Fuses, MCBs, RCDs and RCBOs – what is it all about????

MCB Diagram

Fuses, MCBs, RCDs and RCBOs – what is it all about????

Fuses, MCBs, RCDs and RCBOs – what is it all about????

OK so they are all there to protect, but what is the difference and what do they do?


Firstly fuses and MCBs – that is miniature circuit breakers – they both ‘blow’ when current is exceeded, but an MCB can be reset after the fault is fixed, a fuse must be replaced.
A fuse is basically a piece of wire between two contacts to make a circuit. When current flows heat is produced and the wire gets hot. If too much current it produced the wire melts and the circuit is broken. There is nothing you can do you must replace the fuse.
A miniature circuit breaker on the other hand uses an electromagnet to break the circuit if the current is exceeded. It also has a toggle on the front so that it can be reset. The MCB is generally more reliable than a fuse and reacts quicker.
The Chint range of MCBs are available from stock at Chalon Components in all curves and Amp ranges.

So that is a fuse and a miniature circuit breaker what of an RCD?

Well the fuse and the MCB will protect equipment and wiring, they will not protect people. An RCD is designed to protect people.
The residual current device monitors the current that goes down the live and neutral cables. If the circuit is healthy then the current going down the live cable will be the same as that coming back through the neutral cable. This means that the amount of current going down the earth cable will be nil and all is well.

NE2 Metal SocketIf some of the current leaks from the live to the earth the RCD will detect the difference between the live and neutral and trip. So if you are gardening and accidentally ‘strim’ the cable instead of the grass an RCD protected socket would detect the difference in current flow and trip out.
The Chint RCD protected sockets have a 30mA trip so although they will allow a small amount of leakage they will greatly reduce the risk of fatality. The plastic version can be used as a direct replacement for a standard double socket.


If sockets are fitted outside it is preferable to fit an RCD into the distribution board feeding the sockets. Great, but you’ve not got any space in the board as it is full of MCBs. Enter the RCBO – in short a combined MCB and RCD. This is the answer to the problem as they now come in single module width to fit into distribution boards.

NB3LEU RCBOThe MCB side of the unit handles the current and will trip based on a over-current whilst the RCD side monitors the earth leakage and will trip if it detects an imbalance of the live and neutral. The other great advantage of using the RCBO is that only one circuit is affected in the event of a trip – not the whole board as would be the case if using an RCD as an incoming switch on a distribution board.

Chalon Components Ltd stock the Chint NB3LEU RCBO in both B and C Curve up to 40amps.

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