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!!

 

[odawire]

the plastic waste must be connected to a metal waste that has the fault on it, if its coming from more than one circuit it could be on a main riser? As said before, I think you are going to have to start exposing the waste pipe. Good luck! cant wait to find out what it is!

 

[Silly Sausage]

Cant wait to find out what it is!

Me too.
He'll be the most unpopular member of the forum if he doesn't reveal the cause when he finds it!

 

[chcnorris29]

Take it that the bath is not conductive (metal bath).

 

[KAS1]

Use an analogue voltage tester

 

[purplemaf]

Ok so we have read this voltage so far using a digital fluke probes, a stienel, a robin test kit. Readings have been taken between water-mixer tap, water-pipework bonding, water-driven stake outside, water-final circuit cpc, water-direct to consumer unit MET. These are not electric showers as posted. Bath is not conductive....

Lol! I too cannot wait to discover the solution!!

 

[purplemaf]

Having lifted the drain today I think it's probably fair to say there are 3 downpipes all running down to the same drain as the drain is 4 way with 3 inlets from the house clearly below where each pipe is. So it seems to me that the wastes of each room (bathroom & 2 en-suites) don't even connect till they reach the ground.

 

[Lucien Nunes]

18V well below the level of detection by touch!

I have spent many happy hours working on marine electrical systems soaked in water / seawater / bilge etc. When thoroughly wet, I can tell whether a 12V DC circuit is energised by putting my finger on it. Grasping a 24V terminal with wet hands while sitting on a steel deck in wet clothing can be quite unpleasant. I would not be surprised if they can feel 18V.

What is important here is that the measurement of voltage, in itself, is not terribly informative because the effective source impedance is clearly quite high and the instrument input impedance will likely have a strong effect on the voltage indicated. What is relevant is that the voltage is not negligible, and that ought to be sufficient to make it traceable.

Have you had a chance to take readings, under the same conditions with water flowing, of the following set of three voltages:
a) External test spike to MET
b) External test spike to energised water stream
c) MET to energised water stream
If the conditions are identical and the spike outside the resistance area of the fault, then it should prove conclusively which point is being elevated above true earth.

Also, do you have an IR value for (suspect lighting circuit L+N) to (running water)?

 

[Rockingit]

If that were the case I would never have had the call....I have felt this shock and it can certainly let you know it is there!

Just a thought.....is this place rural?

 

[Tridentsystems]

Get a camera down the waste. Reckon you will find a screw through the plastic that may have nicked a cable on its way. Wood butchers and wet heads !

 

[purplemaf]

I have spent many happy hours working on marine electrical systems soaked in water / seawater / bilge etc. When thoroughly wet, I can tell whether a 12V DC circuit is energised by putting my finger on it. Grasping a 24V terminal with wet hands while sitting on a steel deck in wet clothing can be quite unpleasant. I would not be surprised if they can feel 18V.

What is important here is that the measurement of voltage, in itself, is not terribly informative because the effective source impedance is clearly quite high and the instrument input impedance will likely have a strong effect on the voltage indicated. What is relevant is that the voltage is not negligible, and that ought to be sufficient to make it traceable.

Have you had a chance to take readings, under the same conditions with water flowing, of the following set of three voltages:
a) External test spike to MET
b) External test spike to energised water stream
c) MET to energised water stream
If the conditions are identical and the spike outside the resistance area of the fault, then it should prove conclusively which point is being elevated above true earth.

Also, do you have an IR value for (suspect lighting circuit L+N) to (running water)?

The above checks have been carried out and as you say they are all the same. Have not done IR to water itself to be honest.....can't say ive ever put a test probe in water before this week lol!

 

[purplemaf]

Just a thought.....is this place rural?

It is classed as rural yes. Not exactly out in the country so to speak.

 

[Rockingit]

OK....was wondering if there might be any electric fence energisers in the vicinity??

My thinking is that you actually need quite a fair old voltage to push through that much pipe, and if it's all three stacks then it's clearly communal to something downstream of the join, hence way after the conventional 'bounds' of the place.

Or - what's the sewerage system? Is there a septic tank with a lift pump or a Klargester system or......

If it's a shared lateral sewer, might be worth seeing what's going on at the neighbours, too.

 

[Tridentsystems]

No volts at the drain outside I believe? Gotta be an IR fault in the house somewhere. Been happening for long or just started ?

 

[purplemaf]

No volts at the drain outside I believe? Gotta be an IR fault in the house somewhere. Been happening for long or just started ?

I'm told its recent...like within last 2 weeks. Of course I've already gone through the diy routine and eliminated any possibility there.

 

[Rockingit]

No volts at the drain outside I believe? Gotta be an IR fault in the house somewhere. Been happening for long or just started ?

That's just implausible if considering water/pipe as the conductor - it's all gotta go somewhere!