TT Earthing Systems - Interest by New Zealand

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.

Introducing TT earthing in New Zealand and having electrical installations where fuses will not blow and MCBs will not trip when there is a fault to earth is going to need some getting used to.


Presently with MEN you cannot fit a RCD upfront of the installation or on many distribution circuits, even if you wanted to because the MEN links would make them trip.


I was thinking that TT could be introduced without any issues of having to deal with existing installations that need upgrading, allowing all new TT installations to be installed to the new regulations.


But it is not quite that simple as existing MEN installations could be converted to TT when an EV charging point is installed by removing the MEN links and adding RCD protection, reusing the existing earth electrodes.


The starting point has to be to decide what RCD protection is required Type A or B, 30, 100 or 300 mA tripping current , 63, 80, 100 amp rating or more.


It could be that the preferred choice of RCD may not be available in New Zealand at present, because there is not currently any demand for them as you cannot use them on MEN systems, so the introduction of TT will require consultations with manufacturers and suppliers of RCDs.


My initial thoughts for the layout of a domestic installation based on UK systems would be an upfront 100 amp 300 mA Type A Double Pole RCD to protect the consumer units and distribution circuits, then 30 mA Type A double pole RCBOs to protect the final circuits. Which is to a higher standard than is generally currently installed in the UK.


But the EV charger may possibly need a Type B RCD, in which case availability becomes a bigger issue still.


Andy Betteridge 

Hi Peter,


We have a device that is used for EV Charging in the UK which does not require an earth rod or indeed any reference to Earth to reliably work, the device needs a 3 phase supply but single or 3 phase loads can be protected on the load side. Importantly our technology cannot be set on a fault condition for example a Voltage between N-Vn over 70v as per BS7671 A1


If you wish for more information then please feel to contact me on matt@matt-e.co.uk


Kind Regards


Matt

Andy, thanks for the explanation of the neutral earthing resistor.  However, 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.   


I don't think that we have ever had a TN-S LV system in NZ - if so, it's been lost in the mists of antiquity, even for NZ's comparatively short history.   A few of our EDBs use resonant neutral earthing at zone substations to permit 11 kV feeders to remain in service with a permanent earth fault on one phase - that is a well established distribution practice in some parts of the world.   Expensive to install so it's not widespread.   


As for the UK, the government here in NZ (pushed by our Greens) is also hell for leather in respect to promoting the adoption of EVs and the phasing out of ICEVs altogether in the next 10 to 20 years..  But I'm sure it remains largely ignorant of the problem of EV charging equipment being supplied from MEN systems.    Our NZ EV charging guidelines documents require the use of supplies from MEN installations as there is no available alternative at present.   Our regulator is well aware - even if the ESI here has not been hammering the need for an alternative because of the risks of broken PEN conductors - mainly in services rather than in the LV reticulation. 


We would not look for a wholesale change from MEN, just the ability to use TT for, say, a selected distribution board supplied from a MEN main switchboard, if that is a safe solution to the problem (having due regard to the need to have good separation between the MEN earth electrode and the TT earth earth electrode) .   


So there is bound to be interest in devices that will reliably detect broken PENs and interrupt all live conductors in such an event.   Might be a much cheaper solution!   


I'm aware that, with a three phase supply, it would be possible to derive the TN-C neutral voltage as a reference against the voltage on the PEN conductor but, as has been said, that won't apply for single or two phase supplies to domestic premises.  Hence I would think that an earth reference electrode would be required, located well away physically from the MEN earth electrode.   


It has been a good discussion so far.   It will all help as I'm preparing to write a report on TT for my committee to submit to the regulator.   


Regards


Peter Browne

My house was built in the 1960’s and has a looped service, the cable comes into my garage then loops back out to supply the house next door, as I understand it the house next door will need a new independent supply before either or both houses can have an EV charging point.


Andy Betteridge

Thanks John, I fully understand that from the 7671 perspective but from my reading of the product blurb, if the device is installed as per manufacturers instruction then any concerns with respect to loss of neutral are at least brought to a tolerable risk level. The idea that NZ would dump MEN in favour of TT just to quell the concerns raised about loss of neutral with respect to EV roll out seems superfluous if a simple commercial product is available as an alternative.

I don't think as a matter of principle BS 7671 would include a patented device for anything.


The new 772 now has a provision in 722.411.4.1 in indents iii, iv and v for manufactuers to bring novel products to the market products that deliver the required level of safety. As BS 7671 is not a product standard the Note 5. provides guidance on the requirement for devices to meet product standards.

I was under the impression that the Zappi charger had sorted the concerns re lost neutral on all systems, single-phase and three-phase without the need for electrodes. 7671 didn’t recognise because commercial patents were pending. Perhaps someone more clued in on the EV charging side could confirm or otherwise.