but IEV does define 'island' as a term for use in a power system:

Humm, Depends how you interpret the brackets - I would have read "island (in a power system)" as "in a power system, the term 'island' means..." rather than the term 'island' may only be used in the context of power systems.

so metaphorically speaking, an "island" in a sea of connected-mode installations all around.

Indeed - but to my mind the same metaphor works just as well with earthing systems - the consumer's TT installation as its own little refuge of true earth in a sea of PME...

I'd see no contradiction if we were to have:

island (in a power system) ..., and

island (in an earthing system).....

in the same sense that in BS 7671's own definitions we have:

"Central power supply system (low power output)" which clearly doesn't restrict the use of the term "Central power supply system" to just the context of low power output.

    - Andy.

AJJewsbury said:

Indeed - but to my mind the same metaphor works just as well with earthing systems - the consumer's TT installation as its own little refuge of true earth in a sea of PME...

But that would require separation from the PME (8 to 10 m all-round, including underground), and we don't know what's underground connected to PME.

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 ...

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

I think you are on safer ground in the back garden than you would be in the front garden. Unless next door has built a studio, granny/gramps flat or hot tub at the back of theirs, or intends to. And this would be at the back of the back garden.

The only services usually running through back gardens are sewers, and lifting the lids of inspection chambers shows their routes. And they are usually quite close to the rear wall.

pity, as 'ground island'  is a common and well understood phrase among my RF colleagues, and seems to be understood in pulse power circles too. Perhaps it is just the company I keep ;-)

Also 'island' is used in PCB layout when the ground-plane is cut and tabbed  (creating a region on layer on on a mezznine on a layer above or below that connects to the plane on one edge or corner only ) so cross currents in the main plane do not introduce unfortunate gradients in delicate circuits, The classic example seen in every mobile phone being a PLL with many MHz/volt of VCO tuning on a transmitter board with a PA. On a solid ground-plane the PA keying on twangs the oscillator as the large transmit current introduces many tens of uV between adjacent ground vias on the PCB- but if the PLL is on its own ground island, then the tuning and gnd Iines of the VCO  are not pulled in this way, and the frequency is un-modulated.  This EMI problem and solution is very close to the power situation.

There is a lot of technical vernacular that is not ideal, but if it accelerates getting a complex idea across to those in the know with as few words as possible, it is likely to stay in common use.  The best that the ever-stretched standards teams can do in that case is probably to run behind the new lingo and add to the glossary as required.

M

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.