This weekend I learnt how NOT to replace a ceiling light fitting, and then how to do it properly.
The lighting circuit consists of a number of segments going from the consumer unit to each light fitting in turn. At a fitting there is a spur to the switch and a spur to the light (though the latter is removed with the fitting). So the circuit is something like
+--+ live _______ | \ switch +---==\ \===-+ | | +-==/______/===----+ _________ live | | _______________ C ===\ \=========-+-)---==\ to next U ===/_________/=========---)-+-==/_______________fitting cable neutral | | | | <- neutral appears to be -> | | neutral but is to either off or live light
So you have three cables hanging from the ceiling, one of which is special (it goes to the switch) but which might not be easy to distinguish from the others.
Do not connect all the red wires together and all the black wires together. This means the light is on when the switch is off, and turning the switch on causes a short circuit which upsets the CU. bang