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Shower tray acting as capacitor? Measured 28v!

Asli

First post as I'm hoping someone here might be able to give me a good technical theory (aware without more points of data a true explanation might not be possible) which explains the phenomenon I have experienced recently.

It started with me receiving what I immediately recognised as a small shock from the metal tap in my shower, a small sharp tingle. An important detail is that I could not feel this shock with my bare hand (even soaking wet) only by chance a tiny cut on the edge of my thumb had touched and there, without fail I could feel it. Upon stepping out of the shower the shock disappeared, back in, it returned.

To make sure I wasn't going crazy, I grabbed my multimeter and took a measurement from the waste water to the metal tap, 20V. My gut feeling was some sort of conductive path based on the waste pipework as the voltage dissipated as the water in the waste drained away with the tap off.

Next thing I did was obviously call a qualified electrician out who tested my installation and could not find anything wrong, a real head scratcher for him. At the end of troubleshooting day one, the working theory was that the water and biofilm in the pipework combined with the water at my feet were creating a path to the cast iron waste pipework much further down in the building, that might not be bonded to the building's main earth point as the waste pipes into each property (its a block of flats) are all plastic so no need to bond.

Purely by chance I did some more testing later myself, and noticed that the voltage jumped up before the water had even had a chance to run down the waste for any time at all really, it seemed to occur as soon as there was a decent pool of water in the tray, this is when a ridiculous idea popped into my head, and I thought the plastic shower tray could be acting as a giant dielectric‽

Anyway I plugged the shower and filled the shower tray, so now the waste was completely out of the equation and the water was much more consistently covering the whole base of the shower, we're up to 28v! I also took a resistance measurement of 290kilohm to give a current of just under 1mA. Tiny, and explains why I can only feel it precisely on the tiny point where my skin is broken and nowhere else.

As for the source of the voltage, as no faults were found I am at this point assuming it is all leakage current, I have a few PCs, all smart lights, so both the socket circuit and the lighting circuit had an influence on the measured voltage.

With that in mind, is what I am proposing: a capacitively coupled pool of water in the shower tray technically feasible as a mechanism to introduce a different potential into that scenario to the earth in my flat, bonded to the shower taps via pipework, that is carrying some leakage current?

I still need to investigate under the shower of course, perhaps some structural metalwork, or other cabling? OR a very damp floor with an unknown slow leak that is conductive enough to join to some other structural point that eventually joins true earth independently of my electrical installation.

Fascinated by figuring this out, but also very confused currently.

EDIT:

Got access to underneath the shower, there is metal studwork on 3 sides of the shower. The metal base bar of the studwork runs around the sides of the shower tray base. The left wall to earth reads 33v and the right wall 28v. There is no damp, cabling, screws poking through the floor into the shower tray that might have broken the surface.

Is it technically possible for current induced in the metal studwork from live cables, to be capacitively coupled to the thin sheet of water pooled in the tray, which then passed to the grounded shower tap via me?

Parents
  • 0

    It seems unlikely although most shocks are capactively  coupled,not sure where you got the 1ma current from?

    Is it an ac 50hz voltage present? Is it still there with the main switch off?

  • +1 in reply to Newfutile

    the 'just under' 1mA is 230C divided by 290K .

    For many people 0.1 to 1mA is around the threshold of perceptible shock current, but it varies between people, and with which body parts are in the current path.
    I would be interested if your meter can do it in an AC mA reading - the voltage is likely to be a very odd thing, as the modern digital meter draws almost no current  (input impedance of 10, 20 and 100 Megohm are common values) and will  read some indeterminate voltage based on the voltage division of a high impedance path and the meter input resistance. Current is more closely aligned to sensation.

    In short, yes all objects have some capacitance to free space (i.e. field lines that leave and go off to the 'plate at the edge of the universe' never to return) as well as considerably more capacitance to other objects near by (field lines that leave one object and terminate  on another ) . However a shower tray, even a 1m2 one, would only muster a few tens of pF to free space, but may be some hundreds to a conductive floor beneath it, and a  lot more if the underside were to form the other plat e of the capacitor  becauseit was a conductive surface by dint of being damp .

    What are these voltages measured with respect to ? - as in where are you putting the  other meter probe?
    The shower may actually be earthed and the thing you are using as reference may be the think that is a bit live - this is more likely the more random objects appear to be live.
    Note that house earth terminal, and real earth (stick a fork in the flower bed sort of earth) can be a few volts apart, but more than perhaps ten would be a moment to take a precautionary look at the house earthing or neutral integrity of a PME supply...

    regards Mike.

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  • +1 in reply to Newfutile

    the 'just under' 1mA is 230C divided by 290K .

    For many people 0.1 to 1mA is around the threshold of perceptible shock current, but it varies between people, and with which body parts are in the current path.
    I would be interested if your meter can do it in an AC mA reading - the voltage is likely to be a very odd thing, as the modern digital meter draws almost no current  (input impedance of 10, 20 and 100 Megohm are common values) and will  read some indeterminate voltage based on the voltage division of a high impedance path and the meter input resistance. Current is more closely aligned to sensation.

    In short, yes all objects have some capacitance to free space (i.e. field lines that leave and go off to the 'plate at the edge of the universe' never to return) as well as considerably more capacitance to other objects near by (field lines that leave one object and terminate  on another ) . However a shower tray, even a 1m2 one, would only muster a few tens of pF to free space, but may be some hundreds to a conductive floor beneath it, and a  lot more if the underside were to form the other plat e of the capacitor  becauseit was a conductive surface by dint of being damp .

    What are these voltages measured with respect to ? - as in where are you putting the  other meter probe?
    The shower may actually be earthed and the thing you are using as reference may be the think that is a bit live - this is more likely the more random objects appear to be live.
    Note that house earth terminal, and real earth (stick a fork in the flower bed sort of earth) can be a few volts apart, but more than perhaps ten would be a moment to take a precautionary look at the house earthing or neutral integrity of a PME supply...

    regards Mike.

Children
  • 0 in reply to mapj1

    I posted a similar problem on tis forum many years ago. The main difference was my customer got a tingle when touching metalwork (all bonded to supply earth) in the shower if he had any cracks on the skin of his fingers. After much testing a screwdriver in the lawn  tested to  incomer, often Neutral/Earth (TNCS) of the overhead supply gave, from memory, about  8 Volts. . Yours appears more complex but the screwdriver in the lawn,  as mentioned in previous posts, has often aided me in fault finding by providing a reliable reference point, ofteh via a long length of wire.

    The customer never found a solution but knowing it was not a safety issue he was happy as he was selling the house.

  • 0 in reply to mapj1
    mapj1 said:
    For many people 0.1 to 1mA is around the threshold of perceptible shock current, but it varies between people, and with which body parts are in the current path.

    Those levels of current are correct for dry conditions ... not wet or immersed.

    Some research shows it may be possible, if you were fully immersed, for a current between 0.5 mA to 2 mA to perhaps kill !

    Certainly no current or voltage is provided in IEC/TR 70479-5 that is considered 'safe' for someone who is immersed.

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