A table lamp contains a simple electrical circuit comprising of the conductive route and insulated surrounding parts.
The electricity travels via the brass in the plug, along the copper inside the brown ‘live’ wire to the alloys in the electric bulb, and then returns via the blue ‘neutral’ wire. A switch interrupts the circuit. The third prong for the yellow and green ‘Earth’ wire – not always present on a lamp – is a safety route that encourages rogue electricity to travel through it rather that you if the blue and the brown wires have connected, making a short circuit. Needless to say, understanding how electricity travels is essential in the process of detecting and fixing the fault in any electrical appliance.
You will need:
Long nose pliers
A small flat and small crosshead screwdriver
1) Before you begin, check it’s not just the bulb that has blown by replacing it with a new one, or switching it with a bulb from a working lamp. Test the wall socket too by plugging in a working appliance.
2) Once you’re sure it’s the lamp at fault, make sure you work in a well-lit area and on a flat surface. Unplug the lamp – never work on an appliance that is plugged in.
3) The problem could be a blown fuse. Open the plug. British plugs are safe, but quite fiddly. Unscrew the top part of the plug (the part with the three prongs) while laying the back of the plug in your other hand. Don’t fully remove any of the screws. Remove the fuse and check it using the multimeter (these cost around £10). The simplest testers to use have two leads with metal probes or clips at the end, and a main body with an indicator needle. Choose the OHM setting on the meter, connect the red probe to either end of the fuse and the black probe to the other. If the fuse is not broken, the meter will register by moving the indicator needle from left to right. Always replace the fuse with one of the same rating.
4) If the fuse is fine, next check the wiring. “Bad” wiring is not only incorrect wiring, but also wiring that isn’t tight and secure.
a) Wiring the plug
Check to see that all the wiring is tight and secure. New plugs have instructions on an attached paper – read them first. The wiring is always to the same principle, but all plugs have a cable securing device that differs slightly in some designs. Make sure the white sheathing cable is securely held in the plug, as this will keep the wiring firm if the cable is tugged or tripped over. The brown (live) wire connects to the prong marked “L” with the fuse fitted. The blue wire fits into the prong marked “N” on the plug. If you are rewiring the plug, use wire cutters to ensure that wires are the correct length, as indicated in the instructions, and use wire strippers to expose about 5mm of copper wire. Make sure that all the exposed copper is securely attached under the brass screw on each of the prongs in the plug. Never use kitchen knives or your teeth on bare wires!
b) Wiring the bulb holder
Check the wiring in the bulb holder. Unscrew the fitting and inspect the connections. If in doubt, replace the entire bulb holder with a new one. A metal bulb holder requires a three core, earthed cable. Lamps with plastic fittings need a two core, six amp cable.
c) Replacing the cord
If you see any breaks or exposed wiring, it is advisable to replace the entire cord. Threading a new cable through the inside core of some lamps can be quite tricky, but you can connect a new cord to the end of the old one by joining the exposed wires together after removing the plug – or exposing the wires at a point above any broken part of the cable – and taping them together with electrical tape to create a smooth connection.
5) Lastly, always test for continuity with your multimeter before plugging your lamp back into the socket. Place one tester probe on the brown or blue connector prongs on the plug, and the other probe on one of the spring terminals inside the bulb holder. Make sure the probes are not touching any other part of the plug or bulb holder. If the test indicator needle does not swing to the right, move one of the probes to the other prong. Carry out the same procedure for the other prong. If the needle does not swing, check that the switch is switched on. Finally, place both probes on both prongs of the plug. This time, the needle should not swing. This indicates that there is not a short circuit. Place a working bulb in the bulb holder and plug the lamp into the wall socket.