Alan M Walton said:

So why can't we use the same logic in other locations.

Which ones in particular?

There are some problems with PME in some special locations, that differ from the general "electric vehicle" approach.

Caravan sites - Section 708 - and Marinas and shore connections for inland navigation vessels - sections 709 and 730 - issue here is ESQCR prohibits distributor's PME earthing terminal being used (legal issue).

Caravans and Mobile & Transportable Units ... EV (usually on tarmac) different to caravans and mobile & transportable units on muddy fields.

Diverted neutral currents in water can be problematic at lower levels than those which would lead to touch voltages at 50 V AC or more on PEN conductors (as sometimes noted in swimming pools) being another reason why open-PEN (as set up for EV) may not be suitable there even if legislation permitted.

I'm just opening a debate and getting my head around the subject of automatic PEN disconnections.

Most of these the potential difference measured from an artificial/ calculated base voltage.why can't we make them measure the difference between the actual supply earth potential and the true earth potential. Once there is a harmful difference the supply could be disconnected including separating the different earth's. If there no supply or PME there's no path for the high true earth voltage to leak back In to the installation. 

All I'm thinking is that we rely on a TT earning arrangement and hope the RCD protect us. Whilst still there could still be a high true voltage present. Which  isn't been addressed.  If there wasn't any power it would be looked into.

Sorry If I used the wrong terms. I'm just trying  to get my head around it and looking for the future.  Except places where supply is always needed.ie. Hospital's.  Surely identifying the problem and solving it is better than copping with it.

Hope I'm making some sense in my maddness.looking forward to more educated thoughts.

Healthy engineering debate is always good.  It is how we make things better

Alan M Walton said:

why can't we make them measure the difference between the actual supply earth potential and the true earth potential

If you are going to put in a good enough rod (or mat, etc.) to act as a reference earth, you may as well make it TT in the first place.

Alan M Walton said:

why can't we make them measure the difference between the actual supply earth potential and the true earth potential

If you are going to put in a good enough rod (or mat, etc.) to act as a reference earth, you may as well make it TT in the first place.

Depends what earth is good enough. to trip a 100mA RD for example  - sort of thing you may want at the incomer, you may like 1A of current to trip fast - so electrode resistance of 230 ohms or better. Not too hard in most places I agree. To act as just a voltage reference for neutral wander you need less than a mA so can tolerate a very minimal contact area.

Consider how weedy the probes are on the earth test meter where we are not too worried about current.

If you would want to  do that for a permanent install is another matter - but there will be situations (buried ordnance comes to mind but there are less spectacular examples) where you really do not wish to spike too deeply.

Mike,

Hi Mike, when calculating the resistance of a rod electrodes what is your preferred source to use for soil resistivity values throughout the UK? 

Having measured electrode resistances differing by up to 10:1  from "identical" rods  a hundred yards apart I take it all with a pinch of salt and try and measure a test electrode if the application is for something more critical than firing an RCD. Back-filled ground on a building site in particular may not match the map.

For converting a test electrode result back to a bulk conductivity I often use the Mil Handbook formulae in reverse, but pretty much the same maths  turns up in the BS and IEC docs as well, just not for free.

Mike

e.g. https://www.wbdg.org/FFC/NAVFAC/DMMHNAV/hdbk419a_vol1.pdf

chapter 2.

Yes Mike, but that is a 3 electrode test set.

My point was whether the "actual supply earth potential" with its transformer earth, or the measurement rod potential represents true zero. It's all relative is it not?