Changing light fitting: electrical advice reqd, please

[Matt Barton]

Hi,

I'm changing the light fitting in my bathroom, and I've discovered that there are two mains power cables running out of the ceiling feeding the existing fitting. The two cables run into a splitter unit (small plastic affair) and the three bulbs in the light fitting are individually powered from there.

I only need one of the cables to power the new light, and I'm at a loss to know what to do with the other one. I know that the correct thing to do would be to go into the floorspace of the loft conversion upstairs and remove the spare cable from the system, but for reasons I won't go into that won't be possible for a number of months.

Would I be able to bind the spare cables wires in insulating tape and kep them where they are out of the way as a short-term measure? If not, what would be another solution?

Thanks in advance,

Matt

[Ian Stirling]

Is the light switched on by an external switch? If not, I'd guess that you'r simply seeing the lighting ring main, which should have two cables connected to the splitter unit, as it's a ring. Can you open the "splitter unit"? If so, can you simply disconnect two of the existing cables to the lights and use the third?

[Ian Stirling]

Is the light switched on by an external switch? If not, I'd guess that you'r simply seeing the lighting ring main, which should have two cables connected to the splitter unit, as it's a ring. Can you open the "splitter unit"? If so, can you simply disconnect two of the existing cables to the lights and use the third?

If not, do you have a powered ventilator of some sort? This may be the second wire, going off to this, run from the light switch.

[Steve Firth]

It's not a spare cable. It's (probably) part of the "daisy chain" arrangement of lights in your house, disconencting it will stop some (or all) of the lights on that floor of your house from working.

You need to wire the new light int he same way with a loop in, loop out and the connection to your new fitting switched with the third wire.

[Dave Plowman (News)]

Lighting circuits are usually a spur, not a ring.

[Matt Barton]

The lightswitch is a pullstring affair on the ceiling... by external do you mean external to the light fitting (in which case: yes) or external to the bathroom (in which case: no)

There's nothing to open, so I've got full access. Sorry: I'm a total novice and don't know the proper name for it, but it's v common kit: a translucent plastic strip of input and output terminals where each terminal is sunken into the plastic and has a screw to secure the wires...

If what I am seeing is the ring main, then yes I probably could. If thats the case I owe you a pint, and feel a little sheepish for not thinking of it myself.

No, the only other electrical stuff in the room is the combi boiler (which has it's own dedicated supply from the consumer unit) and the shower. Pretty sure the shower is on a different circuit from the CU than the lights though...

M

[Dave Plowman (News)]

If there are two cables going to the fitting, normally one of them is from the switch. If the fitting has its own built in switch (unlikely) the second cable would be the feed to the next fitting on that lighting circuit.

If you don't need that next fitting (or fittings) you can simply disconnect the cable and forget it - it will not be 'live'.

However, you need to be sure what you're doing with electricity, so perhaps some reading up is in order?

[Matt Barton]

The fitting does not have it's own built-in switch...

Given that the fitting doesn't have its own switch, and there aren't any more fittings controlled by the same switch, then I can't forget about it can I? If I'm following your reasoning correctly...

Mmm, absolutely.

In the absence of a textbook dedicated to my house, could you recommend anything? Everything I have read seems to work from a best-case scenario...in that all the advice they teach is great, but largely useless in practice...

[RichardS]

might give you some idea as to what is going on.

The thing that you will be missing (from the sounds of it) is the third cable that takes the power to the next light on the lighting circuit. All the other light fittings on that floor shoudl have a full complement of the three cables.

Basic idea is that you have a permanently live feed from the lighting circuit that goes to the fitting, but the live is connected via the switch, so a single cable is utilised to take the live feed to the switch, and return the switched live via the other conductor. I believe that the black conductor in the switch cable ought to have a red sheath to identify it as a switched live rather than neutral, but in practise I have never seen this in any house that I have lived in.

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[Matt Barton]

This is the part where theory and practice start to break down. If the set-up was missing the third cable to take the power to the next light on the circuit then I'd already have a problem woudn't I? Ignoring any changing-the-light-fitting issues, the current fitting is set-up like this, so if it was missing a cable to the next fitting then that fitting wouldn't be working... or have I got the wrong end of the stick?

Sorry I'f I'm being a bit dense, I'm just trying to save myself the cost of getting a sparky in without blowing myself up...

Matt

[Ian Stirling]

Usually, yes, which I should have mentioned.

[RichardS]

well, yes, what you say is true. However, for each lighting circuit there must be one fitting which is on the end of the chain, and doesn't supply another one. My guess is that this is the one...

a Digital Multimeter is a good place to start if you're trying to investigate what wire does what. The switch cable is the easiest to detect, because it will have continuity across the conductors when the switch is in one position, and open circuit in the other. (DON'T TEST THIS WITH THE LIGHTING CIRCUIT LIVE!!!!)

One essential consumable to have in your arsenal when doing these minor works is a roll of the green/yellow sleeving for the earth conductors (CPCs, or Circuit Protective Conductors). All exposed CPCs must be sheathed with this, and it's just a good mindset of good practice to get into (and all too often ommitted).

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk

[Lurch]

On Mon, 28 Jun 2004 10:26:30 +0100, "Dave Plowman (News)" strung together this:

ITYM radial. ;-)

[Dave Plowman (News)]

IDI.

[Dave Plowman (News)]

Right. It's common to make the connection from the feed to the switch at the ceiling rose, which provides terminals for doing so. But people often change light fittings, and remove the ceiling rose. And how they then make the connection to and from the switch varies rather a deal.

Just to try and clarify, each lighting point would normally have three cables - the mains feed in, the feed looped to the next fitting in the chain, and one to the switch. The one at the end of the chain would only have two cables, and bathrooms are often the last.

Out of your two cables, are you sure one of them doesn't go to the switch?

Most DIY books cover the basics. And, indeed, many fittings from a DIY store include a wiring diagram. Or get one of those 'how to' leaflets from your DIY store.

[Matt Barton]

No, I'm not, however I suspect that this is probably the case. How would I determine this for definate?

Thanks - the links that have been supplied on here, combined with my DIY book have helped me understand how most ceiling light fittings are arranged, it's just a matter of marrying that theory with what I've got coming out of the ceiling at home.... :)

[Dave Plowman (News)]

There are many ways of doing this, but the simplest would be with a continuity tester. A half decent DVM (digital volt meter) costing only a few pounds will have one. They produce a buzzing noise on a short circuit.

With the power switched off at the main switch, connect the DVM across the two wires going to the switch. Operate the switch. It will buzz when on, and not when off.

[Owain]

"Dave Plowman (News)" wrote | Just to try and clarify, each lighting point would normally | have three cables - the mains feed in, the feed looped to | the next fitting in the chain, and one to the switch. The one | at the end of the chain would only have two cables, and | bathrooms are often the last. | Out of your two cables, are you sure one of them doesn't go to | the switch?

If it's an end-of-radial light fitting, as you say, there should be 2 cables (feed and switch, loop-in wiring), or one cable (switched feed only, joint-box or loop-at-switch wiring).

If it's end-of-radial with switched feed only wiring, there should only be 1 cable to the fitting, unless the switch controls more than one light fitting. This would probably be noticed by the OP, but, if it's a bathroom, the additional cable could be a switched live feed on to an extractor fan or a shaver socket.

If the OP doesn't use an electric shaver he might not notice if his shaver socket stops working.

Owain

[Matt Barton]

Such as this:

?If not, would it be possible to recommend one? Sorry I'm such a PITA, I'm in unfamiliar waters here.

With the lighting circuit switched off at the CU, I take it?

[RichardS]

> ?

Not entirely sure from the sparse spec whether it does continuity or not.

I'd probably go for something like their product code N49AB - that definitely has continuity checking, still only a tenner.

Yes. Absolutely. Definitely. And check that the voltage between live and neutral is zero on both cables before you touch it as well.

I think that there is a safety procedure that you ought to go through when checking that a circuit is in face isolated, and it goes something like:

switch off the circuit, check the light doesn't operate. remove covers and fittings so that the connections can be reached, don't disconnect the wires though. Check voltage is zero. Switch circuit back on again, VERY CAREFULLY test the voltage again, check that you can get a 240 (or thereabouts) reading (making sure that the DVM is on the appropriate AC voltage scale!!!), now switch circuit off again and make sure the reading is zero. I'm sure someone will correct me if this procedure is wrong.

Rationale is that you need to know that you have truly measured a zero voltage and it isn't just that your meter isn't working....

-- Richard Sampson

email me at richard at olifant d-ot co do-t uk