11-12 volts reading across line and earth

[Sting Ray]

trying to trace an earth fault in a friends home today after the RCD in the single ring main suddenly and persistently started tripping
why might I see 11 to 12 volts across line and earth on some sockets?
are there any tips regarding finding the earth fault? have broken the ring and disconnected all sockets and i am working my way along the ring reconnecting one socket at a time until the fault reappears

 

[Marvo]

why might I see 11 to 12 volts across line and earth on some sockets?

Is this with the circuit live or isolated?

are there any tips regarding finding the earth fault? have broken the ring and disconnected all sockets and i am working my way along the ring reconnecting one socket at a time until the fault reappears

Have you got a mega tester?

 

[Sting Ray]

yes i have a megger tester

 

[Marvo]

What IR readings did you get with it before you started removing sockets?

 

[micknew]

*removed*

 

[Sting Ray]

What IR readings did you get with it before you started removing sockets?

less than 0.02Mohms

 

[Sting Ray]

*removed*

don't understand your remark

 

[Sting Ray]

circuit is live when reading 12 volts, yes I have an insulation tester

 

[Marvo]

less than 0.02Mohms

Not a very meaningful answer.....how much less than 0.02 MOhms?

reconnecting one socket at a time until the fault reappears

I don't think I understand what fault you're looking for here. What tests are you doing and what tester are you using each time you connect a socket?

 

[ArseyDee]

The circuit is energised and showing 11-12v across L-CPC at some sockets? Is that what is being said here?

 

[Sting Ray]

The circuit is energised and showing 11-12v across L-CPC at some sockets? Is that what is being said here?

yes that is what my test meter shows (MFT1730)

 

[Sting Ray]

0.02-0.018 MOhms at 500V

 

[Octopus]

0.02-0.018 MOhms at 500V

Between which cores?

 

[Sting Ray]

To restate:
I see 11 to 12 volts across line and earth and a very low insulation resistance at 500V test voltage between line and earth
I know there is a short circuit somewhere and I have cleared all but two outlets on the ring so the ring is presently configured as a radial with the last part of the ring dead ie not connected to the main board or the incoming ring cable
I was curious about why the 11 to 12 volts appeared, nothing else

 

[Marvo]

... why might I see 11 to 12 volts across line and earth on some sockets?

To restate:
I see 11 to 12 volts across line and earth and a very low insulation resistance at 500V test voltage between line and earth
I know there is a short circuit somewhere...

I was curious about why the 11 to 12 volts appeared, nothing else

Okay the way I read this is you're actually worrying about two different faults here. Firstly there's the low insulation that was causing the RCD to trip. Secondly there's a mysterious 11-12 volts appearing at some sockets between the L and E.

Just ignore the 11-12 volts between the L&E, chances are it's a meaningless reading because you're using a tester with an extremely high input impedance which is over sensitive. Concentrate on finding the extremely low insulation problem between N and E.

 

[telectrix]

i hope you're not reading the MFTs battery volts that appear on screen when the MFT is powered up.

 

[Dizzy_Maskell]

Start the fault finding technique either loose cable or damaged cable by screw etc break the ring main down until you find it.

Good luck can be a full in gruler it you could get lucky,

I once found that after British gas had put some pipes in the ring main randomly tripped turns out they had snagged a cable with the new nails they out in to floor boards and if some one walked on them it would trip too ages find can't beat the best in the business

 

[pepparz]

why would british gas put pipes in the ring main?

 

[DPG]

Ah, the importance of a subtle comma in a sentence. Daz

 

[Des 56]

Where has this trend for measuring voltage to do fault finding come from ?

I have noticed quite a lot of fault finding questions being posted, real,induced and phantom voltages are noted, then used to try and trace insulation and continuity problems
Does the training reflect this method these days or is it just the "in" thing to do this