Isolated circuit showing 400V in house

[cake100]

I am a DIYer and started the process of fitting a spur from a socket (and from the ring) in a house. A few days ago I converted an old immersion switch into a plug socket. It was linked to the boiler and I had to isolate the boiler after there was continuity with the N/L.

Back to the spur. Using a voltage and continuity tester (this: https://www.test-meter.co.uk/voltage-testers/handyman-tek900-voltage-and-continuity-tester/ ) I turned off all the MCBs and RCBs plus the main switch in the Consumer Unit. I did the isolation tests at the CU. All showed there was isolation.
I opened up and checked a plug socket using the volt and cont tester. When I tested the ground with the line/neutral the tester showed 400V and beeped. I also checked the shower on/off switch which was the same - 400V. This problem was not seen at the boiler socket spur.

When the CU is turned on and not isolated, the socket and switch which read 400V when isolated is normal at 230V.

Any ideas what issue this could be?

Many thanks.

 

[Geoff1946]

I suspect it's a combination of the tester you are using, and the way you are using it.  I think I'm right in saying that what you see is one more led in the strip is lighting when you say you are seeing 400v, (?) 

The test meter is AC/DC and probably a high impedance input.  I suspect, (again guessing) it can be confused by the capacitive coupling to an isolated conductor.

If you doubt this, connect a small load, say an incandescent lamp across the points you are measuring and see what it reads then. 

 

[cake100]

I am seeing almost all the strips lighting. There are 8 from 12V to 690V. In my case, 7 light up which indicates 400V.

Usually, it should be non of the strips lighting when there is isolation?

 

[Geoff1946]

Usually, it should be non of the strips lighting when there is isolation?

Not necessarily. An isolated conductor adjacent to a live conductor is capacitively coupled to the live one which induces stray voltage in it.  High impedance meters can measure this stray voltage. 

 

[SPECIAL LOCATION]

Are you using two probes, one on the cable you are testing and one on a known good earth reference point?

If you have no known good earth reference to measure against then it is pretty much guess work as to what voltage is or isn't present..

You could think of it as a bit like measuring a distance..

If I asked how far away is Manchester?

without a second point of reference to measure from, then any value is irrelevant.

e.g. Manchester from Lands End, is different to Manchester to Liverpool..

But Manchester to "nothing" is an impossible measurement.....

Its the same with just a single probe on a cable without a known good earth, (0v), reference it is an impossible voltage to measure...

Guinness      

 

[binky]

voltage probes can be irritating to say the least, mine likes to beep even with the circuit isolated. What it is registering is induced voltages in the cable from the cable being run alongside other live cables. It can be rather disconcerting to say the least and I have often double checked becuase of it. If in doubt, just turn the whole board off.

 

[kerching]

During my apprenticeship a couple of years ago, amongst many pieces of sage advice I was told "if in doubt kill the board +/or building NOT yourself!

so far it has worked well,with 100% success 

 

[Sharpend]

I didn’t know Sage were the electric police, It explains why Deke has struggled to find them, he’s obviously been looking in all the wrong places!

 

[kerching]

I didn’t know Sage were the electric police, It explains why Deke has struggled to find them, he’s obviously been looking in all the wrong places!

He was most likely looking for Parsley, Rosemary and Thyme

eeeeeeeeeeey thank yowwwwwwwwwwwwww

 

[SPECIAL LOCATION]

I didn’t know Sage were the electric police, It explains why Deke has struggled to find them, he’s obviously been looking in all the wrong places!

He was most likely looking for Parsley, Rosemary and Thyme

eeeeeeeeeeey thank yowwwwwwwwwwwwww

I heard that due to Lockdown he couldn't go to Scarborough...

So has been spending a lot of time round at 'Mrs Robinson's..

Allegedly providing assistance within her support bubble..

But he may have been 'Fakin It'?

:C

Guinness  

:coat

 

[phil d]

On the subject of induced voltages,or capacitive coupling, I remember a long time ago in my early days I was doing some alterations at my mothers house. I had a cable that I knew was totally disconnected at the mains end yet whilst removing the other end from its connections at an isolator, the neon screwdriver I was using actually lit! in the end to satisfy myself it was dead I resorted to taking a wire from a known good earth and shorting it to the live and neutral conductors, no big bang but the neon stopped lighting. I'll still do it today, if I'm in any doubt then it gets shorted, better to make sure that whatever you are working on is dead and you are still alive.

 

[binky]

On the subject of induced voltages,or capacitive coupling, I remember a long time ago in my early days I was doing some alterations at my mothers house. I had a cable that I knew was totally disconnected at the mains end yet whilst removing the other end from its connections at an isolator, the neon screwdriver I was using actually lit! in the end to satisfy myself it was dead I resorted to taking a wire from a known good earth and shorting it to the live and neutral conductors, no big bang but the neon stopped lighting. I'll still do it today, if I'm in any doubt then it gets shorted, better to make sure that whatever you are working on is dead and you are still alive.

when my volt probes are being irritating, I also touch the cables together - no spark means its definetly dead! 

 

[cake100]

Hello,
Strange yesterdaay. I checked the voltage of a socket and the 400V reading I was getting before was not present today???
I did get some continuity though with N/L. There was some slight lighting of the tester with G and N/L as you can see from the video.
I don't know if it made any difference, but I isolated the boiler switch (as I did last time due to installing a spur from boiler - old immersion heater switch) and doing the same as above there was a buzz with the continuity light.
Plus when I tested the shower switch something strange happened but I only checked when the switch light was in the on position (realised afterwards).

First here are some images of the CU etc.
https://www.dropbox.com/s/q315pqqu947b5hw/2021-01-10 10.54.38.jpg?dl=0
https://www.dropbox.com/s/sx5ckmxeyu95tfl/2021-01-10 10.55.12.jpg?dl=0
https://www.dropbox.com/s/qk1bbm61exr0r9g/2021-01-10 10.55.29.jpg?dl=0
https://www.dropbox.com/s/ll5tl11xi4fd6qo/2021-01-10 10.55.43.jpg?dl=0
https://www.dropbox.com/s/s25zvq3r6m6ahx2/2021-01-10 10.58.25.jpg?dl=0

https://www.dropbox.com/s/b1ezo71qsmnqkcm/2021-01-10 10.59.40.jpg?dl=0

Here are the vids:
CU was isolated along with all MCBs and RCBs
1. socket with boiler switch on
https://www.dropbox.com/s/4acmj8jy1swpxrq/IMG_2962.MOV?dl=0

2. socket with boiler switch off
https://www.dropbox.com/s/8jyng7quulzzuqz/IMG_2971.MOV?dl=0

3. Shower switch boiler switch on
https://www.dropbox.com/s/rxdukuej2zehy4s/IMG_2967.MOV?dl=0

4. Shower switch boiler switch off
https://www.dropbox.com/s/pd3dqxk8kqscch3/IMG_2975.MOV?dl=0

For 3 and 4 why is there that light and buzz?
And for the rest what is happening?
Many thanks.

 

[phil d]

when testing anything you will always have a degree of capacitance, this will cause the light to flash momentarily, on the last video where it's showing a voltage thats just an induced from a nearby cable. all cables have some degree of magnetic field around them, more if the cable is carrying a heavy current, this will induce a small voltage in an adjacent cable which I suspect is what is happening here. If you were to use a tester that presents a bigger load you'd not see what you are seeing presently, I use one of these, granted it doesn't have a continuity function but as a test lamp you can't fault it.

https://martindale-electric.co.uk/product/mtl20-test-lamp/

 

[kerching]

Just a thought but, could it be the quality of the tester?   I don't ever recall this happening with my fluke T150 or any of its predecessors. However it 'may' have happened many decades ago,with my Steinel combi check but this could have been 'side illumination' from an adjacent LED

just saying

 

[binky]

Just a thought but, could it be the quality of the tester?   I don't ever recall this happening with my fluke T150 or any of its predecessors. However it 'may' have happened many decades ago,with my Steinel combi check but this could have been 'side illumination' from an adjacent LED

just saying

last 2 pairs of continuity testers I have had buz when an induced volyage is present, but show no voltage as such, my previous set (can't remember the make) never did this.

 

[cake100]

So if everything is fine at the CU - isolation with no errors, should that be taken as isolation even with the things I had going on in the videos?

Would it make any difference isolating the remote isolation switch?

Cheers.

Locked to prevent being resurrected again