Wiring switch question

alan4cult

Hi

I'm trying to wire a one way switch and in the back box there are two wires, both brown. Obviously one is live and the other is just the connection to the light fitment. How do I know which one is the live one?

The reason I need to know this is I'm wiring a special type of switch that has an inbuilt circuit board and I think the polarity may matter?

Thanks.

KoolKid

Easiest way is to use a phase tester . Holding your thumb on the top carefullt tip the terminal off each wire. When it touches the live it will light up.

alan4cult

koolkid wrote: »

Easiest way is to use a phase tester . Holding your thumb on the top carefullt tip the terminal off each wire. When it touches the live it will light up.

Thanks,

Is this a phase tester?
http://www.handyhardware.ie/product/15925/Faithfull-Mainstester-Screwdriver-Walleted

KoolKid

alan4cult wrote: »

Thanks,

Is this a phase tester?
http://www.handyhardware.ie/product/15925/Faithfull-Mainstester-Screwdriver-Walleted

It is.
Remember safety first. You are dealing with live mains voltage.

alan4cult

koolkid wrote: »

It is.
Remember safety first. You are dealing with live mains voltage.

Ok thanks very much for your help!

alan4cult

Just another question.

If I get one of those voltage testers with the two probes, what do I connect the other probe to if I don't have an Earth wire?

Say this one http://www.maplin.co.uk/Module.aspx?ModuleNo=30569

maxamillius

of the switch is on a metal back box hold it against that!

alan4cult

maxamillius wrote: »

of the switch is on a metal back box hold it against that!

What?

alan4cult

Sorry I read your post again, didn't make sense. You mean "if" instead of "of" at the start of your sentence I guess. Is it safe to hold it against the metal back box even though it's not earthed?

KoolKid

He means the metal box will be earthed. If you meter live to earth you will get a reading of 220vac

alan4cult

koolkid wrote: »

He means the metal box will be earthed. If you meter live to earth you will get a reading of 220vac

How is the metal box earthed. Is it connected to a green/yellow wire thats hidden behind it, or is it just earthed because it's metal?

KoolKid

If its a metal box it should be earthed . There should be an earth wire running into the back box & connected via a terminal near one corner.

alan4cult

koolkid wrote: »

If its a metal box it should be earthed . There should be an earth wire running into the back box & connected via a terminal near one corner.

Ok I'll check for that, really appreciate your help. Electricity baffles me sometimes but just wanna play safe as you said

Buford T Justice

If the box isn't earthed, then best bet is to go with the phase tester.

maxamillius

sorry guys ye meant if instead of "of" ha,agree with fingers,if the box is not earthed go with a phase tester,although i dont like them very much prefer the fluke stick!

M cebee

ya the fluke sticks are indispensible -

.G.

Although not infallible.

Had one tell me once something was dead,it wasn't.

Hence I always check with a meter now too

ted1

i was just going to saw never trust a phase tester. A meter is the only way to go .

KoolKid

ted1 wrote: »

i was just going to saw never trust a phase tester. A meter is the only way to go .

Unless its a live switch wire with no earth.

2011

superg wrote: »

Although not infallible.

Had one tell me once something was dead,it wasn't.

Hence I always check with a meter now too

I would have to agree. I was given my money back on 3 of them because they failed. I have heard of many others that have had issue with them.

Unless its a live switch wire with no earth

My answer to that is if you want to be sure then get an earth! It is not that hard to do. All you need to do is run a temporary cable from the nearest known earth point.

I accept Koolkid's point that it may be enough so that you can tell which wire is live for connecting the switch. But if I used it for that purpose (for whatever reason) I would treat both cables as live because I don't trust this product.

Bruthal

2011 wrote: »

I would have to agree. I was given my money back on 3 of them because they failed. I have heard of many others that have had issue with them.



My answer to that is if you want to be sure then get an earth! It is not that hard to do. All you need to do is run a temporary cable from the nearest known earth point.

I accept Koolkid's point that it may be enough so that you can tell which wire is live for connecting the switch. But if I used it for that purpose (for whatever reason) I would treat both cables as live because I don't trust this product.

Multimeters are not perfect either, they can easilly show 230v at a testing point if there is a very high resistance in the circuit, but we would`t get 230v across us if we now complete the circuit as our impedence is far lower than a multi meter. But at least they are unlikely to say zero volts if there is 230v there.

Also if one probe is touched off each of 2 wires at a switch to see which is live and the other is held in hand, it will show which is live, -i can just see everyone saying is he mental, but the digital multimeter is far higher impedence than a phase tester, as long as its not connected in the Amps setting though. Not that id highly recommend that though.
The best tester i had was one they used to use in ESB, it had a magneto coil in it so was a lot more positive a reading than an electronic type voltage tester. You could hear it sort of buzzing
as well so it was a good indicator. I use a unitest volt tick one now, seems quite reliable, it beeps slowly and speeds right up when close to live sources

Bruthal

koolkid wrote: »

He means the metal box will be earthed. If you meter live to earth you will get a reading of 220vac

Even if the box is not earthed you would still know which wire is live if test between unearthed metal box and live wires, especially if the metal box is screwed to the brick. The impedence of the digital multimeters is so high they will give a reading between live and an unearthed metal box, or even the probe onto a wall, It wont read 230v but will be above zero. So if just using multimeter to identify which wires are live at a switch its not required to go running temporary earths to the testing position. But a metal box should be earthed alright.

2011

Robbie to be fair it depends on what the switch is mounted to. It may be a patrician wall or the house might be timber frame.

If you don't want to take chances use a quality multimeter, test the meter on a known live source and then test the cables with a proper earth.
That is my advice, each to their own.

Bruthal

2011 wrote: »

Robbie to be fair it depends on what the switch is mounted to. It may be a patrician wall or the house might be timber frame.

If you don't want to take chances use a quality multimeter, test the meter on a known live source and then test the cables with a proper earth.
That is my advice, each to their own.

Yes but try a digital multimeter with one leg onto a live then onto a non live wire, with the other free in the air, it can be seen which is the live, no one is taking any chances, if its a switch we are talking about and finding which is the live feed, then no tester should be needed at all, let alone running a temp earth as a test point. I was never suggesting doing that on testing mcb boards or mains panels.

I would`t think we would have to test the multi meter on a known source to find out which of two wires at a switch is the live feed, maybe if we are going to handle bus bars then yes, but hardly to identify a switch feed. I could be wrong again as usual though.

2011

I would`t think we would have to test the multi meter on a known source to find out which of two wires at a switch is the live

Robbie the OP is not a DIY person, not an electrician. Without proper training like you have my advice stands

Bruthal

2011 wrote: »

Robbie the OP is not a DIY person, not an electrician. Without proper training like you have my advice stands

Well if he is not a diy person then advising to run an earth from a known earth point might not be as simple as you said, its simple for you alright, its a 2 wire switch, and will work which ever way its connected more than likely. And its not a safety test on the wires before handling, its to find which is live, so a phase tester is the simplest way.
I take your point though.

Bruthal

Whats the position with the bathroom light now,, its going on the RCD?
Is it the socket RCD they going on?

Wonderwall

I think it has to be put onto its own RCBO. Sure if it was on the socket RCD; if something went wrong with the light then the sockets would go off or vice versa.

Could be wrong though.

Bruthal

Wonderwall wrote: »

I think it has to be put onto its own RCBO. Sure if it was on the socket RCD; if something went wrong with the light then the sockets would go off or vice versa.

Could be wrong though.

No i`d say your right, i was more wondering what the actual rule is, id say its seperate RCBO, thats what i`d do anyway but was just curious what the actual requirement was, although the immersion is on sockets RCD and that is far more likely to cause trips than a light.

Its getting to the stage where everything will be on RCBO`s, and emergency lights etc.

Wonderwall

Very true. I think they're getting too safety conscious nowadays. It's a joke. I know protecting people is paramount but surely all these extra things are upping the cost of wiring homes, therefore making it very off-putting for anyone thinking of getting their home rewired. They could at least wait until the industry is back on track before throwing more costly regulations at us.

I do not have a copy of the most recent regs but i will ask in work tomorrow about the bathroom light.

2011

It has to be a dedicated RCBO/RCD or use suitable 12 volt lighting like this:

http://www.google.com/m/search?q=Bathroom+downlighters&aq=f&oq=&aqi=-k0d0t0&fkt=1762&fsdt=18994&csll=&action=&site=images&gl=us&source=mog&ltoken=54b4d5d2#i=8