Link between one light fitting and socket circuit?

[uksparks]

Hi,

I changed a CU this morning, old 3036 for split board.

I had tested it prior, IR etc, few Zs readings and the like, continuity of rings etc, nothing abnormal, even checked for borrowed neutral on light circuit which there was.

Anyway, changed it this morning, put lights on one rcd and the sockets on another.

Tested everything, R1+R2, IR etc on all circuits and other tests, all absolutely fine, tested every point on each circuit, all readings all ok.

Turn it on after completing all tests...

Everything seems ok, thats good then, walk round the house, switch lights on upstairs so see if they all work as expected, yep, all fine, all fine downstairs, but when it comes to the hall light (not stair light) i turn it on and it trips both RCD's? hmm thats odd. Take the bulb out and its fine.

So i put my meter on it on line neutral and earth, 230 L-N, 230 L-E, 0 N-E, ok so I would imagine the bulb would work.., put it in switch it on again and it trips both.

Anyway, I narrowed it down, it appears there is a link between the socket circuit and that light fitting.

That light fitting is definately on the light circuit.

I even then tested N-E from the socket circuit, so see if theres any reading (dead) at the light, nothing, I tried all combinations, I get zero voltage at the light fitting when the light is off and also when the light MCB is off.

So somewhere, and for some reason that particular light is connected, but cannot seem to do any tests that suggest it is.

Unless I have missed the obvious.

Anyway, so resolve it until monday, I just moved the lights over to the same RCD as the sockets, then on monday will look at it as have got to pull the floor up to add more sockets in anyway.

I was happy to a degree that it was safe to leave for the weekend as I could not get however much I tied any voltage on that light fitting when the lights were off at the CU.

The point to this I guess is, if I had put them both on the same RCD to start with which I may have done, it was just chance I didn't, then I would never have found this problem.

Any thoughts? I can only see that it is connected on the neutral, but can't understand why as when you disconnect the socket circuit at the board, the light still works fine...

Bit of a weird one, I have not had this happen before.

 

[telectrix]

my guess would be that someone has fitted an outside light, possibly from hall light switch, and found that there was no neutral at the switch, so pinched one from the easiest point, maybe a socket.

 

[uksparks]

There are no outside lights, and only one socket in each room upstairs and minimal downstairs, none at all in the hall way.

I just cannot understand it, but the main thing that is different about that light, there is two cables in the switch, a twin and earth, one of which is cut off and only one conductor being used, and the other is a single, these are both red, and the one thats cut is red. And at the light fitting, there's only one cable, black and red...

Above the ceiling there will be a joint at a guess.

Its just chance that I even have this problem as I may not have ever known if I had put them both on the same RCD to start with, I can't really think of a way to get around avoiding this as I have tried everything I can think off.

 

[telectrix]

maybe trhat dissed wire was L feed to some wall lights, since removed, but their N came from another circuit and is still connected?

 

[Leesparkykent]

Have you had all the socket fronts off to have a look?

 

[Octopus]

I did a CU change recently and discovered that the upstairs lighting circuit was connected to the immersion circuit - literally connected via 3 cores! Fortunately on the "load" side of the (redundant) FCU!

Very odd!

 

[uksparks]

yes I have looked at the sockets in the front of the house up and down which I guess would be the obvious place.

Im going back next week anyway so I am sure ill find it.

Thanks.

 

[davesparks]

I love a good game of guess the fault!
My vote goes to borrowed neutral from the socket circuit.

 

[Octopus]

If its in the hall then look "above" in all the obvious places inc the airing cupboard, because thats almost certainly off the socket circuit.

 

[uksparks]

That's my guess, I'll update you when I get to the bottom of it.

 

[uksparks]

If its in the hall then look "above" in all the obvious places inc the airing cupboard, because thats almost certainly off the socket circuit.

The airing cupboard just has one outlet for the boiler on a radial, most annoyingly the rooms all have laminate down and are full of furtniture, it's going to be fun!

The biggest concern Is that I could have never found it if on the same rcd, so would I be at fault? I'd say probably not as I follow dead testing to a t and do the whole job and still didn't pick this up.

 

[HappyHippyDad]

It does sound very odd! If it was a borrowed neutral with that socket circuit then why does the light still work when that socket circuit is disconnected at the CU? Sorry to state the obvious Tom, but when you say disconnected, you did mean the neutral as well?

Perhaps that light is connected to both the neutral of that lighting circuit AND the socket circuit, hence the tripping, but working once the sockets are disconnected!!

You MUST update us Tom.. I cant stand it when we dont get to hear the end of the story. :smile5:

 

[Leesparkykent]

I would drop one of the legs of the ring out at the board and split the legs on a socket at roughly midway point and repeat the process on the sockets until I found what leg the borrowed neutral is connected to.

 

[telectrix]

I would drop one of the legs of the ring out at the board and split the legs on a socket at roughly midway point and repeat the process on the sockets until I found what leg the borrowed neutral is connected to.

far to sensible for a friday evening. STOP IT!.

 

[HappyHippyDad]

I would drop one of the legs of the ring out at the board and split the legs on a socket at roughly midway point and repeat the process on the sockets until I found what leg the borrowed neutral is connected to.

But why does the light still work when the socket circuit is diconnected from the CU?

 

[7029 dave]

If the circuit feeding the sockets has been disconnected at cu, and the light is still working it can't be linked.

 

[davesparks]

Did you try it with all of the other lights switched off and just the hall light switched on?

 

[Leesparkykent]

But why does the light still work when the socket circuit is diconnected from the CU?

I must of missed that, where does it say that?

 

[Octopus]

More to the point why were you changing a CU when you should have been in church?

 

[davesparks]

More to the point why were you changing a CU when you should have been in church?

In church? I doubt very much he'd have a clue where to start on an MICC installation. I'm not sure you'd be able to work in a church today anyway, isn't there some sort of religious nonsense going on this weekend?