Fuses and Circit Breakers Including MCBs, RCDs, MCBOs Explanation

Summary: An explanation about different types of fuse and circuit breakers for electrical safety. What is an MCB, RCD, RCBO, and when do I use a Circuit Breaker? Cartridge and rewireable fuses pictures, information and facts.

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Please also see our project on the New Wiring and Cable Colours.

Make sure you have isolated any circuit you are working on and see our electrical safety project

Please remember when attempting any electrical installations at home that you are obliged to get the completed job tested by a fully qualified electrician and obtain a minor works certificate. Failure to do this may render your house insurance invalid and you may have difficulty selling your home.

Please also see our project Part P Building Regulations

MCB - Miniture Circuit Breaker

MCB - Miniture Circuit Breaker

RCD - Residual Current Device

RCD - Residual Current Device

An MCB is a form of fuse (protective device) which overcomes the traditional problem associated with fuses in so much as when one blows it does not need to be replaced as a fuse does. MCB's operate when they sense an overload, or over current, and become an automatic switch, turning off, or tripping, the MCB when it detects such an overload.

An RCD is a similar protective device which is different in two ways. Firstly it is connected to both the live and neutral wires in the consumer unit making it a double pole switch, whereas the MCB is only connected to the live side of the circuit. Secondly, rather than just detecting an overload of current, it detects the fault which causes the overload.

Many consumer units these days are produced to be split load consumer units. See our project on consumer units. Those circuits which need more protection than others, ie showers, sockets serving outdoor appliances, external 240V lighting, must be protected by an RCD. Other circuits such as lighting and cookers, are protected by MCB's.

Each individual circuit, of whatever kind, is protected by an MCB. The circuits needing most protection are also served by 1 RCD. Each circuit does not need its own RCD in a split load board.

If you wanted to protect every circuit against overload and faults, you can install an RCBO which is a residual circuit breaker with over current protection. This is a combined MCB and RCD.

RCBO - Residual Current Circuit Breaker with Overload Protection

Residual Current Circuit Breaker with Overload Protection

For total safety outdoors if you are unsure about circuits and the like, you can fit an RCD plug to your lawnmower or other outdoor equipment. Even easier is the RCD socket which simply plugs into a normal socket and provides you with all the protection you could need outdoors. RCD protected sockets can be easily fitted in place of your existing double sockets.

RCD plug adaptor

RCD plug adaptor

RCD socket adaptor

RCD socket adaptor

Re-wireable/cartridge fuse socket

Re-wireable/cartridge fuse socket

Other forms of fuse are either re-wireable fuses and cartridge fuses. Cartridge fuses are simply fuse wire contained in an enclosed glass or ceramic tube (such as the fuse in a plug) and re-wireable fuses (slowly becoming obsolete as wiring regulations are upgraded) which are simply two terminals connected by a length of accessible fuse wire of differing amperage rating.

For regulations governing heights of sockets etc, please see the wiring regulations document on the iee.org website

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