This is a very old web page written before 2008 where the rules changed requiring RCD protection on buried cables at less than 50mm and also for bathroom lights. I expect to lose access to these pages due to change of ISP some day I may get around to re-writing but please do check on latest regulations.
Again and again we are asked how to change a light fitting.
The main problem the ceiling rose is not only a light fitting it is also a junction box.
There a four connections:-
The earth this does not normally cause a problem.
The Loop (Line)
The Switch (Line)
The problem is the Loop does not have a connection to the cheap imported lamps and this means the connection block will need changing to one with four ways rather than three.
Second the switch wire is often the same colour as neutrals.
It is therefore important to note all cable connections before the old lamp in removed.
The problem in explaining what wires go where is there are many colours used. Originally all "Line" wires were red and all "Neutral" wires were black, but this was changed so "Line" now is Brown and "Neutral" is blue. However twin reds were and twin browns are more expensive than standard Red + Black or Brown + Blue so it is common to fit sleeves over the black or blue wire showing it should be red or brown. Also Red, Yellow, Blue or Brown, Black, Gray are used for two way strappers so are sometimes also used in the wiring. The sleeves are often either never fitted or fall off. Although Electricians may carry sleeve most DIY people will not want to buy a whole hank of sleeving. If one is lucky they may have used insulation tape instead. I like white tape then I can mark it with a marker pen.
So to return to fault finding first job is to identify the switch wire. If one is lucky it will be one of a pair of "Line" coloured wires. If not then it will need belling out.
To the left is a normal ceiling rose. And to the right is a standard wiring diagram showing the three most common lamp types X1 is a standard lamp, X2 is a two way lamp and X3 a bathroom lamp. It will be noted that X2 still has just two wires coming from the ceiling rose the same as X1. Also both lamps feed with blue wires both for Line and Neutral supply. Using a bell and battery or an ohm meter (Many have a buzzer built in) one has to test each wire flicking the switch on/off until the switch wires are located. The power in and out does not really matter which is which as they just connect together.
One departure is where twin switches have been used to switch two lights in the same room. Instead of running two separate twin and earths a single triple and earth is run from nearest lamp and also triple and earth is run between the lamps. The diagram below to the left shows how the circuit can be modified to allow a single triple and earth cable to control two lights the problem is in theory all the cores should be brown or on old system red so you don't know which wire is which. Also there is an extra wire in one ceiling rose which is not catered for in the rose.
It must also be remembered that shoving wires up the hole in the plaster rather than using a ceiling rose means that if anyone gains access from above either under floor boards or in loft they may grab in error these live wires and therefore great care is required to ensure there are no exposed live parts. A second picture is shown to left which shows the rose wired with the standard three cables. The next picture again shows is wired up. I am not in favour of removing ceiling roses although I have done it in my own house. But in my house all lights are also covered with RCD protection.
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