N-E fault on lighting

[Tom__40]

Hi. I recently had an electrical inspection and N-E faults were found on both upstairs and downstairs lighting circuits. It's a 50s house, but has been rewired in PVC wiring (March 1982 if I'm to believe the newspaper clippings next to junction boxes under the floorboards). The upstairs lighting circuit has a borrowed live conductor from the downstairs lighting circuit (two way lighting for the upstairs landing light) if that's relevant to the below.

I was wondering the best way to track down where the N-E faults are? I have a multimeter with continuity function/low ohms setting, and have done a diagram of what connects where. I was thinking turn all lights on so all the wiring is being tested, then turn both breakers for the lighting circuits off, then split the circuits at convenient locations to narrow it down and test across N-E with the multimeter? I've done this at a couple of locations and think I've ruled out several lights/switches. I did also disconnect the two lighting cables from the consumer unit at the junction box upstairs and got continuity on both between neutral and earth (this is with the power off). Is this expected for a TN-S supply - I know earth is connected to neutral all the way back at the transformer, but should I see this continuity even when everything's turned off?! If not it would seem worth testing the cables between the consumer unit and junction box individually.

I do also have an insulation resistance tester, so was going to split the circuit in the same way and take some readings that way to see what kind of condition the wiring is in, removing all the bulbs. The guy who did the inspection didn't bother to remove the bulbs for the insulation resistance test, and has put down 0.05 Ohms (a dead short I presume) for Live-Earth for both circuits.


Thanks

 

[flameport]

Switch off at the consumer unit and disconnect all 3 wires for both lighting circuits (L,N,E).
Test for resistance between N&E on both cables, make a note of the resistance.
Repeat the test at each light fitting - wherever the smallest resistance is will be nearest to the fault.
That section can then be inspected / disconnected as required to identify exactly where the fault is.

Not necessary to turn the light switches on for N-E faults, as switches should only be in L, so won't make any difference.

 

[ericmark]

There are two ways, one as @flameport says is to test in a methodical manor, the other is to aim for likely places where the fault is, and most likely is a fixing screw on a light switch touching the neutral, still using part methodical manor, test the cable at consumer unit, then remove screws in each light switch in turn and see if fault goes, if it does inspect neutral at that switch.

 

[oldbutnotdead]

Got any dimmer switches in this installation?

 

[Tom__40]

Thanks guys, very helpful

There was one dimmer, I replaced it recently with a normal switch as the LED bulb wasn't liking it, it was replaced after the proper inspection but before the limited testing I've done so far. Think I'll try the methodical way first and see what I can find, will post back when I've had time to do it!

 

[Tom__40]

Hi. I followed flameports method using a multimeter, problem was I couldn't find a problem! After disconnecting all 3 wires for both upstairs and downstairs lighting circuits, I checked N+E resistance and could only get infinity. Same at all fittings/junction boxes. I then thought ok, lets try out the insulation resistance tester I ordered. Reconnected both earths to the earth bar and tested between earth and all 4 other conductors testing at 500V. Maxed out at >200 Megohms every time. Same between L+N after disconnecting earths. The guy who did the inspection supposedly found a L+N fault on the cooker circuit too, but testing the insulation resistance for that in the same way I again max out, it seems fine. Even tested that one at 1000V/2000Megohms, still maxed out. Test leads test ok. Mystified! I checked my ring main too, maxed again, they said 50 Megohms. Starting to wonder if they wanted to rewire (it would look easy to them, I already have most of the floorboards up, chasing and back boxes in place etc). The guy who signed the inspection form wasn't the guy who tested it, I queried this and was told he was an electrician and actually had more experience than the guy I'd been expecting (so why didn't he sign the form...).

I should add, I did replace a couple of junction boxes with WAGO's in WAGO boxes on the lighting circuits (to make it easier to test things). It's conceivable this fixed it, though there was nothing obviously visually wrong before doing this.

 

[flameport]

The guy who signed the inspection form wasn't the guy who tested it

Extremely dubious, those doing the inspection should be those completing the report and signing it.

It's conceivable this fixed it

Unlikely - an N-E fault with a resistance of <1 ohm would usually be obvious, such as damaged insulation, a fixing screw through a wire or wires connected incorrectly.

 

[oldbutnotdead]

Your new IR tester- what voltage does it test at?

 

[Risteard]

Extremely dubious, those doing the inspection should be those completing the report and signing it.


Unlikely - an N-E fault with a resistance of <1 ohm would usually be obvious, such as damaged insulation, a fixing screw through a wire or wires connected incorrectly.

Probably reading through the neutral/Earth link.