Strange voltage readings, has anyone else tried this!!!

[dogwind]

Hi my brother is a plumber and was getting a back feed through a underfloor heating control holding a relay in, he rang me to ask for advice and until i tried it for myself i didnt believe him... he said in the end he disconnected the cable at the wall stat and the control which was getting the back feed and the only wire he had connected was a live 240v, it was multi core cable and he just had choc blocks on the other wires both ends to make it safe, then measured each cable to an earth and was getting 240v on every wire in the multi core, this was on new cable and was 999 on insulation resistance. My first thought was faulty cable so i got a length of cable and tried the same test myself by putting only the brown into the live in a socket, this was twin and earth and that was the only cable connected to anything, the blue and earth open both ends, i also got a reading of 240v back to the earth on the in the socket, the shorter the cable the lower the voltage readings i get, i then tried it with different cable with the same results, i have been a electrician for quite a few years but this was the first time i have ever tried this with only one wire in a cable connected, i still not sure why i am getting 240v on the other cores, i cant see how it can be induction as no current is flowing through anything.
Just wondered if anyone else has tried this and if they could explain the voltage readings.
The fault my brother had found was a faulty control and not the cable in the end.
Thanks

 

[sparkinlee]

What were you using to measure the voltages? My volt stick gets confused by voltages induced on disconnected conductors by an adjacent live conductor, but the voltage setting on my MFT does not.

 

[dogwind]

hi thanks for reply, i tried a basic multi meter and i also have a kewtek multi tester, my brother used a basic multi meter so 3 cant all be wrong, i have asked a few electricians and they couldnt think why either

 

[bob22]

Hi dogwind, i've had this on circuits before, never as much as 240v though. I tried to check it out with a bit of 3 core and earth, i connected it to a plugtop live neutral and earth, and left one wire floating(not connected to anything) connected the floating wire to a lamp holder with the neutral. Without a bulb in, there will be a voltage in the floating wire but as soon as you add a resistance(light bulb) for it to flow away, it will be back to zero volts. This is why on two way circuits when some led bulbs are used there will be a slight glow to them because the induced voltage can't flow through because of poor resistance I think. The longer the wire the higher the voltage gets.