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A common mistake to make when replacing a light fitting is to assume all of the red wires are lives and the black wires are neutrals. It is a common practice for one of the black wires to be a switched live, this should be marked with a red sleeve which is quite often lost or not fitted. It is also common for a light fitting to be supplied with a 3 piece terminal block whereas a 4 piece block is required.
Turn off the power to the circuit you are working on and make sure it is switched off.
Firstly you will need a multimeter as you will need to identify which of the black wires is a switched live.
You will need to replace any dimmer switch with a normal switch for testing, if you have made this mistake the dimmer switch will almost certainly have died!!
In a standard loop in ceiling rose there are either two or three cables, each containing a red, a black and a bare copper wire with a green/yellow stripe sheath. If there are only two cables this can mean there is no loop supply out to another light, just a supply in and a switch wire. If there are three cables this can mean there is a supply in, switch and a loop supply out.
You will now need to disconnect and separate out the red and black wires to identify which one is a switched live. If you already know which is the switch wire then you can jump this step. With your multimeter set to ohms range, connect across one of the red and black wires on one cable whilst operating the light switch on/off. If you see the resistance of the meter go from low to high to low etc when the switch is operated then this is the switched live. If not, try another cable.
Once you have identified which cable is switched live, put a piece of red sleeve over the black wire to signify it is a live. You now need to connect all reds together but to nothing else, the switched live black (with red sleeve) wire to the brown wire or L terminal for the lamp and the other one or two black wires with the blue or N terminal for the lamp. Connect all earths together and to the earth terminal if required.
Notes,
(If you only have 2 cables ignore the supply to next light cable).
Chock block must be enclosed in fitting or in a suitable enclosure above ceiling which is accessable for inspection. The chock block must not be exposed to heat from the lamp.
If a metal light switch is used, connect the earth wire to the earth terminal on the light switch and loop to the back box.