Voltage in shower waste!

[purplemaf]

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!!

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!!

 

[UKMeterman]

This is a PME property? Try putting some earth pegs outside, you don't need to go mad you need to get a Ze of about 1 kohm or better, using a long lead check the voltage between the earth peg and the MET of the property under heavy load. Also check the voltage between the waste and the earth peg first when several buckets of water are being poured through, and then when water is being run from the mixer tap.

It may be there is a metal waste pipe that is at true earth somewhere and the water is bridgeing the gap.

 

[Farmelectrics]

Meterman what would you be hoping to achieve by this the high zs on the ring main wants correcting first but interested to here what your meaning

 

[Marvo]

I feel for you.

http://www.electriciansforums.co.uk...lations/70467-shocks-bath-tap.html#post688922

 

[Farmelectrics]

Agree as above glad it's not me baffling isn't the word book a holiday and disappear for two weeks lol let us no if you solve it and what it was

 

[purplemaf]

LOL,

I just came back from 2 weeks away to this!

 

[purplemaf]

Agreed about the ring main but my priority right now has to be the water issue. I have had every neutral and earth disconnected at the board and the problem still lives. It must be an external anomilie!?!?! I hope...

 

[Hawk]

Are the waste pipes metallic? If so could you bond them?

 

[Marvo]

There are some faults that standard electrical testers will never pinpoint even in the most experienced hands. Sometimes you've just got to bite the bullet, be proactive and get dirty if necessary. If you've done all the tests and none of the results give you any clues where the problem may be then start exposing the shower drain pipe so you can visually check along the full length.

 

[purplemaf]

No I wish they were then it would be easy to remove the voltage! HOWEVER that would not tell me where it is coming from which of course is my biggest concern!

 

[purplemaf]

I hear you Marvo

Ive got all the externals to check through tomorrow then it'll have to be the bad news for the customer... sorry but I'm just going to rip your house apart :/

 

[UKMeterman]

I am starting from basics trying to find out what is causing elevated voltage with respect to true earth. 1Kohm or less should be significantly less than the impedance of the volt meter used to investigate the problem, this Ze is for the test lead ONLY. I am suspecting one or more of the following;


1. MET of the property is at an elevated potential, if this is the case then the DNO need to be got back.
2. Some water heater has got a small earth leak.
3. There could be some metal waste pipes that are either at some high potential or are earthed to true earth.

It should be quick to check point 1. if there are more than a couple of volts phone up the DNO and get them back.

The checks to the waste are to try to find out where the potential is coming from. If it is present when the buckets of water are poured down the waste then there is someting in the waste pipe that is causing the potential. If the volts go away when the main switch to the property is turned off then it is the installation at fault and time to get the megger out. If the volts do not go away get the DNO out again as there is a fault not under the control of the property owner.

Now check the volts between the test lead and taps and so on and see if any are found. If volts are found try turning the various circuits off in turn in the consumer unit. It could be the heater has split and particularly if there is a lack of earth bonding a shock hazard could occour.

 

[7029 dave]

By what you have described I would say external fault, try bonding anything metallic in the said area, also disconnect all circuits completely from db,keep cu off, to prove its worth a try.

 

[purplemaf]

I have carried out the process of turning on/off each circuit in turn to see when the problem arises. It is difficult to gauge exactly but it definately appears clearly on 3 or 4 of the 6 circuits...

The voltage does disappear when the main switch is off.

I'm 100% banking on an external issue.....I DO LOVE A CHALLENGE!

 

[Knobhead]

Have you measured the total leakage to earth for the installation at the service head?

Nothing to do with the MET, it’s a test on the incoming L&N tails.

 

[purplemaf]

Just checked outside electrics (only 1 twin skt on house wall)...no problem there. Also checked for voltage in the actual drainage outside the front door. No voltage present.

 

[UNG]

Have you tried an IR / continuity test on the waste to L & N and the main earth

Also what type of meter are you using to get your voltage readings

 

[Lucien Nunes]

If the voltage is absent with some circuits off, and remains absent even when there is load on other circuits, and the Ze is OK, that would suggest a localised insulation fault rather than external. If the insulation has broken down to some pipework or structural metal that is not bonded your original IR test might not have revealed it. How about earthing the water flow and repeating the IR test on suspect circuits.

 

[newbec]

If the voltage is absent with some circuits off, and remains absent even when there is load on other circuits, and the Ze is OK, that would suggest a localised insulation fault rather than external. If the insulation has broken down to some pipework or structural metal that is not bonded your original IR test might not have revealed it. How about earthing the water flow and repeating the IR test on suspect circuits.

I agree with this. Also have you completely removed all connections to the installation when carrying out Ze and also tested loop impedance of the supply between Line and Neutral and compared it with the Ze result to ensure the PME link in the service head is intact and the external return path is effective?

 

[purplemaf]

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....