Tripping earth leakage in fuseboard with new fitting

[RogH]

New to the forum - looked at several similar entries but I couldn't find an answer to my specific problem.

After having the builder fix a new front porch I've now got around to fitting a new external light unit to replace an old rusty one. We haven't used the old one in years as it was blowing the fuse when switched on - we put it down then to damp getting into the works of the light and shorting...

Well I put the new stainless steel/sealed unit in place and linked up the Live and Neutral wires to the existing connector block and that was when I found the first problem :- The New light unit has an earth terminal but there are only 2 wires coming out of the wall from the switch.
Question 1: Do I need to run another (earth) lead from the light unit to somewhere ? And where should I connect to ?

Pushing ahead and risking the lack of an earth connection I flicked the switch on and the lamp lit up - fantastic. But the earth trip in my fuseboard detected a leakage and turned the power off. Weirdly enough this left the lights on and just turned off the power sockets and cooker etc. which I though a bit strange (like lighting power can't electrocute you or something ?!)....

Suspecting that the lack of an earth to the light unit might be the problem I substituted a simple 2-wire ceiling light fitting I had kicking around (brand new, never used but wired). Same thing happens.

Going a stage further I started to wonder if there was a short in the wiring in the wall from the switch so I removed the light unit and just left the connector block with the 2 wires terminated. Now when I switch on/off there is no reaction from the fuseboard and all is "safe" - so I assume the wiring from the switch to the connector is sound and not shorting....

Whare do I go from here ? I'm completely out of ideas as to why this situation is happening. I've checked the wiring of both light units and can't see why the earth trip is kicking in each time and dropping the power off but leaving the lighting circuits live - especially as the lighting might be the "dangerous" bit causing the leakage !

Baffled - hope you can sort me out.

Rog

 

[jj4091]

My immediate thought would be that you have a neutral to earth fault in the wiring to the outside lamp. Do not under any circumstances use it or leave it connected in this condition without an adequate earth. What are you using to check the wiring?

 

[RogH]

That was quick - didn't expect that ! Cheers ;^)

I don't have a tester so my next step is to borrow one from my mate when he gets home from work tonight. Only tests I've managed are the ones listed "doing it for real" and being VERY careful !

Are you suggesting there is a problem with the wiring in the wall to the connector as I suspected ? If so why doesn't it trip the earth leakage when I turn the switch on but with no light connected ?

 

[jj4091]

That was quick - didn't expect that ! Cheers ;^)



Are you suggesting there is a problem with the wiring in the wall to the connector as I suspected ? If so why doesn't it trip the earth leakage when I turn the switch on but with no light connected ?

Because there is no supply on the neutral until you complete the circuit (have to surmise I'm not there)

 

[RogH]

I'm with you now... Shows how little I know/recall...

I guess I was thinking the leakage would be on the supply TO the connector so might trip when the switch was selected.

 

[Steve]

Look on the fuseboard - the RCD only protects the circuits it needs to.

This is normally all the sockets, the shower, any outdoors appliances, sometimes the cooker.

Lighting is a low risk application, since you cannot plug dangerous appliances into it or extension leads to outside. So yes, a correctly designed and wired lighting circuit cannot shock you.

 

[RF Lighting]

Sounds like a borrowed neutral to me

This is where the live for the circuit has been taken from a convenient place (often a nearby lightswitch), and the neutral for the circuit has been taken from another convenient place (such as a nearby socket)

By the sounds of things you really ought to get this part of the circuit rewired, due to the lack of earth and the fact it is causing your RCD to trip.

 

[RogH]

Thanks to all for the feedback.

It looks like the wiring may well be faulty/shorting - maybe the insulation has been broken when dragged through the wall...

So, it seems I need to install a new cable to sort it out AND provide an earth at the same time.

That easy "5 minutes job to put a new light up" appears to have expanded to use up all my holiday and need a replastering job too !