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Topic Title: VOLTAGE IN SHOWER!
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Created On: 23 October 2013 09:47 PM
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 23 October 2013 09:47 PM
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purplemaf

Posts: 20
Joined: 23 October 2013

Voltage in shower waste!
Hi All,

I am at my wits end here so here goes...

On Monday I was called to a domestic premises with reported shocks occuring while the client was taking a shower. This was apparently first noticed over 2 weeks ago!

We proceeded to do all the relevant checks on Earth Bonding and various testing including IR on all circuits. Most circuits proved to be fine apart from a high Zs (3.06) on the upstairs ring. All bonding was proved to be sound. Install is a PME with a split board with only 1 RCD. Main incomer has a Zs of 0.26. I moved all circuits over to RCD protection to see if any tripping would occur....to no avail.

Upon further investigation we ran a lead out from the board to provide a means of testing in the shower...what is clear is that a voltage of 6v can be meaured between Earth and the actual water in the shower tray but ONLY WHEN IT IS DRAINING THROUGH THE WASTE! This voltage increases to around 18v at least as load is increased around the house.

Upon FURTHER investigation I have now discovered this to be the case on all 3 showers and all basins upstairs. With the basins I actually filled them up, released the plug then could measure the voltage as the water drained...NOT BEFORE. With the showers I ran the water and turned off then measured the voltage again as the water drained. All showers are hot and cold fed with no local pumps.

I have had every circuit apart and individually connected 1 circuit at a time in the board and found that 4 out of the 6 circuits seemed to bring about the voltage. I have also bizarrely placed a lead between the mixer taps and into the water in the plug hole while running the water. This always removes the voltage.

I am left with the only answer that somehow a voltage is finding its way through the waste water and once someone gives it a path then the shock occurs!! The waste is not metal anywhere that I can see.

I have had SSE out today checking everything their end....no problems. Contacted the NICEIC...no advice could be given! I've consulted 2 of my so called guru electricians who are stumped. I am hoping that tomorrows investigation of the external wiring will reveal something but I'm not holding my breath!

Somebody please tell me they have come accross this before!!
 23 October 2013 09:56 PM
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Legh

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Possible causes, that's if the supply neutral is Ok
A Zs of 3+Ohms on the sockets maybe affecting the shower circuit.

My suggestion would be poor MPB and SPB.....

Legh

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http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
 23 October 2013 10:14 PM
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Parsley

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Perceived electric shock caused by voltage drop in the DNOs PEN conductor.
True earth and PME earth are not the same and the voltage difference increases as the load increases.

Was the customer touching the mixer tap when the shock occurs?

Regards
 23 October 2013 11:36 PM
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potential

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Originally posted by: Parsley
.........
True earth and PME earth are not the same ........

Regards


I suggest using a long fly lead connected to an earth spike (or long screw driver) in the garden and see if there is a pd on the waste outlets and/or any metal parts connected to the PME.
My betting is that the wastes will register zero volts whereas the PME will have a voltage present.
 24 October 2013 01:10 AM
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alancapon

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Originally posted by: purplemaf
Voltage in shower waste!

. . . Upon further investigation we ran a lead out from the board to provide a means of testing in the shower...what is clear is that a voltage of 6v can be meaured between Earth and the actual water in the shower tray but ONLY WHEN IT IS DRAINING THROUGH THE WASTE! This voltage increases to around 18v at least as load is increased around the house. . .

. . . I am left with the only answer that somehow a voltage is finding its way through the waste water and once someone gives it a path then the shock occurs!! The waste is not metal anywhere that I can see. . .

You are making a common mistake here. The waste is likely to be at earth potential, particularly if the water (which is conductive to some extent) is running. What you need to remember is that a PME earth is not actually an earth connection. What you have is a DNO that is unable to give you an earth connection and allowed you to connect to the System Neutral instead. As the load changes within the network, and also within the DNO's network to some extent, there will be a voltage between the "PME Earth" and true earth.

Your only solution is probably supplementary bonding, which is what you had when you connected the plug hole to PME Earth. You could of course convert the property to TT, which would probably solve your problem too.


Regards,

Alan.
 24 October 2013 04:54 AM
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Jaymack

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Originally posted by: purplemaf
I am left with the only answer that somehow a voltage is finding its way through the waste water and once someone gives it a path then the shock occurs!! The waste is not metal anywhere that I can see.

The insulation resistance is probably high at the waste trap, being non conductive. When water flows and a voltage is applied, there will be a current flow causing potential differences throughout the circuitry. A rubber mat or a pair of Wellies when showering, is called for.

Is there an immersion heater involved? Has the shower circuit per se been tested for insulation resistance?

There will be an earth path created through the water, albeit of high resistance. If this was a metal waste and equipotentially bonded to the local water pipes, there would be minimal potential difference, and no shock.

Regards

Edited: 24 October 2013 at 05:10 AM by Jaymack
 24 October 2013 10:43 AM
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purplemaf

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Have checked all outside electrics. No problem there. Just tested the water running through waste outside with shower running. No voltage there at all.
 24 October 2013 11:07 AM
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UKPN

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Alan Capon is quite correct in his explanation. there will always
be volts on our PME network neutral. In normal circumstances
a correctly bonded installation will give no problem. However,
this bathroom situation should be likened to a sports pavillion
shower room with a concrete floor. We advise, a grid installed
in the floor to eliminate shocks. A rccd/s is a better solution.
I hope this helps.

Regards
 24 October 2013 11:25 AM
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Parsley

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As I said originally the problem is likely to be volt drop in the DNO's PEN conductor and pd difference between PME earth and true earth. The bathrooms are on the first floor, we don't know the floor construction but it sounds like there are shower trays the user is not standing directly on the floor like you might find in a sports pavilion with a concrete/tiled floor set up.

I don't see how RCD's would solve the problem UKPN?

Regards
 24 October 2013 12:02 PM
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potential

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Originally posted by: Parsley

I don't see how RCD's would solve the problem UKPN?
Regards

Neither do I.
The voltage between true earth and the PME system would not be altered by the additon of an RCD.
Even if the RCD were to trip for whatever reason, the PME voltage would still be present as it is created (for the most part) by other consumers' loads on the supply network.

As has been said, if the shower outlet was bonded the shocks should not happen.

However if this was my problem, I'd be interested in what voltage is actually present on the PME in relation to true earth as we are still not sure that there is not a broken neutral somewhere in the DNO supply.

Edited: 24 October 2013 at 04:20 PM by potential
 24 October 2013 12:31 PM
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daveparry1

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More effective/additional bonding is the answer here i'd say,

Dave.
 24 October 2013 01:12 PM
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Parsley

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Maybe remove a section of the PVC waste where it joins the plug hole and replace with metal and sup bond. I don't think you will get a EC15 on the plug hole.

A local earth rod connected to the MET might help to stabilise the PD between PME and true earth under normal operating conditions.

It is definitely worth carrying out the screwdriver test as potential advised.

Regards
 24 October 2013 02:10 PM
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SherlockOhms

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I'm not sure if this is of any relevance but I had similar a while ago, it may help.

Client was receiving mild tingles in the shower.

I checked everything to no avail then noted my volt stick sqwarked as I walked into the en-suite. This was repeatable by waving the voltstick around the threshold.
Further investigation showed the metal frame of the shower door was bolted into the metal frame of the stud wall.

Light switch cabling inside the wall was nicked on the studding I deduced.

Bypassed the switch drop and hey presto, happy customer.

S.
 24 October 2013 02:25 PM
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weirdbeard

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My gut feeling is it's nothing to do with the earthing arrangements, I would look for a leaking waste pipe that might be dripping over a termination in an accessory or junction box.
 24 October 2013 03:34 PM
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slittle

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I'm with Alan and UKPN on this one.

Screwdriver into terrafirma to measure exactly what's happening.

My money is on PEN being higher than it should be and conductive paths through the waste pipes as has already been suggested.

Let us know the outcome


Stu
 24 October 2013 03:36 PM
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aligarjon

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Is the main incoming water pipe metal. I would be tempted, under controlled conditions to remove the PME earth and use this as a temporary earth to test with, or bang in a temporary spike with the cable literally just going through the doorway to see what you get. As bonding to a plastic waste pipe won't do anything, converting to TT might be your only option. Seems strange that its upstairs basins only . I presume they all share 1 outlet somewhere.

Gary

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Specialised Subject. The Bleedin Obvious. John Cleese
 24 October 2013 04:15 PM
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purplemaf

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Hi Gents thanks for all the posts....unfortunately this is still not resolved.

I have once again worked through all circuits and was convinced I was closing in on he problem. I connected each circuit individually and switched on to see which were giving a voltage. I had done this before. I found that only the upstairs lighting gave me 16v. So I started looking closer at the lighting leg by leg taking r1+r2 readings as I went along. Each room increased the reading by roughly 0.4 ohms. Overall the Earth loop reading was 2.36. I put it back together adding each room to the circuit and testing the water. The voltage in he water started very low and increased every time I added another room. HOWEVER once I finished this process I decided to completely disconnect the lighting and power the rest of the house back up. The voltage again returned at 5/6v

I tried converting to TT.....still there. Ran a bond to upstairs straight from board...still there....
 24 October 2013 04:32 PM
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potential

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Originally posted by: purplemaf

Hi Gents thanks for all the posts....unfortunately this is still not resolved.
.......................
I tried converting to TT.....still there. Ran a bond to upstairs straight from board...still there....


Did you disconnect the neutral link to the MET when you converted to TT?
The earth spike would not make a alot of difference in relation to the low impedance of the neutral link.

When you ran a bond upstairs, where did you connect it?
Did you connect it to the waste?

(Sorry if questions may seem a bit obvious to you. )
 24 October 2013 04:44 PM
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purplemaf

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Removed link on TT and when I ran the bond up I hooked it up to bonding upstairs and checked for voltage and also checked it directly to water..
 24 October 2013 06:10 PM
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Legh

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Main incomer has a Zs of 0.26


1/ I assume that you mean Ze?
2/ Did you loop test between P-N and then P-PE connected at the PME link?
3/ Did you loop test between the P-MET?
4/Did you loop test between P-PE with the PE disconnected at the MET?
5/ Have you checked the effectiveness / continuity of the MPB to the Gas and Water?
6/ Does SPB exist in each of the shower rooms? - no will be the answer then...

The voltage between the MET and ground for a normal PME supply will fluctuate continuously anywhere between 0.1 - 11+V dependent on the load inside and outside of the installation. This part is out of your control and you must follow 701. to protect against micro-shocks.
If, however, you can show an abnormally high Ze then you will have a cause for concern and its back to the DNO.

I have now discovered this to be the case on all 3 showers and all basins upstairs. With the basins I actually filled them up, released the plug then could measure the voltage as the water drained.


1/ I assume that each shower is supplied by the same CH/W boiler?
2/ Have you checked the earth continuity at the CH boiler and controls?
3/ Have you checked the SPBs at the boiler? -with modern boilers cross bonding appears not to be necessary due to a metal fixing plate, however....

Something more for you to think about

Legh

-------------------------
Why do we need Vernier Calipers when we have container ships?

http://www.leghrichardson.co.uk

"Science has overcome time and space. Well, Harvey has overcome not only time and space - but any objections."
IET » Wiring and the regulations » VOLTAGE IN SHOWER!

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