devices together. For example, sockets should be on a separate circuit from the hot water tank to avoid overloading.

MCB Selection: Choose MCBs with appropriate ampere ratings for each circuit. Ensure they are compatible with the electrical system voltage. Typical MCB ratings for campervan applications range from 6A to 16A.

Selecting a Circuit Breaker Based On The Following:

  • The circuit breaker construction standard, BS EN 60898
  • Current rating for overload protection, with preferred values: 6 A, 10 A and 16 A.Rather than go to 16 A, however, the designer would be better specifying more circuits at 10 A, or even 13 A.
  • Sensitivity for fault protection, with Type B or Type C; but NEVER Type D
  • Short-circuit capacity – probably between 3000A (3kA) and 6000A (6kA)
  • Energy limiting class – preferably 3
  • That the circuit breaker must disconnect both line and neutral conductors (2-pole), to satisfy Regulation 721.43.1 of BS 7671 for touring caravans and motorhomes.

721.43.1 Final circuits: Each final circuit shall be protected by an overcurrent protective device which disconnects all live conductors of that circuit.

Double Pole Miniature Circuit Breakers

As we have highlighted above in the last bullet point, circuit breakers used for the protection of circuits in touring caravans and motorhomes must be double-pole, including the main disconnector/main switch. (Regulation 721.537.2.1.1 of BS 7671). 

721.537.2.1.1 Each installation shall be provided with a main disconnector which shall disconnect all live conductors and which shall be suitably placed for ready operation within the caravan. In an installation consisting of only one final circuit, the isolating switch may be the overcurrent protection device fulfilling the requirements for isolation. 

Some confusion lies here for installers and van builders as the obvious choice is a single-pole device, making and breaking the line conductor only. 

Since a touring caravan or motorhome is considered a special location by the IET wiring regulations, we are to adhere to slightly stricter guidelines. A double pole overcurrent protection device being one of them. 

Since owners of motorhomes and caravans are likely to travel to countries where polarity of supply is not as rigorously enforced as the UK, the designer will want to specify 2-pole devices providing protection for both the line and neutral conductors. 

Types of MCBs and Alphanumeric Codes:

Type B MCBs: These are the most common MCBs used in residential and commercial applications. They provide protection against overloads and short circuits caused by general-purpose equipment. The alphanumeric code for Type B MCBs starts with the letter “B.”

Type C MCBs: These MCBs offer increased protection for circuits with equipment that have a higher inrush current, such as motors or transformers. The alphanumeric code for Type C MCBs starts with the letter “C.”

Understanding MCB Markings

MCBs are labeled with letters and numbers that provide vital information about their specifications. Let’s decode these markings:

  • Current Rating (In Amperes): This indicates the maximum current that an MCB can handle without tripping.
  • Trip Curve: The letter associated with an MCB (e.g., B or C) denotes its trip curve, which indicates the response time of the circuit breaker to an overload or short circuit.
  • Breaking Capacity: Represented in kiloamps (kA), this value signifies the maximum fault current that an MCB can safely interrupt without causing damage. Typically 6kA (6000a) or 10kA (10,000a). 

How do Miniature Circuit Breakers Work?

MCBs operate based on the principle of thermal and magnetic tripping. They consist of a bimetallic strip and an electromagnet, which respond to different types of electrical faults:

Thermal Tripping: The bimetallic strip within the MCB is designed to heat up when the current flowing through it exceeds the rated value. As the temperature rises, the strip bends and eventually trips the circuit breaker, disconnecting the circuit. This mechanism protects against overloads.

Magnetic Tripping: In the presence of a high magnitude short-circuit current, the electromagnet within the MCB generates a magnetic field. This magnetic field rapidly pulls the trip mechanism, causing the circuit breaker to trip. Magnetic tripping provides rapid protection against short circuits.

The combination of thermal and magnetic tripping ensures that MCBs can respond to a wide range of electrical faults, thereby safeguarding the connected circuits and devices.

*Can’t find miniature circuit breakers in our online shop? That’s because all of our 230V distribution boards are built in house by competent, qualified personnel, to your specific needs, enhancing safety and ensuring that you are being matched with the correct product for your installation.  

Get in contact today and we’ll be happy to size and design your board. 

Cable Terminal

Integration of MCBs into Distribution Boards:

Distribution boards, also known as consumer units or fuse boxes, are key elements in electrical installations. They provide a centralised location for housing MCBs and other protective devices, enabling efficient power distribution and electrical system control.

Mounting: MCBs are mounted onto DIN rails within the distribution board. DIN rail mounting allows for easy installation, removal, and replacement of circuit breakers.

Circuit Configuration: MCBs in distribution boards/consumer units are typically organised to cater to various circuits in the campervan. Each MCB is connected to a specific electrical circuit, such as power outlets, or specific appliances. This arrangement allows selective disconnection of individual circuits in case of a fault, minimising downtime and ensuring safety.

Circuit Identification: To simplify maintenance and troubleshooting, MCBs in distribution boards are labeled or numbered to correspond with the connected circuits. This labelling helps electricians and campervan owners quickly locate and address any issues.

Additional Protection: In addition to MCBs, distribution boards may incorporate other protective devices such as residual current devices (RCDs). These devices provide additional protection against electrical shocks and ground faults, enhancing overall electrical safety. 

Read more about RCD’s and choosing the correct type here.


The regulations specify the selection and coordination of MCBs to ensure proper protection against overcurrents. They outline the maximum permitted Zs (impedance of the circuit) values for different types of MCBs, taking into account various factors like cable size, protective device characteristics, and fault currents.

The Wiring Regulations also address issues related to discrimination, which is the ability to isolate a faulty circuit without affecting the supply to other circuits. Discrimination ensures that only the circuit experiencing the fault is disconnected, minimising disruption to the rest of the electrical installation. MCBs should be selected and coordinated in such a way that discrimination is achieved.

Furthermore, the regulations emphasise the importance of proper installation, labelling, and periodic testing of MCBs to maintain their effectiveness. Qualified electricians are responsible for ensuring compliance with the Wiring Regulations and conducting the necessary inspections and tests.

Need help with understanding the regulations surrounding Miniature Circuit Breakers? 

Contact us here at Tiny Build Electrics and we will be happy to help. 


Miniature Circuit Breakers are vital components of electrical installations. They protect circuits and electrical equipment from overcurrents and short circuits, mitigating the risk of fires and damage. 

By understanding the different sizes of MCBs and adhering to the regulations outlined in the British Standard 7671, you can ensure the safe and effective operation of electrical systems.

Remember, when dealing with electrical installations, it is essential to consult with qualified electricians who are knowledgeable about the regulations and possess the necessary expertise to install and maintain MCBs in compliance with the requirements outlined in the Wiring Regulations. 

Safety should always be the top priority in any electrical project, and we at Tiny Build Electrics can help you achieve just that. Contact us for a consultation and we’ll be happy to help. 

Tom Alderdice

Hey, I’m Tom – founder of Tiny Build Electrics

My mission is to help sustainable-minded folks develop their electrical knowledge, giving them the confidence to do their own tiny build electrics.

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