Do NZ domestics often have 3-phase supplies? (It's rare in the UK - hence the lack of appetite for that kind of cut-off device here). Three phase is a bit more common on the European continent for domestic supplies - but far from universal I think, and some places (e.g. France) I believe has been phasing it out in favour of single phase - but France (along with much of southern Europe) much prefers TT anyway.


TT'ing an entire installation, since we're trying to avoid the influence of a broken MEN, can be tricky since any metallic services (e.g. water or gas pipes) that are continuous to neighbouring properties could import the very MEN voltage you're trying to avoid. TT'ing a small area, such as a detached garage, is sometimes simpler. Another option which I don't think has been mentioned yet, but has often been done in the UK is to TT just the charge point itself - i.e. insulate the installation's PE conductor at the charge point and connect it to a local rod instead - where the charge point contains its own RCD (as most seem to these days) it can be as simple as that. As with any transition to TT it needs a little care to ensure there can't be faults between the incoming live conductors (before the RCD) and TT earthing system, which can be easier if the charge point has an insulating enclosure, or an additional RCD upstream can mitigate that risk.


 

the MEN system and no doubt the PME system provides safety from line to earth faults by the high fault current that returns back to the transformer via the PEN conductor that then operates the MCB or blows the sub-circuit or service fuse to interrupt the phase supply.

That does raise the "interesting" problem of mixed disconnection times. I presume you have requirements for disconnection times not unlike ours for TN systems - e.g. max 0.4s for small final circuits, but anything up to 5s for anything else - and potentially longer again for faults on the public supply network. During a L-PE fault the PE/MEN/PEN conductor will be dragged up to something approaching half the line voltage (a pretty hazardous 115-120V say) at the point of the fault until the overcurrent device opens. I doubt that a few extra electrodes of tens or even hundreds of Ohms each will help much (other than to raise the voltage on the soil immediately surrounding each electrode) since the main potential divider (the L and MEN/PE conductors feeding the fault) are likely to be far less than one Ohm. That voltage in then imposed on any metalwork connected to the earthing system - including our EV.  Normally "importing" an earth fault from upstream of the final circuit or outside of the installation it mitigated in two ways - firstly by the interior of most buildings being substantially insulating so there's no widespread true earth potential to complete the shock circuit, and by anything metallic that might introduce an true earth potential (e.g. gas & water supply pipes) being solidly bonded to the installation's earthing system. Outdoors however, on damp ground and random metallic things (like fences & gates) unlikely to be bonded - those mitigations don't apply and so people could be subject to the full fault voltage for the full disconnection time. A TT system has advantages on that score - not only should it be immune to 'importing' earth faults from TN parts of the system, any earth faults with the TT system (even upstream of the circuit feeding the EV charge point) typically disconnect far more quickly than by fuses - even a time delayed RCD should open with 200ms or even 150ms at a decent earth fault current.


  - Andy.

Do NZ domestics often have 3-phase supplies? (It's rare in the UK - hence the lack of appetite for that kind of cut-off device here). Three phase is a bit more common on the European continent for domestic supplies - but far from universal I think, and some places (e.g. France) I believe has been phasing it out in favour of single phase - but France (along with much of southern Europe) much prefers TT anyway.


TT'ing an entire installation, since we're trying to avoid the influence of a broken MEN, can be tricky since any metallic services (e.g. water or gas pipes) that are continuous to neighbouring properties could import the very MEN voltage you're trying to avoid. TT'ing a small area, such as a detached garage, is sometimes simpler. Another option which I don't think has been mentioned yet, but has often been done in the UK is to TT just the charge point itself - i.e. insulate the installation's PE conductor at the charge point and connect it to a local rod instead - where the charge point contains its own RCD (as most seem to these days) it can be as simple as that. As with any transition to TT it needs a little care to ensure there can't be faults between the incoming live conductors (before the RCD) and TT earthing system, which can be easier if the charge point has an insulating enclosure, or an additional RCD upstream can mitigate that risk.


 

the MEN system and no doubt the PME system provides safety from line to earth faults by the high fault current that returns back to the transformer via the PEN conductor that then operates the MCB or blows the sub-circuit or service fuse to interrupt the phase supply.

That does raise the "interesting" problem of mixed disconnection times. I presume you have requirements for disconnection times not unlike ours for TN systems - e.g. max 0.4s for small final circuits, but anything up to 5s for anything else - and potentially longer again for faults on the public supply network. During a L-PE fault the PE/MEN/PEN conductor will be dragged up to something approaching half the line voltage (a pretty hazardous 115-120V say) at the point of the fault until the overcurrent device opens. I doubt that a few extra electrodes of tens or even hundreds of Ohms each will help much (other than to raise the voltage on the soil immediately surrounding each electrode) since the main potential divider (the L and MEN/PE conductors feeding the fault) are likely to be far less than one Ohm. That voltage in then imposed on any metalwork connected to the earthing system - including our EV.  Normally "importing" an earth fault from upstream of the final circuit or outside of the installation it mitigated in two ways - firstly by the interior of most buildings being substantially insulating so there's no widespread true earth potential to complete the shock circuit, and by anything metallic that might introduce an true earth potential (e.g. gas & water supply pipes) being solidly bonded to the installation's earthing system. Outdoors however, on damp ground and random metallic things (like fences & gates) unlikely to be bonded - those mitigations don't apply and so people could be subject to the full fault voltage for the full disconnection time. A TT system has advantages on that score - not only should it be immune to 'importing' earth faults from TN parts of the system, any earth faults with the TT system (even upstream of the circuit feeding the EV charge point) typically disconnect far more quickly than by fuses - even a time delayed RCD should open with 200ms or even 150ms at a decent earth fault current.


  - Andy.