TNCS weirdness

[griftech]

Long time lurker but this is my first post here.

I’ve got a weird situation where I’ve got a voltage difference between the shower and the drain, and the bathroom sink tap and the metal drain plug. I’d appreciate some advice on what might be causing this, and what to do next.

I noticed in the shower earlier this week that I got a ‘tingle’ when lightly touching the chrome plated shower mixer. A cheap multimeter indicates that there is 2.5v AC between the shower mixer and the chrome plated drain in the shower tray. There is a similar 2.5v AC between the adjacent bathroom radiator and the shower drain. So basically any of the water or heating pipes and the drainage system.

The house is late 1990s, with a split load dual RCD board, on a TNCS supply. Every circuit is on a RCD, but there has never been any nuisance tripping. The gas and water are both brought into the house in plastic, but are bonded in 10mm as they enter the house regardless. There are no outbuildings and no metal pipes buried in the ground anywhere. All of the bonding is intact and connected at the MET.

Having let the drain/sink dry out for several hours the voltage reduces, which implies that the stream of water down the soil pipes is carrying the voltage. If it turn off the supply at the consumer unit the voltage disappears completely. The voltage caps out at 2.5v AC, even if I switch on some big loads like the kettle and range cooker.

It feels like the TNCS supply is floating above ‘true’ earth and the voltage is travelling through the soil pipes, but I don’t have the experience to diagnose this further. Presumably this isn’t normal and expected? Any guidance would be appreciated before I call out the DNO for something this is a non issue.

Many thanks in advance for any guidance or suggestions!

 

[JohnW2]

2.5V is not enough to give even a 'tingle'.

The most likely explanation is that you are getting spurious readings due to use of a multimeter with a very high input impedance, so that the voltages you are measuring are just things picked up by capacitive or inductive coupling to the wiring. Have you got some sort of 'load' (like an old-fashioned incandescent light bulb) you could connect across the meter. If you did that, the very small voltages you are measuring might well disappear.

It feels like the TNCS supply is floating above ‘true’ earth and the voltage is travelling through the soil pipes, but I don’t have the experience to diagnose this further. Presumably this isn’t normal and expected?

Even if the TN-C-S earth were appreciably above true earth potential that wouldn't matter, wouldn't represent a danger within the house and wouldn't allow you to find any voltages (potential differences) between 'anything and anything else' within the house IF all the required main bonding is in place, rendering the entire house an ';equipotential zone. Do you know if all required bonding is present?

Kind Regards, John

 

[flameport]

Every circuit is on a RCD, but there has never been any nuisance tripping.

Have those RCDs been tested recently, or ever? Including pressing the test button?

Having let the drain/sink dry out for several hours the voltage reduces, which implies that the stream of water down the soil pipes is carrying the voltage.

Inevitable, since the waste pipes are plastic.

Presumably this isn’t normal and expected?

It isn't. Any sensations of voltage on items in the house must be investigated and the problem fixed. These things will not get better on their own, and could get a lot worse very quickly.
Realistically, you won't be finding the problem or fixing this yourself.

 

[Harry Bloomfield]

All of the bonding is intact and connected at the MET.

I would check, or better have someone qualified check that bonding, especially around the bathroom.

 

[JohnW2]

I would check, or better have someone qualified check that bonding, especially around the bathroom.

indeed - as I asked ...

.... Do you know if all required bonding is present?

 

[ericmark]

A loop impedance tester checks the volt drop with a load and from this works out the resistance of the loop. So it should show any problems with the earth, but this

is the cheapest one I can find, at around £70, so likely better to get an electrician to test.

As to RCD again the meter costs around £35 to test leakage, and if the voltage is DNO earth to true earth the RCD will not switch it off, or detect it, the old ELCB-v did but these are no longer used.