Hi I've fitted new lights & switch in the kitchen about 4 weeks ago. The switch is a 2 way & everything was working fine until yesterday when turning on 1 switch it would trip but the other switch works fine. I checked the wiring in the light & the switch thinking it might be a loose wire but they were ok can anyone advise me on this problem?
I have considered what you have said and there are normally three wires going from first switch to second switch and these will be. Com permanent line L1 line when far switch is switched to L1. L2 line when far switch is switched to L2. So if a nail, screw or other item was to penetrate the cable and damage just L1 or L2 then it would do as you describe. Unlikely to be switches as there is no earth or neutral connected to most switches. I have found in the past where a nail or screw has nearly penetrated a cable enough to cause a short to earth or neutral but not until a little damp gets in to cause tracking did it cause a fault large enough to trip the protective devices. I would be very careful as anything may be live with this type of fault although the neon screwdriver is not a safe test device in this case it may be use full for testing every screw or nail in the wall. It will not prove them as safe but may locate the bad one. Floor boards are often the problem and you need to consider where were the wires run and what may have gone through them. Electricians use 500 volt to test which is more likely to find a fault than the 330 volts peak of a 230 volt supply. I even have a tester using 2500 volt which is really for testing appliances after re-assembly but again the DIY guy will not have this equipment.
What type of lights are they? It is possible that if the lights are on SELV transformers, that the transformer is causing that part of the circuit to trip. Other than that, if it is MCB/Fuse that is tripping, you would be looking for a fault, generally a sort circuit of line to earth fault. This could be down to a snagged/trapped wire (could have happened when replacing switches, a loose connecting making contact with other conductors or metal parts of accessories. Or failure of the cables insulation. So would be wise to isolate the circuit and do a little investigation.