Is a 2 bedrooms flat.i don t think he did this kind of work.
The bathroom heater is working fine.I ll check it again tomorrow to see if is not connected somehow with the lights.
hopefully tomorrow i will manage to find the fault.
I ll ask once again- can i connect neutrall from the another working light to see if is the bathroom lights are working with another neutral?
If the light is from another circuit then NO! as it would not comply with Reg 314.4 page 39 of BS7671!.
If the light is from the same circuit.. yes you could... but IMHO if the fault is on the "circuit" just taking the neutral from another point on the same 'faulty' circuit neutral.. will make very little difference to your symptoms!?
If the original circuit was wired incorrectly, (e.g. borrowed neutrals as already mentioned by others), then need to trace & rectify the problem.. not just pick a random light to guess at getting it working.
That said.. if you think you do have a faulty circuit I would be going back to Dead tests, Not random live tests.
General observations...
I have read down the thread, (apologies if i have missed it but), I have NOT seen any mention of any meter readings or tests carried out?
Which is IMHO the first things that should have been done..
1/ You said you changed the RCD, why?
was it proved faulty...?
i.e. what were your results for the trip tests when following the guidance in chapter 11 on site guide, page 91 onward,
(paragraph 11.1 test as near to RCD as possible with all loads disconnected).
What are the trip times you got for the original & replacement RCD's
2/ If you have proved the RCD was within spec, what are your test readings for the "suspect light circuit"
e.g. Disconnect ALL conductors for the circuit from CU,
I would then start with a 250v insulation resistance tests..
L-E
N-E
L-N
Also I would test each circuit conductor L,N & E
back to the N and E bars of the CU in turn.
a good circuit with all conductors disconnected should be completely open circuit back the the CU, no electrical path, high resistance!
Any low readings would point you toward the cables/conductors you need to start investigating.
HTH
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