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Yep, I agree with Will, the reason I asked if low voltage or mains voltage was because if they had been low voltage, the transformer would have come into play and may have been faulty, but as they are mains this does not apply.
How old is your wiring?

It's the RCD tripping, as transformers are generally class 11 how are they going to trip the RCD?
 
Its been a fairly wet week across Britain so damp on that part of the lighting circuit might be an issue. Seeing as you have changed the switch and lamps its pointing towards the wiring instead. Basic Insulation Resistance testing would find the fault but remember to disconnect your brand new accessories. This is simple fault finding for any decent Electrician, call one.
Why disconnect the accessories?
If the RCD is tripping there is no point in L-N tests, therefore no point in disconnecting accessories and potentially removing causes of the tripping during testing resulting in wrong conclusions.
 
It's the RCD tripping, as transformers are generally class 11 how are they going to trip the RCD?
I agree under normal circumstances a class II device should not trip an RCD but we have a fault situation and possible dampness, if a transformer is sitting in a puddle of water in the ceiling void would this not cause the RCD to trip?
As the OP had changed the lights and the fault still existed, I was thinking what else could there be, it has been eliminated anyway as they are mains powered.
 
Why disconnect the accessories?
If the RCD is tripping there is no point in L-N tests, therefore no point in disconnecting accessories and potentially removing causes of the tripping during testing resulting in wrong conclusions.

The point would be to separate L and N so that you can establish whether the fault is L-E or N-E, which may (or may not) assist in the fault finding process.
 
I agree under normal circumstances a class II device should not trip an RCD but we have a fault situation and possible dampness, if a transformer is sitting in a puddle of water in the ceiling void would this not cause the RCD to trip?
As the OP had changed the lights and the fault still existed, I was thinking what else could there be, it has been eliminated anyway as they are mains powered.

It would hopefully cause something to trip if it's sitting in a puddle of water but then you'd expect the ceiling to be noticeably wet and discoloured. This wouldn't be unique to transformers though as that much water would likely have got in to joint boxes etc.
Also I wouldn't expect connections sitting in water to present such a delayed tripping action.
 
Why disconnect the accessories?
If the RCD is tripping there is no point in L-N tests, therefore no point in disconnecting accessories and potentially removing causes of the tripping during testing resulting in wrong conclusions.

OP said he had already swapped the dimmer and lamps and the fault is still occurring. He has then ruled out that the aceesories are not at fault. He should remove them anyway to properly carry out insulation resistance testing.
 
It would hopefully cause something to trip if it's sitting in a puddle of water but then you'd expect the ceiling to be noticeably wet and discoloured. This wouldn't be unique to transformers though as that much water would likely have got in to joint boxes etc.
Also I wouldn't expect connections sitting in water to present such a delayed tripping action.
Not quite sure what I have done to deserve the tone of response on this one, fault finding remotely is always difficult at the best of times!
Maybe my statement of a puddle was a bit OTT, but I was trying to make a point. I have been in this game for 40 years and have not failed to find a fault yet!
 
Not quite sure what I have done to deserve the tone of response on this one, fault finding remotely is always difficult at the best of times!
Maybe my statement of a puddle was a bit OTT, but I was trying to make a point. I have been in this game for 40 years and have not failed to find a fault yet!
If you've been in the game 40 years a bit of confrontation/ banter should be water of a ducks back. I don't think it was meant in the way you have read it
 
What is going to cause this carbon build up, and where's the carbon coming from?
It's an analogy to burnt cooking , degraded insulation becoming heat sensitive .
Well cooked , brittle , as it warms some of flexibilty is removed , so may be suffering compression . The main effect , insulation failure begins to make its own
heat , causing more degradation, in the same place ,like burnt dinner .
( Chemicals found in damaged PVC / plastics probably more worring to mention)
Regs about low smoke cable , etc ,are for special cases .
 
The point would be to separate L and N so that you can establish whether the fault is L-E or N-E, which may (or may not) assist in the fault finding process.

Quite frankly it wouldn't make any difference whatsoever to the process of finding a fault. Disconnecting equipment for no reason will ultimately prolong the process and remove possible causes of the fault leading to false conclusions.
 
Quite frankly it wouldn't make any difference whatsoever to the process of finding a fault. Disconnecting equipment for no reason will ultimately prolong the process and remove possible causes of the fault leading to false conclusions.

I don't see how you can say that categorically without seeing the job and knowing what the fault is.
 
ok ... had planned to get an electrician in, but hampered by Sports day ... and my electrician does not do weekends. So I decided to have another look. Turns out it is a N-E fault. Opened up the switch and managed to find the N wire that is causing the problem. Turns out that it is the N wire coming into the switch, shorting with a few E wires. Hope my electrician can find the fault quickly and fix it.
 
No ... Not connected into the switch (only live wires connected to the actual switch) ... But located in the switch box. There are a lot of N wires screwed together using a terminal block. Identified the N connected to the lights and the N coming into the terminal block. The N connected to the lights are ok ... No N-E fault ... But the N coming in has a short to some of the E wires.
 

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