Vexing Wiring Problem

[tipper258]

Hi,

Have had a good look round the wiki and other threads and can't see anything that looks like this problem. Am struggling trying to figure out how the light in the front room we're renovating is wired up. It all worked, and then I got the "looks like there's a loose wire in the switch" call, and indeed one was flying loose. Now trying to work backwards to a way it will all work and need a better brain than mine to figure this out!

At the switch there is a single 3-core and earth cable, colours are red, yellow and blue. Red is in C, blue in L1 and yellow in L2. When I found it, the yellow was free, and others loose. Could have been in L2, or tied into L1 with the blue but please read on.

At the ceiling rose there is a single 3-core and earth cable, colours as above. There's also the usual blue and brown 2 core cable down to the actual bulb. The red and yellow are in the same connector block as the blue to the bulb, i.e all connected switched live. The blue is on the brown connector block, i.e. the neutral side.

With the help of a meter, the red at the switch is the live power, ok so far.

With the switch set so power is applied to blue, I can detect power on the yellow at the ceiling.
With the switch set so power is applied to yellow, I can detect power on the red at the ceiling.
With both the blue and yellow disconnected at the switch, no power detected at all at the ceiling rose.

At no point does the bulb light!

My current thinking, no pun intended, is whatever is hiding in the inaccessible junction boxes in the ceiling, it seems applying power to the switched live side of the ceiling rose, never causes the bulb to light, therefore the problem must be on the neutral side of the ceiling rose back somewhere up in those junction boxes.

Is there any other possible explanation please?

Lots about this is confusing me, such as, given there is only one switch, why would 3 core and earth be used anyway, at both ends of this? Also, this all worked, so how could all of sudden there be a problem on the return side of the circuit that doesn't go near the switch, it seems, and I'd hope!

Can anyone shed some light on this for me please? (ouch - not a pun either).

Thanks
Alan

 

[ColJack]

a picture paints a thousand words my friend..

with a photo we might be better able to tell you where it's supposed to go..

it definitely sounds like there is a hidden JB somewhere...

it worries me that you think that brown is the neutral side...

 

[securespark]

With the power off and the meter set to continuity, attach a flying lead to one of the probes. Put this flying lead to the cores at the switch and the other to the cores at the rose. This will help you discover which the switchwires are.

With the power on, use the MM to check for live.

This should now have identified live and switchwires. With a live at the rose, now use the MM to find the neutral.

You have cpc's?

 

[tipper258]

Thanks folks - ColJack - sorry about the typo in the text - you're absolutely right - a pic is worth a 1000 words and it doesn't help when I get my colours wrong!

Hope these help the diagnosis.

Untitled

• tipper258
• 9 May 2009
• 1

Untitled

• tipper258
• 9 May 2009
• 1

Securespark - Will find myself a longer lead to see if I can make those direct circuit tests between the cores on the rose and switch.

Cheers

 

[ColJack]

there definitely has to be a JB somewhere...


time to start lifting floorboards..

 

[Space cat]

The red and yellow are in the same connector block as the blue to the bulb, i.e all connected switched live. The blue is on the brown connector block, i.e. the neutral side.

Your bulb is wired backwards for a start. The convention is brown for live and blue for neutral. I know it makes no difference to a light bulb but it's thoroughly bad practice and a source of confusion you, and also we, can do without. It doesn't take much effort to get two wires the right way round.

With the switch set so power is applied to blue, I can detect power on the yellow at the ceiling.

With the switch set so power is applied to yellow, I can detect power on the red at the ceiling.

These two statements strongly suggest that you are not looking at two ends of the same cable! Certainly the red in the rose cannot be the same live feed as the one in the switch unless ---

Beware of the hidden capacitor. When you use a high impedance meter on a wire that connects to nothing you can get a significant voltage reading if that wire is in the same cable as another one that's live. Try those measurements again but have the bulb connected to the wire you're checking. What do you get this time?

 

[securespark]

Is that the only switch controlling that light?

If so, I suggest that there was another that has now been removed.

Done those long-lead dead tests yet?

 

[tipper258]

Was at Wickes as soon as they opened, a few pounds lighter I was back home with a good length of cable.

Circuit tests confirmed there's a JB in there as the cores colours swapped as per above.

So having changed the switch, checked the connections, tested circuits and worked all this out, the one simple thing I hadn't done, was replace the flex that drops from the rose to the bulb, I mean, how can they fail right?

It was rotten inside, the action of removing the bulb must have been the last straw. Makes me think it's been running too hot a bulb in there over the years it must have been up there.

Swapped for a spare one and it all works. The only remaining nagging doubt was the use of the 3-core and earth and how to resolve that.

I think you're right folks, there must have been a second switch at some point which was removed. I tied the appropriate cores together at both ends and all works well. What's the preferred way of doing this, should I have left the spare core disconnected (and insulated) at both ends ot tied together?

Anyway, thanks to you all for the advice.

Cheers
Alan