Thank you for the continuing contributions to the above subject, which are grist to the mill for us where the requirement for LV earthing is limited to driving in a spike electrode where bonding to concrete reinforcing is not possible.   


The warnings concerning the interaction of earthing electrodes in close proximity is also appreciated as if we were to convert part of a MEN installation to TT to supply EV charging and its close environs, there would need to a good separation between the driven earth and the constructed TT earth.    I take it that the reference to a neutral earthing resistor was to an IT system, which I understand is commonly used (not in NZ) to provide a reliable supply to important loads such as hospital theatres or some industrial premises.     


Lyle, I have heard of bentonite as an earthing enhancing material to use around an earthing electrode but not "chemrod".  I wonder if that might be helpful here.      


In respect to EV charging, which is a main driver of the possible move to permit the use of the TT system in NZ, I have noted, having been advised of it, that there is an amendment to BS 7671 that permits the use of a device that effectively monitors the integrity of the PEN conductor by measuring its voltage to earth and disconnects the supply in the event of that voltage exceeding 70 V for more than 4 seconds.   If such devices were reliable, the provision of such would likely be a much cheaper solution to converting part of a MEN installation to TT operation.   TT may still be preferable for some installations but it is the fear of a broken PEN conductor while an EV is plugged in to a charger that is a major fear here.   With the great majority of our electricity generation being from renewable energy sources, our government is keen to promote the uptake of EVs but needs EV charging to be a safe operation to the extent that can be provided.    We shall continue to monitor the possibilities.   


Regards


Peter Browne

I take it that the reference to a neutral earthing resistor was to an IT system

Similar to IT systems, but not quite the same in some details. Most IT systems either have the live conductors entirely isolated from Earth, or have a deliberate resistance that's usually high enough that any shock current is limited to safe levels; also the consumer's earth electrode can be separate from the source's.


With the neutral earthing resistor approach, the resistance might be much lower - so as to limit an L-PE fault to perhaps several amps (rather than a few milliamps) - but the source and consumer's earthing systems are solidly connected together (as well as to Earth) - the result being that the earth fault current doesn't raise the voltage on any earthed parts by more than a few volts (instead the voltage on the (insulated) N is raised instead). So there's no hazardous voltages either between different exposed-conductive-parts or between exposed-conductive-parts and Earth (as there can be with both TN and TT systems). I originally found the proposal in an old paper (I think it was one of the Cahier Technique ones) where, if I recall correctly, they described it as modified TN-S or hybrid TN-S/TT system (neither of which properly describe it to my mind). As I said it won't be much use if you're starting with a distribution system that doesn't have separate PE and insulated N conductors (let alone existing installations that rely on low L-PE loop impedances), but if it's of academic interest I can see if I can dig the article up again. I believe it is used in some HV systems for earth fault detection - but as far as I know it's rare for LV.

 

there is an amendment to BS 7671 that permits the use of a device that effectively monitors the integrity of the PEN conductor by measuring its voltage to earth and disconnects the supply in the event of that voltage exceeding 70 V for more than 4 seconds.

BS 7671 permits a couple of different designs of such devices - one type references the PE/PEN voltage to a local electrode, the other just monitors L-N voltage and tries to infer a broken PEN from that.


The first suffers the disadvantage that the local electrode has to be independent of the MEN earthing systems - otherwise it can be fooled if the ground it's planted in has a raised potential due to the broken PEN - so similar issues to creating a local TT system (although the electrode resistance can safely be higher). The second approach as the disadvantage that if the system is fed from a 3-phase distribution system (even if the installation itself is entirely single phase) it's quite possible (if not particularly likely) for the combination of unbalanced loads to pull the severed PEN conductor to a hazardous voltage above true Earth without the L-N voltage being outside a normal range (e.g. 230V+10%-6% for the UK).


If you have a 3-phase supply and can verify that all three L-L voltages are "normal", an artificial N/Earth point can be deduced from them, which in turn can be used to compare with the voltage on the PE/PEN conductor - which is far more reliable - but obviously of no use for single phase installations (which the vast majority of domestics and small commercials are in the UK).


   - Andy.

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. 

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.

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.

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

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

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