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1. outside lights. both L and N need to be disconnected to rule them out.
2.Insulation testing should give high readings, in the MegOhm range, not 0 as your neighbour says.
3.this is not a job for a handman with a miltimeter. it requires an electrician experienced in fault finding with the correct tester/s to investigate.
4. rewiring a "suspect" cable can cause untold damage to decor and you may still have the fault after.
Hi Telectrix

1. Is disconnecting the live wires from the dimmer switches to the outside lights sufficient? If not, what constitutes disconnecting them?
2. I think what he was showing me was voltage.
3. I agree
4. Absolutely this too. I am not thrilled at the thought of this at all. The person I'm caring for doesn't seem to mind so much as long as it fixes it and it is the only solution...but we're both not so sure. I think trunking would be the least damaging solution despite being unsightly.

Is it possible that the Immersion heater, which is on Fuse 3 of the OP photo, could have caused this, despite not tripping? The heater and pump on the same fuse are still working as normal, and the water heater sounds like it's working still (though the water isn't as hot).
 
The plot thickens, you most definitely need a professional electrician to look at this, does you friend have home emergency insurance? if so call them out.

What is the ceiling construction if its a suspended ceiling then the temporary fix is a simple matter.
Quite! I just asked his wife and she said he did use an MFT but I've just googled and I don't think he did - I think he used a Fluke T6-1000, LCD Voltage tester.

It's not a suspended ceiling. Which makes life very difficult even for diagnosis, on top of everything else.

This is a real FML moment, things are so stressful right now. But thank you all for your advice and tips so far. I know you're all just internally screaming the obvious 'get a proper electrician', I wish were as simple as that. Sorry.
 
re-read my point #3 in my last post. it's the only viable option.
 
Short answer to your post 23 "NO"

Newbie101. I think you have now exhausted the amount of help that we can give you on here, as has been said in quite few post, you need a professional electrician to look at this.
 
I know - I'm working on it. But in the meantime, could you give me a clue to number 1? Is what he did enough to 'disconnect' the outside lights, thus isolating them?
no. as you could have a N-E fault which would still be there if you only disconnect L.
 
So disconnecting all the wires within the dimmer switches for the outside lights would suffice for full disconnection? I'll try that to see if that isolates it.
not necessarily. you maynot have the neutral at the switch.
 
The instrument in the centre of my post above will not measure insulation resistance, only voltage and continuity in a limited sense, it is used by an electrician only to test if voltage is present any other tests are performed with the instrument on the right hand of the above pic, which is a MFT.
 
The instrument in the centre of my post above will not measure insulation resistance, only voltage and continuity in a limited sense, it is used by an electrician only to test if voltage is present any other tests are performed with the instrument on the right hand of the above pic, which is a MFT.
That's what I thought :(
not necessarily. you maynot have the neutral at the switch.
Interestingly, I just disconnected the two dimmer switches that are able to control the outside lights. Despite the fuses being off at the box, when I put started unscrewing the neutral in the first dimmer, the screwdriver lit up, which surprised me. But live wasn't live so I carried on unscrewing and was careful not to touch anything just in case. With both switches 'completely' disconnected, the remaining lights still don't work(!) BUT the fuse is no longer automatically tripping. Progress!? :)
 
The false indication on your screwdriver is why no electrician would ever use one, what you have proved is that with the circuit disconnected the lights don't work.

There is a need for a methodical regime of testing each part of the circuit to find the problem an electrician could probably do this in very much less time (3hrs 16mins) than it has taken to post all of these questions and answers and your lights would be back on.

Don't forget that if the problem is water ingress just replacing the cable will not prevent this happening again unless the water short circuit is also addressed, please get an electrician in to sort it for you.
 
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The false indication on your screwdriver is why no electrician would ever use one, what you have proved is that with the circuit disconnected the lights don't work.

There is a need for a methodical regime of testing each part of the circuit to find the problem an electrician could probably do this in very much less time (3hrs 16mins) than it has taken to post all of these questions and answers and your lights would be back on.

Don't forget that if the problem is water ingress just replacing the cable will not prevent this happening again unless the water short circuit is also addressed, please get an electrician in to sort it for you.
You're right. Thanks everyone for all your help. We're decamping to my flat for a few days which gives someone an opportunity to look at it properly.
 
Hope that someone is an experienced electrician.

Replacing cables can be done without major work, an experienced electrician can run new cables alongside an existing cable without taking down the ceiling, in most case's above a ceiling, especially if the aforesaid is a drop false ceiling, which is generally the case in top floor apartments.
 
Hope that someone is an experienced electrician.

Replacing cables can be done without major work, an experienced electrician can run new cables alongside an existing cable without taking down the ceiling, in most case's above a ceiling, especially if the aforesaid is a drop false ceiling, which is generally the case in top floor apartments.
This is a converted Georgian mid terrace. No drop ceiling and the wires are not housed in tubes/pipes like regulations now dictate. There is obviously some space between flat ceiling and roof, but it's small, just big enough for the housings of each ceiling light. There are no choc boxes isolating any of the lights, they all seem to be wired together.
 
Conduits are not mandatory if using T&E, most electricians have a set of push pull rods that can follow the existing cables from one light to another, it is then easy to pull another cable through the void if needed without pulling down ceilings, possibly without making too many, if any, access points through the ceiling to get around possible obstacles. This would be a simple job for an experienced electrician without too much disruption.

The only possible additional work could be the connections to the lights which may need to be changed to comply with regulations the individual lights should be connected with something like this, it is a Wago lighting box and Wago connector, ignore the gold coloured thing inside the box it's to help prevent flashing of LED's. RCD Trip - insulation problem from too much load? Imports - 1 of 1 (2) - EletriciansForums.net
 
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Late to the party, but I too believe this problem is moisture getting into the external lights, or rodent damage?

Get the handyman neighbour to fix it for a price, as he’s going to be there days replacing bits and pieces with no guarantee of success.

Also, first picture of board, I see a plug top with a short length of flex just cut off.
Remove the flex from the plug for safety.
 

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