Simple answer - check to see where the permanent-line is located, at the switch or at the lamp (advice below):
Common Wiring (Loft Lamps)
In a loft, the permanent-line will often go to the switch first, with the lamp connected directly to the switch.
- Common way to tell: the neutral conductors (between supply cable & lamp cable) are usually joined in a connector-block inside the switch.
Alternative Wiring (Loft Lamps)
Sometimes the permanent-line goes to the lamp first, with a switch-wire run between the switch and the lamp.
- Common way to tell: The lamp will have a 'loop' connection which the permanent-line is connected to. There should also be visible brown-sleeving seen on some neutral-conductors in both the switch and lamp to indicate they are being used as line-conductors.
Side Note (Considerations)
- Make sure the circuit Ib (design current) is suitable for the additional equipment, small current-demands can add up quickly and result in potentially dangerous over-currents (and the circuit may have been modified in the past).
- Check the cable-calculations and voltage drops to be sure.
- Find the breaker you're using for the new equipment connection and label it in the consumer-unit for future reference.
- Carry out the necessary inspection & testing before going live.
- Remember that although the new equipment current-demand is low meaning that small-conductor sizes would be ok (under normal conditions), the protective device is calculated to protect the cables under fault-conditions and a small conductor size would not satisfy the requirements. Consider installing a FCU (Fused Connection Unit) in this case.
I hope this has helped. I've explained things simplistically just in case you're unsure but don't mean to seem rude (if it comes across that way). Always happy to answer any questions as best I can.
