Hot Tub - small potential differance between water and the ground

[Sparks]

Hello all,

A friend of mine has just (Yesterday) had a hot tub installed at his house, in the garden.

All seems to work fine.

However, We (I) just (out of curiosity) put one probe of a multimeter in the water and the other one into the grass next to the tub.

I got a reading of about 35v AC @ 0.12mA between the two.

We can't feel any shocks etc. from the water (with bare feet on the (wet!) grass)

His earthing is provided by the armour of the main feed to the house.

He has about 3m of 2.5mm T&E from the house CU to a plastic box, then about

15m of 2.5mm armoured cable between there and the CU in the shed (Three core, using one core for the earth), then from a 20A MCB, a FCU with a 13A fuse (Via about 40cms of 2.5mm T&E) then from there (Via a metal box) 2m of 2.5mm armoured cable (Three core, using one core for the earth) to a triple pole isolator near the tub (like this) from there, about 4m of 2.5mm armoured cable (Three core, using one core for the earth) to the tub.

It is protected by two 30mA RCD's (One in the house, then a second one in a consumer unit in the shed next to the tub)

We are not getting any RCD tripping.

Is it just because this path to earth is favoured slightly, as it is a shorter distance than via the main supply armour? - If so, should we install an earth rod from the shed consumer unit, but leave the house earth connected too? (This seems the logical thing to do in my mind) If so, would

10mm cable between the CU and the rod be the right choice?

Any advice would be gladly received!

[meow2222]

Kill the power to it immediately. Something serious is wrong.

NT

[Guy King]

The message from [e-mail] contains these words:

Probably because the earth has been exported to some way from where it's actually earthed.

[Sparks]

That is what I was thinking, I assume the earth supplied by the power company is actually connected to planet earth at the substation, this, I assume, is quite a way away from the house, so some current is flowing to planet earth via the shorter route. So the solution is to install an earth rod near the tub?

Sparks...

[The Natural Philosopher]

Possibly.

In my installation here, with a substation (sic!) in the garden. My own very own substation (did you want 3 phase with that, sir?)..the outer sheath of the underground cable I had to lay is neutral AND ground both. I assume they connected that to teh frame of teh equipment in the cabinet.

That leaves a potential (sic!) way for local house earth to be very different, if large currents flow, so I ran a bloody thick cable to an earth rod whacked 5 ft into the wet clay, from the consumer unit area...I am not sure whether or not the outer of the incoming is connected to earth somehow or not...I suspect we left it as neutral only.

I managed to not destroy a valve radio once that some cheerful co apprentices had wired up with the loudspeaker across the mains, because it hummed as soon as it was plugged in, (but not switched on) a meter revealed that there was 15v AC between neutral and earth..and they had missed the fact that they had connected the switched live to the EARTHED side of the loudspeaker..and the neutral to the 'hot' side of it.

What teh effects of this crass vandalism would have been I do not know.

I had the satisfaction of getting out my soldering iron and removing all the b'modifications' , and turning it up to 11....;-)

[Guy King]

The message from "Sparks" contains these words:

That'd depend on which earth system is in use. In our house there's a tag beside the incoming cable which says which sort of earth it's got. There are several.

[meow2222]

That could explain 3.5v but not 35v.

NT

[Just Another DIY Fan]

Is it not possible that the water itself is acting as a weak electrolyte and that is where the REAL voltage being read is coming from? I.e .it's nothing to do with the mains.

Easy to test for this. Just totally disconnect ALL wiring to the mains and see if the reading is still there.

[Staffbull]

electrolytic action would supply a DC current and not AC I would have thought.

[Guy King]

The message from "Just Another DIY Fan" contains these words:

If you can find an electrochemical pair that can make 35 volts with a single cell you're going to be rich beyond the dreams of avarice!

[meow2222]

Possibly, but more likely from pyrotechnics than battery manufacture. The higher the cell voltage, the faster things fry up. Cells of 4v or so are usually over in seconds, so 35v would be a firework or bomb.

And an ac cell would certainly interest everyone :)

NT

[Just Another DIY Fan]

True but it would be interesting to know how the extremely low current was measured as well before saying that the meter was displaying the truth. Many digital meters display all sorts of rubbish under adverse conditions.

It would be interesting to put a frequency counter or a 'scope in place

to see if it was indeed 50 Hertz mains ac. It could even be picking up stray rf.

[Staffbull]

".It could even be picking up stray rf."

At 35v !! that would be some new kind of power transmission, needs to be marketed, overhead cables will be no more !!! :-)

[Guy King]

The message from "Staffbull" contains these words:

Wasn't it Tesla who wanted to transmit power by RF?

[Staffbull]

Tesla coils, yes. Transmitted across small distances as the air breaks down, lightning does it fairly well over greater distances!! not the safest option for power transmission me thinks :-)

Be a nice option though, would save me having to channel the cables in the extension :-)

[Just Another DIY Fan]

Dunno if anyone still does it but years ago was experiments with transmitting power through ground itself.

[meow2222]

in this case its displaying correct, as the source is low impedance.

It probably is, but that will contribute only microvolts. Unless someone's hidden 3 car batteries under there and put them in the earth lead, its mains. And whats worse there are 2 dangerous faults. First mains is flowing through the water due to bad insulaiton or wrong wiring, and 2nd the thing isnt earthed. Whoever wired it up should kill themselves by using it.

NT

[Grimly Curmudgeon]

We were somewhere around Barstow, on the edge of the desert, when the drugs began to take hold. I remember Guy King saying something like:

The Soap and FartGas Cell?

[Nick2]

Measuring from the earth connection itself would determine that (doubtful as you say). What is the measurement between water & earth connection? Between earth connection & ground?

Might be worth running an extension lead out from the house & measuring earth to earth continuity. Expect a few ohms reading from the length of cable. (Check volts reading first - there shouldn't be any!)

Nick

[Guy King]

The message from Grimly Curmudgeon contains these words:

Hmmm, methane fuel-cells. Won't the sulphur contamination poison the cell like it does the rest of us?