Some of my work involves high frequencies and short pulses, where it is quite possible for many reversals of voltage to exist in the 1m or so distance of a step voltage - if you like not only is there a voltage gradient between your feet, there is also a phase shift, of more than one rotation .My view and description on what an earth electrode is for is coloured by that view.


I do not think we should talk about 'the' earth voltage as if there is some 'plate at the end of the universe' (apologies to fans of Douglas Adams)  at zero volts and we must connect to that by putting an electrode far far away. This is a  simplification that makes  text books easy to write.

Rather I think there are many  things we can refer to as 'an earth', but these are not always at the same voltage as each other.

What I think is more important for the shocked user  is the local voltage gradient - we need the car, caravan or boat to have  the smallest sensible  offset voltage relative to whatever else is in touching or stepping range.

To me the most useful 'earth' is the potential of the terra-firma beneath your feet - but by that definition, your earth does not have to be at the same voltage as mine, and generally will not be.


(Rather like the birds that perch on the 11kV line  - their local earth is approx 6kV RMS different from mine, but two birds on the same wire can come along side each other without incident, however if I being earthed to terra-firma were to touch the wire, or touch the birds for that matter, it would be very bad indeed for both of us - to the birds I am live and 'off earth' . To me however, their "earth" is what the RADAR  power supply folk and some particle accelerator texts call a "flying deck"  - they could, if they were not birds, build sensitive electronics things into little metal boxes and earth them to the wire and as far as the kit in the boxes was concerned no voltage gradient would exist. )


Now Annex H4 is about car charging and correctly points out that a leaking PME service (or indeed anything injecting a voltage ) under ground creates voltage contours on the surface., and if you place the TT electrode in that region of influence, the voltage on the electrode  bounces up and down in time with the voltage on or near the surface.

Thinking cars, but realizing metal framed caravans are similar (a wooden one would not matter so much )

If you park your car such that is straddles such a fault, then it will couple to the surface like a parallel plate capacitor from the under body, and via the tyres resistively.

Both of these are weak effects, at 50Hz capable of passing only tens of microamps for 230V.

But if you stand beside the car, you too are at this local terra-firma surface  voltage and what you do not want to do is to earth the car body via a CPC to some potential widely at odds with your own.


The PME earth voltage is obviously not the right thing, nor is a TT  electrode voltage from several metres away.  Ideally the electrode would be under the car, but  sensibly near is the best we can do.

Some of my work involves high frequencies and short pulses, where it is quite possible for many reversals of voltage to exist in the 1m or so distance of a step voltage - if you like not only is there a voltage gradient between your feet, there is also a phase shift, of more than one rotation .My view and description on what an earth electrode is for is coloured by that view.


I do not think we should talk about 'the' earth voltage as if there is some 'plate at the end of the universe' (apologies to fans of Douglas Adams)  at zero volts and we must connect to that by putting an electrode far far away. This is a  simplification that makes  text books easy to write.

Rather I think there are many  things we can refer to as 'an earth', but these are not always at the same voltage as each other.

What I think is more important for the shocked user  is the local voltage gradient - we need the car, caravan or boat to have  the smallest sensible  offset voltage relative to whatever else is in touching or stepping range.

To me the most useful 'earth' is the potential of the terra-firma beneath your feet - but by that definition, your earth does not have to be at the same voltage as mine, and generally will not be.


(Rather like the birds that perch on the 11kV line  - their local earth is approx 6kV RMS different from mine, but two birds on the same wire can come along side each other without incident, however if I being earthed to terra-firma were to touch the wire, or touch the birds for that matter, it would be very bad indeed for both of us - to the birds I am live and 'off earth' . To me however, their "earth" is what the RADAR  power supply folk and some particle accelerator texts call a "flying deck"  - they could, if they were not birds, build sensitive electronics things into little metal boxes and earth them to the wire and as far as the kit in the boxes was concerned no voltage gradient would exist. )


Now Annex H4 is about car charging and correctly points out that a leaking PME service (or indeed anything injecting a voltage ) under ground creates voltage contours on the surface., and if you place the TT electrode in that region of influence, the voltage on the electrode  bounces up and down in time with the voltage on or near the surface.

Thinking cars, but realizing metal framed caravans are similar (a wooden one would not matter so much )

If you park your car such that is straddles such a fault, then it will couple to the surface like a parallel plate capacitor from the under body, and via the tyres resistively.

Both of these are weak effects, at 50Hz capable of passing only tens of microamps for 230V.

But if you stand beside the car, you too are at this local terra-firma surface  voltage and what you do not want to do is to earth the car body via a CPC to some potential widely at odds with your own.


The PME earth voltage is obviously not the right thing, nor is a TT  electrode voltage from several metres away.  Ideally the electrode would be under the car, but  sensibly near is the best we can do.