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Topic Title: Erratic Voltage readings
Topic Summary: Any ideas?
Created On: 03 December 2013 03:11 PM
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 03 December 2013 03:11 PM
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Banners

Posts: 117
Joined: 06 October 2013

Hello folks,
At the weekend I carried out an EICR. All Zs readings on the socket circuit tested out fine with the highest reading 0.74? and the voltage around 240V. However, one surface mounted double socket spur in a bedroom gave no reading when I plugged in my faithful Megger for a Zs reading. I turned the dial to voltage and, starting at 247V the reading quickly progressed up to 257V and stayed there flashing. I have never seen this before.

This socket is wired in new colours whereas every other socket in the building is in red & black.


Further information: System is TNS Ze 0.10?, PFC 2.6kA. Fuse board is tidy - Mem 3036s and looks like it has never been touched since the day it was installed. The socket circuit is wired as a ring but I was unable to do IR tests as the neutrals on the socket circuit had fused together with the neutrals on two other circuits (the insulation only, not at the din rail). In my opinion this has simply happened over time (30 years+?) as the polymers have migrated. I do not think this has any bearing on the readings because when the circuit was tested socket circuit fuse carrier was the only one plugged into the board.

By the way I am returning this week-end to replace the board. I have disconnected the cables to the spurred socket (they are still live) and advised the client that the joint needs to be found and made good or disconnected. What I would like from you learned folks are suggestions as to what is causing the odd voltage readings.

edit: Supposed to be 74 ohms not a question mark
edit 2: I am assuming it is still a ring though I have not yet been able to test this due to fused neutrals

Edited: 03 December 2013 at 03:19 PM by Banners
 03 December 2013 03:39 PM
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AJJewsbury

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neutrals on the socket circuit had fused together with the neutrals on two other circuits (the insulation only, not at the din rail). In my opinion this has simply happened over time (30 years+?)

30 years isn't old for PVC - if the insulation has melted together I'd strongly suspect heat - probably a loose connection (tell-tale charring doesn't show up too well on black).

starting at 247V the reading quickly progressed up to 257V and stayed there flashing

I'd suspect an open circuit c.p.c.

More exotic guesses include something like a PV inverter connected to that circuit, or a dimmer in the supply to the socket (was it for a table lamp?), but try the simple things first. Did you prove c.p.c. continuity & polarity with dead tests?

I guess you could have 247V L-N and 257V L-earth if the supply N is a bit above 0V - not impossible, especially on a TN-S systems if the distribution network was heavily loaded. I guess it depends on what your meter is measuring on the V setting.

- Andy.
 03 December 2013 03:57 PM
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Banners

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Thanks OMS, after looking in the board and discovering the neutrals I decided I was not going to start pulling cables out and testing them because I was going to fail the system for several reasons (No equipotential bonding to water, 2.5mm to gas, no RCDs to shower, no supplementary bonding, Class I light metal fitting within easy reach when standing in bath tub and showering......) Property is for rental so client sees sense of providing adequate protection hence the new board. Property was empty and only load connected were a separate fridge and freezer built in to kitchen furniture and socket was behind them and not disconnected. I will do full tests this week-end and probably discover more. As their were only 12 sockets in the property I had pulled the fascias of each and inspected them and all were soundly connected (sleeving missing on some earths). I can only imagine it has something to do with a dodgy junction box under floorboards but I was still surprised at 257V. I will report back once I have changed the board.

Very sorry, I had just read an OMS post before I replied and his name was stuck in my head; many thanks Andy!

edit: I will get the hang of this one day - I only wanted to add a smiley face!
 03 December 2013 04:51 PM
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OMS

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Thanks OMS,


No problems - I'll pass on your thanks to my sub consultant Mr Jewesbury

Regards

OMS

-------------------------
Failure is always an option
 03 December 2013 05:06 PM
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leckie

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Originally posted by: OMS

Thanks OMS,




No problems - I'll pass on your thanks to my sub consultant Mr Jewsbury [IMG][/IMG]



Regards



OMS


LOL, that nice Mr. Jewesbury will be delighted I'm sure
 03 December 2013 05:13 PM
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OMS

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LOL, that nice Mr. Jewesbury will be delighted I'm sure


He will be when I send him a very generous 30% of the design fee, 90 days in arrears

Regards

OMS

-------------------------
Failure is always an option
 03 December 2013 05:26 PM
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weirdbeard

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Originally posted by: Banners

System is TNS Ze 0.10?, PFC 2.6kA.


V = I x R?
 03 December 2013 05:58 PM
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AJJewsbury

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V = I x R?

Two different Rs on TN-S as supply N isn't the same as supply PE.
So you could quite validly have a 2400A fault current L-PE but a 2600A one L-N - so you can't deduce the supply voltage from that.
- Andy.
 03 December 2013 06:00 PM
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AJJewsbury

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Originally posted by: OMS

LOL, that nice Mr. Jewesbury will be delighted I'm sure




He will be when I send him a very generous 30% of the design fee, 90 days in arrears



Regards



OMS


Hopefully you'll spell my name correctly on the cheque then
- Andy.
 03 December 2013 07:22 PM
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OMS

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Ooops

OMS

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Failure is always an option
 04 December 2013 03:43 PM
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weirdbeard

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Originally posted by: AJJewsbury

V = I x R?


Two different Rs on TN-S as supply N isn't the same as supply PE.

So you could quite validly have a 2400A fault current L-PE but a 2600A one L-N - so you can't deduce the supply voltage from that.



Thanks for pointing that out Andy.

My next guess as to why the voltage was apparently rising within the installation - flat batteries on the tester?
 04 December 2013 04:00 PM
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AJJewsbury

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My next guess as to why the voltage was apparently rising within the installation - flat batteries on the tester?

Possibly. I'd maybe consider a dodgy N in the distribution system as well - presuming the supply is derived from 3-phase - an increasing load on another phase could drag N voltage towards that phase and so away from the local L - giving an apparent increasing voltage. All conjecture of course.
- Andy.
 04 December 2013 08:04 PM
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Banners

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Nowt wrong with the batteries - I started testing at the sockets and then proceeded to test the rest of the installation. M
 04 December 2013 08:10 PM
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Banners

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???????? To continue.... My megger gives a good indication when batteries are failing.Interesting what you say about the neutrals Andy - there are two separate cables rising up the wall into the cut out (not a fused neutral though, typical bakelite type with 60A bullet. Perhaps there is a loose neutral connection in the board. However, the problem is that one socket only and as there was no earth reading (Zs) and therefore a definite poor connection my feeling is the high voltage is in some way associated with the junction point..
 07 December 2013 06:30 PM
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Banners

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Hello, good points is dodgy junction box found and readings now nice and happy. Neutrals at board had actually overheated at busbar - four cables were jammed into a terminal and it must never have been quite tight enough or worked loose. Bad point is that ring circuit live not continuous although every single socket is live with just the one good leg connected suggesting a break between board and a first socket - circuit now on a 16A breaker. If manufacturer does a 25A breaker I might pop one in as a compromise. Thanks to those who made suggestions. Interestingly at the cut out there are two live cables coming into the fuse although only one is connected through the fuse. Anybody know why it might have been done like this - suggests provision for a two phase supply although it is a small 3 bed house.
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