Floating Earth on Whole House - Normal?

[FiscoKing]

Hi,

Is it normal for a residential PME earth system to 'float' 4-5V AC above real ground?

I'm measuring the 4-5V with a high impedance Volt meter. Red probe to metalwork on an earthed appliance, and the black probe held in my hand. When I touch the appliance with my hand the 5V AC drops to 0.

Every appliance in the house is like this. Wall sockets, fitted kitchen cookers etc.

I wondered if there was a break in the earth cable going back to the consumer unit, so I tried the same experiment again probing the earth rail inside the consumer unit and got the same result.

I then wondered if there was a break in the earth cable going back to the DNO fuse carrier, so I tried the same experiment again at the fuse carrier as there's a bit of exposed eath cable poking out, and I get the same result.

I get that there might be some voltage on an earth connection to real ground as all cables have resistance, so if any current flows there will be a voltage drop, but the fact that the voltage drops to nothing when touched means the earth system is floating.

DNO fuse carrier has a 'PME System Earthing Terminal' skicker on it.

Is this normal?

Thanks.

 

[endecotp]

Is it normal for a residential PME earth system to 'float' 4-5V AC above real ground?

What do you mean by "real ground"?

I'm measuring the 4-5V with a high impedance Volt meter. Red probe to metalwork on an earthed appliance, and the black probe held in my hand.

Are you imagining that your hand is the "real ground"? What kind of shoes are you wearing?

Suggestion: bury a metal object in the ground, and pour a bucket of water over it. Now measure the voltage between that and your appliance metalwork.

 

[JohnW2]

What do you mean by "real ground"? .... Are you imagining that your hand is the "real ground"? What kind of shoes are you wearing? .... Suggestion: bury a metal object in the ground, and pour a bucket of water over it. Now measure the voltage between that and your appliance metalwork.

Indeed so - but, as the OP might discover if he did all that, to have a 4-5 volt potential difference between a TN-C-S 'earth' and 'true earth' would be far from unusual, and nothing to be concerned about, wouldn't it?

 

[flameport]

I'm measuring the 4-5V with a high impedance Volt meter. Red probe to metalwork on an earthed appliance, and the black probe held in my hand. When I touch the appliance with my hand the 5V AC drops to 0.

You are seeing a tiny amount of capacitive coupling between your body and the earthed metal.
This goes away when you touch the metal, because you and the metal are then at the same voltage as they are connected together.

Is it normal for a residential PME earth system to 'float' 4-5V AC above real ground?

There is usually a small voltage difference due to voltage drop on the combined N&E conductor which supplies the building.
This is unrelated to what you are doing.

 

[JohnW2]

You are seeing a tiny amount of capacitive coupling between your body and the earthed metal.

The capacitive coupling is presumably between his body and something 'live' (rather than the 'earthed metal') ...

This goes away when you touch the metal, because you and the metal are then at the same voltage as they are connected together.

Indeed so - but it presumably would not have been there in the first place had the capacitive coupling been between his body and the earthed metal.

 

[Harry Bloomfield]

I'm measuring the 4-5V with a high impedance Volt meter. Red probe to metalwork on an earthed appliance, and the black probe held in my hand. When I touch the appliance with my hand the 5V AC drops to 0.

You are not measuring voltage on the earthed metalwork, rather you are measuring the voltage induced in your body, against the earth.

 

[FrodoOne]

Here (again) we have an "newbie" poster (using a [now cheap] High Impendence Digital Voltmeter)
discovering "Ghost Voltages" - due to capacitive/inductive coupling, or whatever.

Many on this site could write out a description of "why this is so",
that
one should use a "Low Impedance" meter for this purpose,
and
how to make a "Digital" meter "Low Impedance" etc.

Is there any existing "site" where this is described
or
can someone, better qualified than I, institute one?

 

[Harry Bloomfield]

one should use a "Low Impedance" meter for this purpose,
and
how to make a "Digital" meter "Low Impedance" etc.

Is there any existing "site" where this is described
or
can someone, better qualified than I, institute one?

Simple - all it needs is some sort of load, applied between the two probes. A 100,000 Ohm resistor would achieve that.

 

[FiscoKing]

You are not measuring voltage on the earthed metalwork, rather you are measuring the voltage induced in your body, against the earth.

Ah, OK.

If I've understood you correctly.....my slightly conductive body is one plate of the capacitor, the earth wiring is the other plate and my high impedance meter between the two is acting as the capacitor insulating layer. Electric fields from unshielded mains wiring causes charge to build up on me and that creates a potential difference on the plates of this human capacitor, and that's what I'm measuring. I presume the reason the charge on my body doesn't disipate to real ground (concrete) is becuase of the insulating rubber of my shoes. So my body is insulated on one side by the high input resistance of the meter, and on the other by my shoes.

To test this theory, I tried the same experiment as in my first post on this thread in another house, on the exposed earth block of the meter board. The AC voltage on the meter increased as I moved closer to the meter tails, and decreased as I moved further away.

If this is so then I've learned something new.

 

[JohnW2]

If I've understood you correctly.....my slightly conductive body is one plate of the capacitor, the earth wiring is the other plate and my high impedance meter between the two is acting as the capacitor insulating layer. Electric fields from unshielded mains wiring causes charge to build up on me and that creates a potential difference on the plates of this human capacitor, and that's what I'm measuring.

Not quite. As I wrote in response to flameport's comment, one side of the 'capacitor' is your body and the other side is the unshielded mains wiring around you. Through that 'capacitor' the potential of your body is being raised to a little above earth potential, and what you are measuring is the difference between that potential and the potential of the earthed appliance you are measuring to (i.e. potential of'earth wiring'). When you actually touch that earthed appliance, there is then obviously no potential difference between it and your body, so your measurent falls to zerpo.

I presume the reason the charge on my body doesn't disipate to real ground (concrete) is becuase of the insulating rubber of my shoes. So my body is insulated on one side by the high input resistance of the meter, and on the other by my shoes.

Even the concrete (unless soaking wet) would probably be a pretty good insulator, so you might well get the same results even if you stood on it with bar feet but, as above, if you did stand, with naked feet, on something that really was at earth potential, then the voltage you've been measuring would disappear.

To test this theory, I tried the same experiment as in my first post on this thread in another house, on the exposed earth block of the meter board. The AC voltage on the meter increased as I moved closer to the meter tails, and decreased as I moved further away.

As you say, that 'confirms' it

If this is so then I've learned something new.

We all do, nearly every day

 

[Harry Bloomfield]

If this is so then I've learned something new.

Something like that, but your body can also build a static charge, of quite a high voltage, from simply walking across a carpet. Hence, working on sensitive electronics, you need to discharge yourself.

 

[JohnW2]

Something like that, but your body can also build a static charge, of quite a high voltage, from simply walking across a carpet. Hence, working on sensitive electronics, you need to discharge yourself.

Very true, but static electricity is obviously a totally different kettle of fish, and would not show itself with any of the 'usual' measuring equipment.

Having said that, I've been playing with 'sensitive electronics', without 'taking any precautions', for decades, starting in the days when they were much more susceptible to static discharges than they are today, and have never knowingly 'lost' a semiconductor because of that!

 

[SUNRAY]

Very true, but static electricity is obviously a totally different kettle of fish, and would not show itself with any of the 'usual' measuring equipment.

Having said that, I've been playing with 'sensitive electronics', without 'taking any precautions', for decades, starting in the days when they were much more susceptible to static discharges than they are today, and have never knowingly 'lost' a semiconductor because of that!

Ah but...

A lot depends on a lot of things, such as what shoes you wear, what fabrics you wear your carpet, bench, the amount of water in your skin etc.

One guy I used to work with was a shocker, by that I mean contact with him usually resulted in a static shock.

 

[JohnW2]

Ah but... A lot depends on a lot of things, such as what shoes you wear, what fabrics you wear your carpet, bench, the amount of water in your skin etc. ... One guy I used to work with was a shocker, by that I mean contact with him usually resulted in a static shock.

I think that (in terms of what I wear etc.) I may well have qualified (and still qualify!) as a 'shocker' - but, as I said, I can't recall ever having knowingly destroyed a semiconductor as a result, even though I have been playing with them for many decades.