Fault Finding A Lighting Circuit

[Murdochcat]

Problem with lighting circuits is that they are rarely logical and often have branches off branches in the circuit

 

[Taylortwocities]

I grabbed a load of Rockwool to try and smother the flames,

You should have tried Celotex. That's fire- resistant, isn't it?

 

[securespark]

Trouble is, I have it under my bedroom floor. What do I do?

One BC guy said that the Rockwool I was planning to use was not good enough and said I had to use Celotex. Then a second guy came to sign off and said he'd have been fine with Rockwool....

 

[thomp1983]

So I've started looking at this after dropping my daughter off at school.

Swapped the mcb's for both lighting circuits over, no change trips as soon as energised.

Following the advice on here I've split the j/b that's roughly in the middle of the circuit and also just after where I think the wiring is most likely to have been disturbed by the cat.

That's the jb in question, I've removed the cable at the top left which feeds the next light on the circuit. Top left cable is the feed in from the previous light, bottom left cable is the switch, bottom right cable is to the light.

Testing with a multimeter on continuity I've a reading between the neutral terminal and the earth terminal so it looks like my fault is neutral to earth but I'd of expected that to trip the rcd aswell not just the mcb?

I'll now go and see what's happening at the next junction box back from the one I've opened.

Thanks,
Chris.

 

[EFLImpudence]

There is likely to be continuity between N and E. That is normal.

 

[thomp1983]

I thought that they joined before the meter and should be seperate after that, it would explain why the rcd hasn't tripped though.

 

[EFLImpudence]

A tripping MCB means a fault between L and N or E.

A fault between L and N would not trip the RCD.

 

[thomp1983]

I understand that a fault to earth would trip the rcd and in this case it isn't so that's why it's believe to be a line to neutral fault.

Ive managed to isolate it down to a fault in the downstairs bedroom lighting, this consists of 2 lights and 2 2 way switches so that both sets of lights can be operated by the switch by the door or the one next to the bedside table.

 

[thomp1983]

So the issues sorted but it's going to lead to more fault finding as it's shown something isn't right.

So having narrowed the issue to the downstairs bedroom circuit I then removed the feed out to the second light and the issue remained. I then removed the cable to the light and switch one at a time which showed the issue was with the switch or it's cable.

Being quicker to remove the switch plate than trace the cable I opted to look at the switch first and found the below,

The old classic cable nipped by the faceplate screw, now that's a line to earth fault so should of tripped the rcd which leaves me with more work as to why the rcd didn't trip and also the question of why it's been fine for the 10 months since that room was done.

The rcd itself works as if I plug my multitester into a socket that's on that rcd and use the rcd function it trips so I think that rules out an issue in the consumer unit.

My best guess is a gap in the earth part of the lighting circuit, likely where a class 2 fitting has been used or plastic switch and the circuit hasn't been completed to allow for the accessory not using an earth but having said that given the circuit is done using jb's the earth should be complete for every loop in and out cable unless one has got the earth sheath trapped under the screw.

So overall one issue resolved and another found.

Having done the work I still don't understand how you would of tested for the issue found with the power isolated.

I understand you could of performed a visual inspection of all the fittings and fixtures but I'm interested in how you could find the result just with test gear as at no point could I find continuity between earth and live with a multimeter.

Thanks
Chris.

 

[EFLImpudence]

Please - it is could have, would have.


Perhaps the nick in the insulation has not been touching the screw since last time the switch was fitted.

 

[DetlefSchmitz]

Please - it is could have, would have.

" could've, would've " might be more accurate, as it reflects the pronunciation.

 

[Harry Bloomfield]

I understand that a fault to earth would trip the rcd and in this case it isn't so that's why it's believe to be a line to neutral fault.

You will get continuity between N and E, they will either be joined at the cut-out, or back at the substation, which will not trip your RCD.

Your MCB trip, is either due to a simple overload of the circuit (unlikely), or a L to N fault. A L to E fault would also trip the RCD.

 

[Harry Bloomfield]

My best guess is a gap in the earth part of the lighting circuit, likely where a class 2 fitting has been used or plastic switch and the circuit hasn't been completed to allow for the accessory not using an earth but having said that given the circuit is done using jb's the earth should be complete for every loop in and out cable unless one has got the earth sheath trapped under the screw.

Just spotted this post. I would suggest checking your earth continuity throughout that lighting circuit, because as you suggest - the RCD should have picked that up, assuming the RCD isn't faulty. I would also check the insulation resistance of the circuit, especially between the N and E, because the current to trip the MCB, obviously went earth, then via neutral.

 

[SUNRAY]

Please - it is could have, would have.

Sadly, and I hate it too, you're out of touch; would of is now being offered in some dictionaries as an alternative to would have

 

[EFLImpudence]

Sadly, and I hate it too, you're out of touch; would of is now being offered in some dictionaries as an alternative to would have

NO!

We really are run by idiots and morons.