Adding a new one-way light to a two-way switch

[hellopaul2]

Hello,
This is my current setup:

• I have some exterior lights in my garden, which can be operated by a switch in the house or the garage.
• I want to add a light inside the garage, just a normal, one light, one way thing.

Is there any way I can take a kind of spur from the existing two-way switch to power the garage light? I've just checked, and regardless of any switch position, there is about 200V between L1 (brown) and an unconnected blue wire. Is it likely that connecting the blue directly to the new light, and making a new connection from the L1 (brown) through a new switch to the light will give me what I need? Or does this all sound too dodgy to risk?

I'm using a digital multi-meter, and know that it can give "false" readings due to it detecting induced current, so those 200V readings might not be 200V with enough amperage to actually power anything. The garage is a long way (about 15 metres) from the consumer unit, so some voltage drop (and induced current) is inevitable.

I suppose I could just poke a couple of wires from a light across the blue cable and the brown L1 cable and see if it goes bang...

Alternatively, there are some mains sockets in the garage. Maybe my best bet would be to wire into those? But I know lighting and mains circuits should not be mixed.

 

[JohnW2]

Is there any way I can take a kind of spur from the existing two-way switch to power the garage light? I've just checked, and regardless of any switch position, there is about 200V between L1 (brown) and an unconnected blue wire. Is it likely that connecting the blue directly to the new light, and making a new connection from the L1 (brown) through a new switch to the light will give me what I need? Or does this all sound too dodgy to risk?

Are you saying that the blue wire just 'passes through' the switch box (from supply to outgoing cable to light)? If so, then you are very probably right in thinking that it is a neutral, and that the incoming brown going to L1 is a 'permanent live' - and do I take it that the 200V is always present between blue and L1 is always there, regardless of whether the switch is on or off? IF that were all the case, then doing as you suggest would probably work. For what it's worth, nothing disastrous would happen if you did that and it didn't work, although some here would criticise you for utilising a 'trial and error' approach!

200V is rather low, but maybe that's just your meter - what type is it?

Kind Regards, John

 

[ericmark]

I would expect more than 200 volt line to neutral so as you suspect the blue wire could be connected to nothing or earth rather than neutral. I suggest you really need to test not just connect and hope.

 

[JohnW2]

I would expect more than 200 volt line to neutral so as you suspect the blue wire could be connected to nothing or earth rather than neutral. I suggest you really need to test not just connect and hope.

That's why I asked questions. If the blue wire, and a wire (probably brown) from L2 of the switch are the only wires/conductors going to the existing lights (which I assume are working), then it has to be a neutral - in which case, as I implied, I would suspect the voltage measurement.

Kind Regards, John

 

[ericmark]

If your house is total RCD protected then trail and error method may be OK. However if not totally RCD protected then since one should tie unused wires to earth, if this has been done you could end up raising some earthed items to a higher voltage which could prove dangerous. So I would not assume.

 

[ericmark]

I agree with JohnW2 it is likely neutral where I don't agree is assuming it is neutral without investigating. Neutral is normally near to earth polarity and the 200 rather than 230 volt has alerted me to the possibility it's earth not neutral. As said if RCD protected and it is earth then the RCD will trip. But if not then specially if using an earth rod you could make things in the house live it's just not worth the chance.

Even with RCD protection not really a good idea relying on the RCD but likely if mistake is made it will trip.

 

[JohnW2]

I think that the concerns eric is raising, whilst being a theoretical possibility, represent a pretty unlikley scenario. Can you measure the voltage between blue and earth, with and without the existing lights switched on?

Kind Regards, John

 

[hellopaul2]

Thank you, everyone for your super fast and very helpful replies!

Are you saying that the blue wire just 'passes through' the switch box...
...200V is rather low, but maybe that's just your meter - what type is it?

No, the blue wire was just coiled up in there. It comes from the same cable as one of the two-way switch cables. I stripped some insulation off the end so I could test it.

My meter is a modern one with an LCD digital display.

I would expect more than 200 volt line to neutral so as you suspect the blue wire could be connected to nothing or earth rather than neutral. I suggest you really need to test not just connect and hope.

Can you measure the voltage between blue and earth, with and without the existing lights switched on?

Ah yes - I've just checked the voltage between that blue wire and earth (without lights on), and that blue wire is almost certainly earthed - it's around 0 to 5V. So that rules out using it for anything. The voltage (without lights on) between the blue wire and one of the other connections (L1 probably) is still about 200V.

So, plan B: taking some kind of spur off one of the mains sockets in the garage. Any thoughts on why I should/should not do that? I don't have the option of feeding another cable back to the main house (too many walls / patios / distance / vaulted ceilings / recently decorated rooms!).

 

[ban-all-sheds]

For what it's worth, nothing disastrous would happen if you did that and it didn't work,

Have you actually been round there, and established that the blue conductor isn't being used for something where connecting it to 230V through a lamp would be disastrous?

although some here would criticise you for utilising a 'trial and error' approach!

And rightly so - it's a stupid thing to do.

 

[JohnW2]

No, the blue wire was just coiled up in there. It comes from the same cable as one of the two-way switch cables. I stripped some insulation off the end so I could test it.

Ah - that rather changes things! How many cables enter this switch box - and can you describe what is connected to what (or, better, provide photos)?

Ah yes - I've just checked the voltage between that blue wire and earth (without lights on), and that blue wire is almost certainly earthed - it's around 0 to 5V. So that rules out using it for anything. The voltage (without lights on) between the blue wire and one of the other connections (L1 probably) is still about 200V.

That doesn't actually prove that the blue wire is 'earthed'. It doesn't even necessarily prove that it is not a neutral. Do you know where the cable comes from, and can you inspect the other end?

So, plan B: taking some kind of spur off one of the mains sockets in the garage. Any thoughts on why I should/should not do that? I don't have the option of feeding another cable back to the main house (too many walls / patios / distance / vaulted ceilings / recently decorated rooms!).

Sure, you could connected a spur via an FCU (probably with a 3A fuse) from a socket. Depending on locations etc., you may even be able to use the switch of a switched FCU as the light switch.

Kind Regards, John

 

[hellopaul2]

From a bit of research on feeding a light from a spur, I found this: //www.diynot.com/diy/threads/lighting-spur-from-a-ring-main-socket.113061/
...so if I added a 5 amp fused connector unit between a spur from the sockets and my lightswitch, I should be good to go?

 

[JohnW2]

For what it's worth, nothing disastrous would happen if you did that and it didn't work,

Have you actually been round there, and established that the blue conductor isn't being used for something where connecting it to 230V through a lamp would be disastrous?

I cannot really think of anything (credible) that it could be being used for which would result in anything disastrous happening if one connected it through a lamp to 230V. Anyway, the goalposts have now been moved.

Kind Regards, John

 

[JohnW2]

From a bit of research on feeding a light from a spur, I found this: //www.diynot.com/diy/threads/lighting-spur-from-a-ring-main-socket.113061/
...so if I added a 5 amp fused connector unit between a spur from the sockets and my lightswitch, I should be good to go?

As I just wrote, yes - although, as I said, a 3A fuse would undoubtedly be adequate. As I also said, you might even be able to use a switched FCU as the lightswitch.

Kind Regards, John

 

[Iggifer]

I see no point in having two switches either, use the FCU as the switch

 

[ban-all-sheds]

I cannot really think of anything (credible) that it could be being used for which would result in anything disastrous happening if one connected it through a lamp to 230V.

So how incredible does a scenario have to be for you to decide, on zero evidential basis, that it is safe to use an unknown conductor for a LV application?