Identifying earth with clear 3 core flex

[Heidi Hi]

Hi started adding extension to lamp flex. New purchase from John Lewis it has clear 3 core flex for metal bayonet fitting.
Unfortunately set off gung-ho and took off plug only then realised not the usual coloured core. One core has blue thread - neutral? Another either dark green or black thread? The third no identification. I'm now struggling to identify the earth. Tried dismantling the bulb holder to check there, but worried I'm going to damage it. Help please, should the unmarked be earth?

 

[bhm1712]

Do you have a multimeter?

If so, set it to ohms, null the leads (touch then together to verify you get a reading at or around 0 ohms) and then hold one lead tip against the metal on the bayonet lamp holder and then use the other lead tip to touch each of the three wires in turn. When the reading is at or close to 0 ohms you know you have found the earth wire.

 

[EFLImpudence]

Do you have a multimeter?

 

[Taylortwocities]

Untitled

• Taylortwocities
• 5 Jan 2010
• 1

The only way to be sure is to use a multimeter set to the ohms range, or a continuity tester, and check the connections of each wire.
A lot of light fittings are made in foreign parts where the usual cable colour conventions may not apply.

You must make sure about this, you have a 1:3 chance of connecting the line supply to the metal parts of the fitting, and you do not want to do that.

 

[ban-all-sheds]

A lot of light fittings are made in foreign parts where the usual cable colour conventions may not apply.

And it's legal for JL to sell things with cables like that, is it?

 

[Heidi Hi]

Absolutely Taylorteocities. So cross quick job turned into a marathon!! Check twice etc etc!!!
I will have to wait for brother with multimeter to come and roll eyes then help!!
Wondered if the coloured threads followed a recognised system.

 

[Taylortwocities]

Wondered if the coloured threads followed a recognised system.

They may do, but which system shall we choose? The USA, for instance, uses black to denote a live conductor. What is used in the various factories in China/Korea/Indonesia may be something completely different.

 

[Heidi Hi]

Can't argue Taylortwocities, yours is sound advice.

 

[JBR]

Sorry to intrude, as I'm not an electrician. I hope I don't get shouted at!

You could ascertain which conductor connects to the different parts of the bayonet fitting without the need of a multimeter. By using a small circuit of battery/cell and lamp in the same way, just connect one end to the outside of the bayonet and the other to each conductor in the cable end in turn. The conductor that causes the lamp to light is the earth, assuming it is properly connected to the bayonet (which, if it's a Chinese product, is anybody's guess)! For a lamp, I don't think it really matters which is line and which is neutral.

 

[winston1]

Sorry to intrude, as I'm not an electrician. I hope I don't get shouted at!

For a lamp, I don't think it really matters which is line and which is neutral.

If it has a switch it matters. If it has an ES lamp it matters.

But as you are not a spark I won't shout.

 

[DaveHerns]

Does this lamp comply with UK regs for electrical safety? I thought John Lewis sold quality products

 

[Taylortwocities]

It almost certainly does, as provided to the customer.
Unfortunately the OP has modified it, IMO it's not JL's problem.

 

[ericmark]

Sorry to intrude, as I'm not an electrician. I hope I don't get shouted at!

You could ascertain which conductor connects to the different parts of the bayonet fitting without the need of a multimeter. By using a small circuit of battery/cell and lamp in the same way, just connect one end to the outside of the bayonet and the other to each conductor in the cable end in turn. The conductor that causes the lamp to light is the earth, assuming it is properly connected to the bayonet (which, if it's a Chinese product, is anybody's guess)! For a lamp, I don't think it really matters which is line and which is neutral.

The idea of "Belling something out" is as old as it can get. Using something like a door bell which has a couple of batteries in it to show which wire connects to where is a very valid method of working out which wire is which.

Earth should go to the touchable metal work, with a bayonet type bulb the line should be switched if a switch is fitted, but if not line and neutral can be either way around, with a screw bulb line is always to centre pin, so any child putting there finger in the bulb holder is sure to touch it, clearly not really the reason, with bulb in place you may be able to touch the screw threaded part so that is always connected to neutral.

So yes your method is very valid.

 

[ban-all-sheds]

And can be a useful method if it saves you having to juggle and observe a visual indicator (e.g. a multimeter) whilst up a ladder.

 

[JBR]

Sorry to intrude, as I'm not an electrician. I hope I don't get shouted at!

You could ascertain which conductor connects to the different parts of the bayonet fitting without the need of a multimeter. By using a small circuit of battery/cell and lamp in the same way, just connect one end to the outside of the bayonet and the other to each conductor in the cable end in turn. The conductor that causes the lamp to light is the earth, assuming it is properly connected to the bayonet (which, if it's a Chinese product, is anybody's guess)! For a lamp, I don't think it really matters which is line and which is neutral.

The idea of "Belling something out" is as old as it can get. Using something like a door bell which has a couple of batteries in it to show which wire connects to where is a very valid method of working out which wire is which.

Earth should go to the touchable metal work, with a bayonet type bulb the line should be switched if a switch is fitted, but if not line and neutral can be either way around, with a screw bulb line is always to centre pin, so any child putting there finger in the bulb holder is sure to touch it, clearly not really the reason, with bulb in place you may be able to touch the screw threaded part so that is always connected to neutral.

So yes your method is very valid.

Thank you. As he said it was a bayonet fitting, I assumed that the pins could be either way around although, as has been said, if a switch is included then strictly it does matter which is line and which is neutral. Sorry, I should have mentioned that. I'm sure that testing for the earth conductor was valid, though. As the OP said he didn't have a multimeter, I thought I could save him some time.