Lighting Fault - mystery 3 core

[davidjtown]

I am a relative newbie so excuse me if this might be obvious...

I am trying to find a fault with the lighting in one room. Apparently, the light was very dim before the client had disconnected it. There is a 3 core in the light switch which when tested supplies about 150V. There isn't another switch so apparently, the 3 core isn't from a 2 way. The other two twin and earth cables supply the wall lights and pendant. There is no other feed. I lifted the floorboards above and there is a 3 plate choc block but no power when tested. The client does have a problem with his doorbell not working too. No MCBs have tripped. They have had other builders in doing electrics before and they did a runner!!

See photos of the dimmer switch in question. Any advice greatly appreciated.

 

[UNG]

Your post doesn't tell us where the other end of the 3+e cable is terminated.
Have you checked the light fittings and more importantly have any of the light fittings been replaced recently
Sounds like you might have a couple fittings wired in series

 

[davidjtown]

Your post doesn't tell us where the other end of the 3+e cable is terminated.
Have you checked the light fittings and more importantly have any of the light fittings been replaced recently
Sounds like you might have a couple fittings wired in series

Sorry, forgot to say. There is a 3 core at the choc block above the pendant, but no power present there. I just find it strange that there doesn't seem to be a 240V feed anywhere. The 150V at the 3 core in the switch seemed odd, especially as there wasn't another 2 way switch anywhere. ?‍ The light fittings haven't been changed recently but builders have been putting a loft conversion in above.

 

[Baddegg]

What are you measuring across/to and what with mate?

 

[davidjtown]

What are you measuring across/to and what with mate?

I was trying to determine where the feed was and the only voltage present was the 150V across the 3 core. I was using the Fluke T5-1000.

 

[Vortigern]

There are a few more wires than the three core though. It looks like someone had a go at fitting a nice new switch and got lost with the wires. The idea of some series wiring sounds plausible and the voltage getting divided. So you will have to trace all the wires and identify them.

 

[davidjtown]

Yes, I think that is very plausible. The owner had been playing around with them. I will investigate further. Any other tips on this fault finding issue would be very much appreciated as this is a new one on me. Thanks for the replies so far ?

 

[DPG]

I was trying to determine where the feed was and the only voltage present was the 150V across the 3 core. I was using the Fluke T5-1000.

'150V across the 3 core' is a bit vague. As per badhead's post, what voltages have you got between which cores/points.

 

[davidjtown]

'150V across the 3 core' is a bit vague. As per badhead's post, what voltages have you got between which cores/points.

150V across the yellow and blue on the 3 core and nothing at all anywhere else.

 

[Baddegg]

Some better pics would be good mate, I can see the 3 core, a black and brown core that seems to come down from the top which would indicate either 2 other cables or another (newer) 3 core with only 2 cores used, although the size look wrong, and on the left coming up from the bottom appears to be another live on its own, which could be the same cable as the black core at the top but again size look different and it coming from a different direction.... It’s a bit hard to tell what is going on mate, I think I’d safely and correctly identify the feed in and fed out at the light and isolate them then work out which is the switch drop and start a fresh from there, but may become clearer with better pics ?

 

[Baddegg]

Just out of interest what were the builders doing to the electrics??

 

[davidjtown]

Some better pics would be good mate, I can see the 3 core, a black and brown core that seems to come down from the top which would indicate either 2 other cables or another (newer) 3 core with only 2 cores used, although the size look wrong, and on the left coming up from the bottom appears to be another live on its own, which could be the same cable as the black core at the top but again size look different and it coming from a different direction.... It’s a bit hard to tell what is going on mate, I think I’d safely and correctly identify the feed in and fed out at the light and isolate them then work out which is the switch drop and start a fresh from there, but may become clearer with better pics ?

Ok, thanks for that. Here are some other pics. When I turned up the client had already removed the yellow from the dimmer. The first image here with just the yellow disconnected is how it was.

 

[davidjtown]

Ok, thanks for that. Here are some other pics. When I turned up the client had already removed the yellow from the dimmer. The first image here with just the yellow disconnected is how it was.

They had been doing a loft conversion above, including complete first fix but not connected up the CU. Client said the light started acting funny after they had done a runner. All sounds like a very odd story to me but just need to get to the bottom of this fault find. I thought maybe the builder had damaged some cable and then realised but who knows ?‍

 

[Baddegg]

Ok so we can see the neutral to the red core on the left now, but you still a black and brown coming down from the top....is that a newer 3 core that someone has used a t&e and not sleeved?

 

[Baddegg]

They had been doing a loft conversion above, including complete first fix but not connected up the CU. Client said the light started acting funny after they had done a runner. All sounds like a very odd story to me but just need to get to the bottom of this fault find. I thought maybe the builder had damaged some cable and then realised but who knows ?‍

I’d say without doubt they’ve done something ?.....be careful here mate as once you start working on it you are assuming responsibility for the installation, and homeowners also won’t buy it that although yes it worked before it’s not my fault it’s not now ?.....and what led you to this light after fault finding what bob and his mates did??