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 

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.

It’s a bit of a sod if you decide that TT installations should be protected by a RCD that is no available or even made.


An upfront 100 amp 100 mA Type B S-type time delayed RCD could be considered as a suitable main switch, but as far as I know no such device is made.


If you need a 30 mA Type B RCD to protect an EV charger is there a RCD that you can install upfront of it that will afford discrimination?


Andy Betteridge

Double insulated cars only allowed to be imported to NZ?
https://communities.theiet.org/discussions/viewtopic/1037/25909#p137872

Thank you for the continuing comments on the above theme.    Three phase supply to a domestic installation would be very rare and would be justified only by a mansion very heavy electrical loading or else a three phase industrial load in the garage or workshop.   The norm is a 60A single phase supply and unless in new subdivisions is likely to be overhead by open wires (less common these days) or a neutral screen cable.   At least the overhead neutral screen cable is inherently safe but its neutral tail connections may not be.   Ideally a double clamp on the screen would be wise but does not always apply.   i


The concept of a Class 2 EV is interesting and, who knows, we may all be driving around in EVs with fibreglass or plastic chassis in a few years' time.  Not sure about carbon fibre - might be conducting!   


Regarding RCDs, the need for a Type B RCD when supplying power electronic equipment is appreciated and is certainly advised here.   I had not contemplated the use of a Type B RCCB for front end property protection purposes.   My thoughts are that we could have a 300mA or 100mA Type S RCCB as the main switch on a TT main switchboard (or on a TT distribution board fed from a MEN main switchboard) with a 30mA Type B RCBO supplying the EV charging equipment and 30mA Type A RCBOs supplying the close by electrical equipment so there is an equipotential zone in the vicinity of the EV.    But there would  be several possible solutions and we have plenty of time to think about it while we change the regulations - never an easy matter!      


By the way, we require all RCDs in MEN installations to interrupt all live conductors as a matter of course, which should avoid any problem with an earth faulted neutral conductor.  Same will apply to any TT installation.  


Regards 


Peter Browne

If domestic properties generally have 60-amp supplies that allows the use of RCDs rated at 63-amp made for the European markets, so immediately avoids one of the problems we have in the UK with having 80 and 100-amp domestic supplies.


Andy Betteridge

My thoughts are that we could have a 300mA or 100mA Type S RCCB as the main switch on a TT main switchboard (or on a TT distribution board fed from a MEN main switchboard) with a 30mA Type B RCBO supplying the EV charging equipment

RCDs and d.c. leakage is yet another can of worms.


As I understand it, B-type RCDs aren't blinded by d.c. residual currents, but may not trip for less than 60mA d.c. (compared with 30mA a.c.) - whereas A and AC type RCDs can be affected (blinded, or in some way made less sensitive) by d.c. residual currents as low as 6mA. So putting  B-type downstream won't necessarily protect an upstream A (or AC) type (whether time delayed/selective  (s-type) or not). So the rule of thumb seems to be, if you need a B-type anywhere, then everything upstream also has to be B-type.


An alternative approach is to arrange for the downstream protection to trip at (max) 6mA d.c. - sometimes that's built into the charge point itself, or is sometimes available as a separate unit (sometimes called an RDC-DD (Residual Direct Current Disconnection Device)), and some RCD manufacturers offer them built into what otherwise would be an A-type RCD and call it something like an EV type.


(The next question is whether the d.c. leakage is a feature of normal operation, or just faults, so in cases where you have more than one charge point (or other similar electronic or d.c. containing loads) in the same installation, whether you need to consider the cumulative effect of d.c. residual currents...

    - Andy.