Indeed, though there is an important  difference, - in TT the RCD protects you against insulation faults that pass current to ground, within your installation - you do not care if L or N is live and an interruption of neutral loses the supply but makes nothing immediately  live.

The PEN protection is there in case the company supplied earth floats off to a dangerous potential, and is protecting you from faults outside your installation causing an immediate danger in the form of live metalwork.

Arguably if you have PME you may actually want the benefit of protection against faults both inside and outside your premises, indeed for sockets you probably have an RCD anyway. One could have an O-PEN type device at the origin of all PME installations - and some accidents would indeed be avoided. It is just the the EV charging problem increases the danger associated with the unlikely fault occurring - the chance of it happening is unchanged,

Mike

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?

Alan M Walton said:

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

Good questions ... and certainly no problem with that, because it's how we all move forwards. The issues you raise have been considered for quite some time.

The supply earth potential varies with that of the 'general mass of the Earth' (and in PME, what do you exactly mean by 'supply earth potential' because there are differences in potential along a PEN conductor !).

What exactly do you mean by "true Earth" and where can we find that potential, given the amount of buried metalwork connected to the PME earthing system, that also carries a proportion of the neutral return current under normal conditions ! And coupled to that, is the issue that voltage can eb transferred from one place to another by extraneous-conductive-parts connected to METs of other installations in the PME network.

Given this, it's only a short step (no pun intended) to see that the voltage at your feet ... or the voltage on extraneous-conductive-parts you are touching ... is not always "true Earth".

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.

You can - that's option (iii) of reg 722.411.4.1 (rather than option (iv)). The issue is that it means banging in a local rod - in ground that's likely to have buried services already in it (electric, water, gas, drainage, possibly landlines) - hand digging and backfilling isn't ideal either from the point of view of good contact with solid earth nor installation costs.

   - Andy.

Picking up on Mike's point:

Also electrode-less PEN fault sensing  relies on detecting the L-N voltage going wild during the fault. It is not perfect - there will be a range of conditions where the earth voltage rise may be dangerous without the LN voltage going far enough out of limits to guarantee the fault being detected.

There was a discussion here open-pen-detection-for-722-411-4-1-iv which attempted to illustrate the problem with these devices are used downstream of 3-phase systems - under certain conditions it can permit persistent touch voltages far in excess of the 50V that's normally the limit. On totally single phase systems (e.g. rural caravan site fed from a single phase pole transformer) the risks should be less but those installations are very often TT anyway.

   - Andy.

Detecting an open pen is one thing but surely the risk can be further mitigated if it does happen by fitting current detection in the cpc going to the car and interrupting it at a certain threshold. I think this is built in to the Myenergi Zappi. 
Or you could all jump on the ferry to Ireland. Most of the country is PME but no one worries about open pen conditions. The take up of EVs is substantial here and you don’t see too many bodies lying in driveways, at least not during the week when the bars are less busy!