3 core and earth - all wires are live - WEIRD!?

So you have both red and yellow to 'live' connection, somewhere. :(
If you really want to rule out capacitive coupling, then you generally need a test device that will draw some current. I have a bulb holder on a short length of flex that occasionally gets pressed into service to sort out low resistance connections to live from the ghost voltages caused by 'runs parallel to live for a bit and couples into it' the key is that modern voltmeters, and neon lights draw so little current they have difficulty telling if there is a real connection or not.
However, usually,as the induced core is equidistant from a live one and a ground, if it were capacitive coupling, I'd expect to see 120 odd volts not 250, so it sounds like in this case a real connection. check what happens at both ends of the cable, and if yellow is tied well out of the way as unused cores should be then look for cable damage.
If your meter has an ohms range then with the power OFF, you could measure the red to yellow resistance at both ends - the connection is near the end with the lower reading. (check by shorting the probes before you start that a low or zero resistance is recorded.)
AC (alternating and all that) does not care about meter polarity - it is like connecting half a cycle earlier or later so red/black makes no difference to the meter.
 
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BJS_Spark said:
I thought the old neon screwdriver was the offending test gear as this will show up induced voltage due to unterminated wires. It doesn't take much current to light the neon hence you don't get much of a shock when using the rubbish things.

Ground one end of the yellow to earth and it will go.

Cheers mate. So the results check out fine I presume?

Map, sorry mate, bit too technical for me. I only found out how to use a multimeter today :oops:

Ill earth the yellow core and that should sort it.

edit, actually map, this was very helpful, cheers!

mapj1 said:
AC (alternating and all that) does not care about meter polarity - it is like connecting half a cycle earlier or later so red/black makes no difference to the meter.

Cheers all.
 
OK chaps sorted it. Thanks for all your help.

One more question. If the mains supply is 230 volts, then how comes when I connected up the multimeter I got a reading ot 249/250 volts?
 
its 230V+10%-6% iirc

that is its allowed to be anywhere between 216.2V and 253V
 
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the nominal voltage is 230 volts +10%/-6% (as stated by the powers that be) so can be 216.2V - 253V
 
Ahaa!! Brilliant stuff chaps!

That also explains why I once also got a reading ot 253 which then lowered to 250.

Nice one.
 
Missed this one last week - co-incidenece as a friend was asking me about the same issue (fellow electronics engineer so he should have been able to work it out for himself!).

I've seen this induced voltage situation several times, first time it caused some head scratching - only when I used the neon on the genuine live did I see the difference in brightness and start to twig. Now-a-days I'll get the meter out and check.

One thing I have noticed is that the fact that modern DVMs (well mine at least) are very high impedance can make the results confusing - as mapj1 says you wouldn't expect 230V to be measured, but with a DVM I've seen it. Using an old fashioned deflection meter the impedance of the meter is low enough to slug the voltage down to between 50 and 100V.

So it would seem that trying the old fashioned light blub test is the only way to be sure.

Although this has got me thinking do "pro" electricians meters include the option to switch a dummy load on (e.g. a 10K resistor) inorder to distinguish a induced voltage from a real one? A sutable adaptor could be easily made (time for a bit of maths to work out the best resistance and wattage).
 
ban-all-sheds said:
What's wrong with also having an analogue meter available?

Can't beat an old AVO meter with FSD meter (can even remember the ones with P & Q buttons), wasn't same when they went digital. The problem is with modern electronic circuits you require a high impedance meter however for household wiring newer is not necessarily better.
 
ban-all-sheds said:
What's wrong with also having an analogue meter available?

Personally I haven't got one and a 15K 1/2W resistor is lot cheaper!

AVOs with P&Q buttons, that takes me back :cry:
 

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