DIY How To

Circuit Protection

DIY Projects - Fuses

Working with electricity can be dangerous, please read the safety pages before you begin any work. If you are in any doubt always consult a qualified electrician.

If a lighting circuit or sockets are not working, the first thing to do is to check if the fuse has blown.

Consumer unit Always switch off the power using the main switch on the consumer unit. The main switch can be up or down for off depending on the make of unit, so always double check that it is off.
Consumer unit fuse Remove the fuse cover if there is one and then pull out the fuses one at a time until you find the one that has blown. Replace the fuse if it has not blown before pulling out another, this way you will not get them mixed up. The fuse holders are colour coded so they can be easily recognised, 5 amp (white), 15 amp (blue), 20 amp (yellow), 30 amp (red) and 45 amp (green).
Blown fuse A close look at a blown rewireable fuse will normally show a broken wire or even no wire at all if it has completely vapourised. There may also be scorch marks on the fuse holder.
Replacing fuse wire Loosen the two terminal screws and remove the old broken wire. Thread some new fuse wire making sure it is the correct rating through the tube until it is visible at both sides. If the fuse holder does not have a tube for the wire to go through you can connect direct to one terminal.
Replacing fuse wire Wrap the fuse wire around the terminals in a clockwise direction to make a loop as shown and tighten the screw. It is important to wrap it clockwise to prevent the loop undoing when being tightened by the screw. Leave enough slack to attach the wire to the other terminal in the same way.
Good fuse Once the wire has been connected to the terminals at both ends, replace the fuse in the consumer unit, replace the cover if there is one and then switch the power back on. If the fuse continues to blow you should consult a qualified electrician.

Some manufactures have designed minature circuit breakers (MCB's) which will fit into existing consumer units, these allow you to do away with rewirable fuses. Always consult a qualified electrician for guidance


DIY Projects - RCD


Principles of an RCD

When a Load is connected to the circuit supplied through an RCD the incoming supply, phase and neutral, passes through an iron core or toroid, which acts as the primary winding of a current transformer.

Diagram of how an RCD works

Note: The new cable colours are used in the above picture.

A secondary winding around the toroid is used as a sensing coil to detect any out of balance between the current flowing through the phase and neutral conductors.

Under normal conditions the phase and neutral currents are equal and opposite so no flux is induced in the toroid and hence no current flows in the sensing coil.

If a fault occurs and current flows to earth, the phase and neutral currents will no longer be balanced. A flux will be induced in the toroid and a current will flow in the sensing coil which if of sufficient magnitude and duration will cause the actuator to function and trip the RCD thereby disconnecting the supply.

A test circuit is also incorporated whereby a connection is made from the load phase to supply neutral via a Test Coil and Test Resistor and activated by a Test Button.

This test circuit is designed to pass a current well in excess of the related tripping current of the RCD in question.

It is recommended to use an RCD when using portable hand operated 240v electrical equipment. Ideal uses in the garden are lawnmowers, hedge trimmers, leaf shredders etc. In the home it is wise to use one with power drills, electric tile cutters etc.

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