Pulsing on test leads

[Colonel]

Hi there, I was at a customers premises today following water damage to the lighting circuit. When I put my tester on the supply at the switch and ceiling the voltage is pulsing 120-240. When I test on the sockets the supply is steady at 240. The breaker hasn't tripped, so ice isolated the lighting meantime. Any ideas or any similar experiences?

 

[Marvo]

Was the light fed from a dimmer?
What tester are you using?
Did you test L-N at the switch and at the fitting or L-E?

 

[Colonel]

Hi, I'm using a set of RS components 424-333 test leads. No dimmer involved and tested across L&N no reading to earth. Same pulsing happens at switch and at ceiling at a joint I can access. The premises is old 100+ and lath & plaster ceilings in original part of building and has PVC cable routed through metal conduit in places. Can't get to a junction box or joint near the faulty lighting because of a flat roof.

 

[Marvo]

I was after info on the actual tester rather than the leads attached to it. Was it a digital tester or analogue?
You say you tested L-N, were the neutrals run through the switch box?
Did you do the usual circuit tests like a Zs maybe? Sounds like you may have more than one fault, you need to fully dead-test the circuit and give us the results.

 

[Colonel]

Sorry, I should clarify, the tester is the RS model I quoted. They're test lamps, rather than a testing unit. 30 years old and have never failed me yet. I'll need to get back in when the place has dried out and get my test meter to take proper readings as you suggest. My suspicion is there's dampness or water causing the breakdown. The problem is the roof repairs can't take place until the weather improves, so it's unsafe to turn the supply back on until that works completed. Thank you for all your help. I'll update readings when I can.

 

[Farmelectrics]

As marvo said I think you need to do some dead testing first