Help needed again pls - Lighting fault tracing

[AJB Temple]

I need your help please once again in tracing a fault. Am DIY but not clueless. Converting former kitchen into 2 rooms: bathroom and laundry. The room had 24 cheapo ceiling spots, originally tungsten then 240v GU10 LEDs that I am replacing. Circuit spilt into two halves with two gang wall switch controlling (this will be in the laundry side). There is also a redundant smoke detector wired in to the same circuit.

I pulled down the spots in what will be bathroom in order to repair ceiling, knowing I will be replacing with good quality IP65 (astro). This should not be notifiable work as it is just replacement of existing with upgraded fittings. Taking the spots down (not brutally!) has caused the lights operated from one side of the two gang switch to trip the MCB with a flash. Neutrals are present in switch box and all joined together (unused) as expected.

MCB does not trip if I remove the wires from the switch on what will be the bathroom side, so the laundry side has no fault and MCB must be OK, but pops instantly if the other circuit is put back in. RCD does not trip at all in Hager CU (which happens to be in the laundry room).

Can’t get access to the floor above the joists, and it’s difficult to see visually how the spots have been wired as they span several joists. All spots on the bathroom side have now been removed and temp wagos fitted. BTW I'm removing some of the lights permanently as well. They are definitely not daisy chained and from gentle tugging I would guess there are at least two junction boxes up there somewhere and one of the light fittings was also used as a mains junction by the look of it as there are two white flex T&E ends going to this fitting point. I expect one junction box must be near where the smoke detector was fitted.

Not had chance to do any tests yet, but would appreciate some pointers as to what to look for that would cause the instant trip and flash of the MCB. It's presumably a short circuit somewhere as I can’t see any faults at the spots, nor any evidence of overheating at the spot terminations. I have suitable Megger MFT.

When I find the fault and fix it, the plan is to replace the switch with a 2 gang Quinetic with built in receiver, so that I can pair the bathroom side with wireless switches in bathroom and bedroom, as it is no use having the bathroom switch in the laundry room.

Thanks, AJ

 

[westward10]

I presume you were removing these lights without the power isolated.

 

[timhoward]

Normally neutrals all joined at switch suggests power to the switch, and then from there to the lights, with the N's all joined together.

Instead of repeatedly tripping the MCB I'd turn the power off, then label, separate and safely terminate the switch wiring.
I'd then test what I suspect to be the feed to the faulty lights from the switch. Assuming a fault will show itself, I'd then check terminations on every light. If the issue is found, great. Otherwise you are into trying to find and pull joint boxes through downlight holes or finding a way to isolate the faulty section and rewiring the lighting.

(Ultimately if you can find the cable from the switch or a different permanent-live cable, and find fault-free cables spanning the joists, then you can very likely replace the wiring for the remainder. )

 

[AJB Temple]

I presume you were removing these lights without the power isolated.

No. Power fully isolated and dead tested using Megger TPT 420 (not the MFT). I intended to keep using the lights for the time being as it is a working area and it protects the new ones. Lights just pulled down to release the clips from the ceiling. This triggered the fault when I tried to switch the lights on again. So I then went on to remove the lights fully with circuit again dead tested.

 

[westward10]

Sounds like something has shorted out when you took the lights down this could easily be the joint boxes you were pulling on.

 

[AJB Temple]

Thanks Tim. That's really helpful. I always habitually safely terminate everything with wagos anyway as soon as I disconnect and I have done that in the switch box and at the wires in the ceiling. I think I confused myself a bit as there seems to be T&E that is not from the lighting switch into the ceiling, as the former smoke detector is up there and at least one of the lights appears to be wired in to that JB. If I can find where the supply comes in to the ceiling I may just follow the suggestion of rewiring the whole lot, as I could get into the ceiling from underneath enough to bridge the joists, without creating too much repair work.

I have spare MCBs in the CU so at a push I can just put a complete new circuit in though would prefer not to as it is very awkward due to structural beams.

 

[AJB Temple]

Sounds like something has shorted out when you took the lights down this could easily be the joint boxes you were pulling on.

Thanks. Yes, that is what I figured. If only I knew where the JBs are!

 

[SJD]

Thanks. Yes, that is what I figured. If only I knew where the JBs are!

So was this room originally wired with 2 light fittings, one on each switch, then converted at some point to downlights? If so, the junction boxes typically might be close to the orginal 2 lighting points. You can often see evidence of old lights in the plasterwork, assuming the ceiling was not skimmed at the time.

 

[AJB Temple]

So was this room originally wired with 2 light fittings, one on each switch, then converted at some point to downlights? If so, the junction boxes typically might be close to the orginal 2 lighting points. You can often see evidence of old lights in the plasterwork, assuming the ceiling was not skimmed at the time.

Hmm. Don't think so. It was a barn conversion back in the 80's (long before we bought it) and although I can see evidence of the old kitchen layout being altered / refurbed I can't see any evidence of a ceiling rose point, especially on the laundry side. However, you do make a good point as there was at one time a pulley light fitting over a table and that is on the same segment between joists as the smoke detector.

The last owners did a lot of DIY electrical quackery with great aptitude. They showed amazing expertise at twisting wires together to form a connection and protecting them with insulation tape. It was not necessary to attach CPCs to metal boxes, and on some sockets it was perfectly OK just to shove wires in without expending unnecessary effort tightening up screws. Orange cable used for garden tools can in fact be used anywhere in house wiring apparently.

 

[timhoward]

I think I confused myself a bit as there seems to be T&E that is not from the lighting switch into the ceiling, as the former smoke detector is up there and at least one of the lights appears to be wired in to that JB.

The smoke detector is actually a clue as that will have a permanent (unswitched) supply.
If a light is wired into the same joint box as the former smoke detector then an assumption can be made that there's both permanent and switched lives within the box.