So, I'm not sure the concept is really appropriate to earthing in urban areas ...

There are a lot UK installations that aren't urban though (at least in the sense of city centre or inside the medieval town walls) (cue arguments of the meaning of the word suburban). I reckon any greenfield newbuild since the 1980s is pretty unlikely to have buried metalwork in contact with the soil and connected to the PME system - both gas and water pipes will be PE and cables will be plastic sheathed. Even if a concept isn't useful everywhere it may well be useful in some circumstances.

A closer analogy in urban areas where you can't get proper separation would be a TT pontoon bobbing up and down on the PME GPR (generally low, but can get choppy). Not that you'd necessarily feel anything because you might be bobbing up and down with it ...

Yes indeed. That chimes very nicely with a little idea I've been working on, of which later. The word equipotentiality might be inexact as a principle, but the concept is still quite useful.  I might even be tempted to steal your words to describe it!

   - Andy.

Indeed - though 'raft'  might be better than calling it a pontoon as that would usually attach to land at one end. To scale with the planet its more like ripples in the skin on custard but we just need a language everyone agrees on. 

I agree totally, the main thing is of course the voltage difference between the thing in the hand and the local terra-firma ground beneath the local feet - not the absolute voltage with respect to some fictional electrode at the end of the universe. The problem is that folk say 'earth' or 'zero volts' and give it no further though, when in voltage contour terms the ground is actually quite noticably lumpy, rising and falling where conducting parts inject or remove current. Indeed surveys of this non-uniformity can be used to find the lost foundations of walls and demolished buildings- though where it is possible for surveys it is more common to inject your own test current for this, and treat the 50Hz component as a nuisance.

M.

Indeed - though 'raft'  might be better than calling it a pontoon as that would usually attach to land at one end. To scale with the planet its more like ripples in the skin on custard but we just need a language everyone agrees on. 

I agree totally, the main thing is of course the voltage difference between the thing in the hand and the local terra-firma ground beneath the local feet - not the absolute voltage with respect to some fictional electrode at the end of the universe. The problem is that folk say 'earth' or 'zero volts' and give it no further though, when in voltage contour terms the ground is actually quite noticably lumpy, rising and falling where conducting parts inject or remove current. Indeed surveys of this non-uniformity can be used to find the lost foundations of walls and demolished buildings- though where it is possible for surveys it is more common to inject your own test current for this, and treat the 50Hz component as a nuisance.

M.