• If the TT electrode is too close (say within 1 m), it's effectively touching PME, so you've not got separation and therefore no point in the electrode. You also need to allow for ground subsidance.

• You might well then want to argue about the person not standing at the "general mass of earth", but sometimes the voltage drops off quite quickly, even a couple of metres, and there's no guarantee the potential at the feet of a person will be that at the TT earth electrode.

• Conversely, and because of the previous point, in small curtilage properties, you might well be standing over some metalwork (say incoming gas or water pipe) connected to the PME earth, and in certain cases, you simply return the  PME touch voltage ... this of course means that going to the trouble of TT in many dwellings is pointless !
  
   

I'd like to comment on  that.

If that was really true, then we would have a safety problem getting into the car, even when the charger is not present, as you are suggesting that significant step voltages are  present due to the house earthing in normal operation, and the car is bridging them.

Where there have been I think 3 deaths of humans from exposed step voltages, and more dogs and some famous horses in the last decade or so, as far as I recall,  these have all be associated with damaged or unterminated underground cables exposing live. As regards earthing faults,  There has been 1 plumber killed by  live buried water main that was acting as CPC until he interrupted it.

In contrast there are a some  hundreds  of  lost neutral events per year in the UK, and this compares with other countries that use aluminum armour on their underground cables.

In practice the surface potential appearing due anything buried at all but the shallowest level is not present as a narrow stripe above the object, rather the surface potential  is a sort of diffused average value with ripples due to sources below, and changes gradually.  Surface finish such as tarmac or concrete are significant, hence the preponderance of free-draining gravel at substations.


It is also worth remembering how RCD blinding works - the core of the current transformer saturates in one direction, so once all the magnetic domains have rotated to align with the external field, and no more can move, it behaves more or less  as an air core, rather than a ferromagnet, and does not 'come unstuck' and start to act as a transformer again, except for those parts of waveform where  the sum of the AC and the DC is back in the linear part of the magnetisation curve, i.e. within a few tens of mA of zero.When the AC term is large, the detector side waveform is largely un affected by the DC  (and the secondary voltage is far from sinusoidal.. ), while when the DC is larger than the AC, no signal is  detected. What happens  in between is slightly unclear as it depends on how well   the detector circuit responds to a 'chopped-up' sinewave where half cycles of one polarity are much peakier than the other.  

I suspect that manyt A type RCDs are just AC designs revisited,  perhaps with a slightly bigger core. Certainly the sensitivity to half wave rectified DC is about half that to a sine wave which would be consistent with this.


                

• If the TT electrode is too close (say within 1 m), it's effectively touching PME, so you've not got separation and therefore no point in the electrode. You also need to allow for ground subsidance.

• You might well then want to argue about the person not standing at the "general mass of earth", but sometimes the voltage drops off quite quickly, even a couple of metres, and there's no guarantee the potential at the feet of a person will be that at the TT earth electrode.

• Conversely, and because of the previous point, in small curtilage properties, you might well be standing over some metalwork (say incoming gas or water pipe) connected to the PME earth, and in certain cases, you simply return the  PME touch voltage ... this of course means that going to the trouble of TT in many dwellings is pointless !
  
   

I'd like to comment on  that.

If that was really true, then we would have a safety problem getting into the car, even when the charger is not present, as you are suggesting that significant step voltages are  present due to the house earthing in normal operation, and the car is bridging them.

Where there have been I think 3 deaths of humans from exposed step voltages, and more dogs and some famous horses in the last decade or so, as far as I recall,  these have all be associated with damaged or unterminated underground cables exposing live. As regards earthing faults,  There has been 1 plumber killed by  live buried water main that was acting as CPC until he interrupted it.

In contrast there are a some  hundreds  of  lost neutral events per year in the UK, and this compares with other countries that use aluminum armour on their underground cables.

In practice the surface potential appearing due anything buried at all but the shallowest level is not present as a narrow stripe above the object, rather the surface potential  is a sort of diffused average value with ripples due to sources below, and changes gradually.  Surface finish such as tarmac or concrete are significant, hence the preponderance of free-draining gravel at substations.


It is also worth remembering how RCD blinding works - the core of the current transformer saturates in one direction, so once all the magnetic domains have rotated to align with the external field, and no more can move, it behaves more or less  as an air core, rather than a ferromagnet, and does not 'come unstuck' and start to act as a transformer again, except for those parts of waveform where  the sum of the AC and the DC is back in the linear part of the magnetisation curve, i.e. within a few tens of mA of zero.When the AC term is large, the detector side waveform is largely un affected by the DC  (and the secondary voltage is far from sinusoidal.. ), while when the DC is larger than the AC, no signal is  detected. What happens  in between is slightly unclear as it depends on how well   the detector circuit responds to a 'chopped-up' sinewave where half cycles of one polarity are much peakier than the other.  

I suspect that manyt A type RCDs are just AC designs revisited,  perhaps with a slightly bigger core. Certainly the sensitivity to half wave rectified DC is about half that to a sine wave which would be consistent with this.