Shock from fully earthed sink!

[PeterDawes]

Here's a strange one for you geniuses.

My brother has a house near the shoreline. Some people have complained they get a shock from the kitchen sink, especially when they've come in from the beach in wet flip-flops or bare feet.

I put a sensitive voltmeter between the sink and a damp cloth on the quarry-tiled floor. It revealed a PD of anything up to 4.2 volts - surely enough to produce a tingle on damp skin (which of course is reported as a "jolt" or "shock" when you're not expecting it).

Time to call in sparky. The guy couldn't find anything wrong with the wiring, even though it's quite old, but said he could cure it with an earthing rod. He drove a four foot (so I'm told) rod into the ground right beside the sink and strapped it directly to the stainless steel.

(The house is built on what we believe is heavily compacted sand and clay. The rod went straight down into it.)

Some cure! People are still complaining of shocks and a subsequent check shows typically 3.5 volts on the same test.

What goes on here? In theory we have a dead short between sink and ground yet it's still producing a voltage. At first I thought there might be some electrolytic action from the salty "earth" below but it seems unlikely this could generate enough current to cause a noticeable jolt, especially through quarry tiles.

I'm beginning to wonder if there is something up with the mains supply itself. Oddly enough, the guy next door has complained that he gets a "tingle" off an outside tap (house is fed by a separate overhead cable from a shared pole).

Any clues, gents? Incidentally, I'm well acquainted with microelectronics but less familiar with the black art of mains power supply.

 

[westie101]

Call your DNO emergency line now, do not pass go do not collect anything. Inform the that both premises have a problem

 

[Spark123]

Driving a rod in and bonding the sink isn't a good idea, in essence the rod is now part of an extraneous conductive part so ought to be main bonded back to the Main Earthing Terminal.
Kitchen sinks on the other hand are not extraneous on their own and do not usually require bonding. What materials are the pipes supplying it and the drain?

 

[bernardgreen]

As Westie says "Contact the DNO emergency line and report it "

What is probably happening is that the sink is "earthed" by a connection to the "earth" of the electrical system. Very often this earth is "created" not by a connection direct to earth as in an earth rod but instead by a connection to the incoming neutral which in a good system is very close to ground voltage. But if the supply network is not at its best then the neutral can be some way away from ground potential so between the "earth" inside the house and real ground there is a voltage difference.

If this voltage is large enough to be noticed as a shock ( or tingle ) then the DNO need to be told as if the fault on the network gets worse then today's tingle could become tomorrow a very nasty shock.

 

[PeterDawes]

Spark123: I think the pipework is standard 15mm copper tube. Will have to check as the house is some distance from here. The drain is definitely plastic all the way back to the gully.

Bernard: That makes sense. Evidence of a fault in the supply is growing.

Like a twit, I didn't take DC readings when I was there - to eliminate the possibility it's caused by something else - but I think we can assume it's stray mains-borne voltage.

What beats me is that, assuming there is virtually zero resistance between the sink body and the point where the rod enters the ground, we are effectively finding 3-4 volts across the surface of a three-foot stretch of quarry tiles laid on cement laid on sand.

Brother has been reluctant to call in the leccy company because he thinks they will condemn the whole house wiring (which as I said is pretty old and botched up over the years). I've told him he may have to bite the bullet and get it rewired.

Meanwhile, do you know if are they obliged to investigate regardless of the state of the internal wiring?

 

[holmslaw]

..

 

[PeterDawes]

Thanks for your advice, guys. Very helpful.

 

[RF Lighting]

Have you rung them yet?

Seriously you need to do right this second.

Someone could get killed if things are left as they are right now.

As for your 'electrician', find one who knows something about electricity for next time you need one.

 

[sparkticus]

Time to call in sparky. The guy couldn't find anything wrong with the wiring, even though it's quite old, but said he could cure it with an earthing rod. He drove a four foot (so I'm told) rod into the ground right beside the sink and strapped it directly to the stainless steel.

Call your DNO as many others have suggested. The electrician who installed the earth rod had no clue what he was doing. Suggest you/your brother contact him and ask for money back.