Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Helpful Answer
  • Replies 21 replies
  • Subscribers 42 subscribers
  • Views 477 views
  • Users 0 members are here
Options
You may also be interested in

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

    Hey folks, thanks so much for the responses, lots of interesting stuff.

    I used this as an excuse to buy a slightly more feature-full meter (not that I need much of an excuse to buy new tools anyway ha!) My current multimeter (used for wiring guitars mostly) a Fluke 101 did not have a Low impedance (LoZ) mode. I got one that did, that also happens to be a clamp meter capable of measuring AC current at a resolution of 0.001A. I unfortunately did not have the budget to stretch to something more sensitive than that like some of the earth leakage meters I'd seen.

    With the new meter I took new readings:

    Stud to shower: 36v

    Stud to shower (LoZ mode): 0.5v

    I then ran some crude temporary 'supplementary bonding' from the stud work to the shower out of curiosity to see if this would help:

    Stud to shower: 0v

    AC current with clamp around bonding cable running to the shower: 0.000A (likely outside of the range of this meter I assume, if it were 0.1mA it would not read this but sounds like it is in the perceptible range from what you say @Mike? Definitely possible with broken skin I would assume

    To answer some of the questions, the voltage disappears gradually as circuits are switched off, one of the first tests the electrician did was isolate circuits one by one, we found that the sockets contribute 10v, the lighting contributes 12v, smoke alarm 8v and a few others a few volts each, there was no single obvious source that might indicate a circuit that had a nick in the live against some metalwork or anything. It was this gradual buildup of the voltage and no obvious problems and nothing alarming on the insulation resistance tests he did (on every circuit) that makes it seems like an induced voltage, it was just the mechanism of transfer, and crucially what was coupling to the water that was baffling. I now know that to be the studs.

    The readings were taken with one probe on the stud, and one probe on the shower metalwork, from which the electrician already checked the resistance back to the CU to see if the bonding was sufficient and he was happy with the reading. The original reading was taken with one probe submerged in the plughole directly and the other problem on the shower.

    I could not feel it at all when standing on the shower with my normal, unbroken (but wet) skin, stood in the pool of water. It was exactly as @kfh mentions, a small crack on the side of my finger where the skin was broken AND wet. In my old job we used to repair computers and sometimes needed to troubleshoot them whilst on with exposed non enclosed power supplies, so we had some electrical safety training and I remember the resistance of broken skin being massively lower than other scenarios but don't remember the ranges given! So this made me think back to that.

    Anyway my conclusion so far is that:

    • Bonding helps, will discuss this with the electrician
    • Regs don't require bonding of metal studs and the like if they are completely enclosed in a wall and insulated that way
    • The current is tiny, the voltage appears to be an induced voltage on the studs and this is probably safe?
    • The mechanism of the coupling of the water to the studs is possible, albeit incredibly unusual?
Reply
  • 0

    Hey folks, thanks so much for the responses, lots of interesting stuff.

    I used this as an excuse to buy a slightly more feature-full meter (not that I need much of an excuse to buy new tools anyway ha!) My current multimeter (used for wiring guitars mostly) a Fluke 101 did not have a Low impedance (LoZ) mode. I got one that did, that also happens to be a clamp meter capable of measuring AC current at a resolution of 0.001A. I unfortunately did not have the budget to stretch to something more sensitive than that like some of the earth leakage meters I'd seen.

    With the new meter I took new readings:

    Stud to shower: 36v

    Stud to shower (LoZ mode): 0.5v

    I then ran some crude temporary 'supplementary bonding' from the stud work to the shower out of curiosity to see if this would help:

    Stud to shower: 0v

    AC current with clamp around bonding cable running to the shower: 0.000A (likely outside of the range of this meter I assume, if it were 0.1mA it would not read this but sounds like it is in the perceptible range from what you say @Mike? Definitely possible with broken skin I would assume

    To answer some of the questions, the voltage disappears gradually as circuits are switched off, one of the first tests the electrician did was isolate circuits one by one, we found that the sockets contribute 10v, the lighting contributes 12v, smoke alarm 8v and a few others a few volts each, there was no single obvious source that might indicate a circuit that had a nick in the live against some metalwork or anything. It was this gradual buildup of the voltage and no obvious problems and nothing alarming on the insulation resistance tests he did (on every circuit) that makes it seems like an induced voltage, it was just the mechanism of transfer, and crucially what was coupling to the water that was baffling. I now know that to be the studs.

    The readings were taken with one probe on the stud, and one probe on the shower metalwork, from which the electrician already checked the resistance back to the CU to see if the bonding was sufficient and he was happy with the reading. The original reading was taken with one probe submerged in the plughole directly and the other problem on the shower.

    I could not feel it at all when standing on the shower with my normal, unbroken (but wet) skin, stood in the pool of water. It was exactly as @kfh mentions, a small crack on the side of my finger where the skin was broken AND wet. In my old job we used to repair computers and sometimes needed to troubleshoot them whilst on with exposed non enclosed power supplies, so we had some electrical safety training and I remember the resistance of broken skin being massively lower than other scenarios but don't remember the ranges given! So this made me think back to that.

    Anyway my conclusion so far is that:

    • Bonding helps, will discuss this with the electrician
    • Regs don't require bonding of metal studs and the like if they are completely enclosed in a wall and insulated that way
    • The current is tiny, the voltage appears to be an induced voltage on the studs and this is probably safe?
    • The mechanism of the coupling of the water to the studs is possible, albeit incredibly unusual?
Children
  • 0 in reply to Asli

    I'm relieved that in the low Z mode, the voltage falls away. It is a bit odd that the voltage rises the more circuits are active, but perhaps there is wiring from each circuit clipped  or tied to the metal studs and so capacitvely coupled L_E  
    Regs dont need the studs to be earthed but, like foiled plasterboard they often are, kind of fortuitously ,by the back boxes for light switches and so on. If it eliminates a risk, then a length of wire held by a self tapping screw into the stud at an exposed end, and then connected to the CPC ('earth') of the nearest light switch or socket would remove that voltage difference.
    Personally I'd suggest do the earth in the garden test at a convenient moment, as it is always reassuring to know that the company supplied 'earth' is still connected to the real thing somewhere.
    Mike

Community Rules & Guidelines