TT Earthing Systems - Interest by New Zealand

When solar panels first came to the UK, the inverter designs with a 50Hz transformer were common. Now, as powers have increased, and budgets have not, and perhaps as power semiconductors have got cheaper and more reliable, most inverters are 'transformerless' so the DC is in effect referred to whichever pole of the incoming mains is negative at that half cycle - from a shock  point of view the silicon of the panels  is connected to L and N on alternate half cycles.

In such designs,  the DC side can not be earthed. For EMC reasons it is important to keep 'flow and return' currents in both the AC and DC paths to the inverter close to each other, remote sites with radio equipment as well may need common mode chokes on the DC lines if the local noise floor is not to be raised.

Advice on the wisdom of using the mains earth for the frames  and mounting hardware of the panels has varied over time and can depend on what other metalwork within reach is or is not earthed to. RCDs on the AC side of the inverter are on pretty much de-riguer on all domestic set-ups although how many are type AC, or A which would be better, is not so clear.


We do not generally allow domestic solar inverters to run as an 'island' - if there is no external mains to sync to, the solar inverter shuts off. The idea is to prevent accidental injection of mains onto external bare  lines that may be down on the ground, or even being worked on at the time.

If stand alone operation is required for any reason then all the same isolation, local NE bond  and change over switching  is required as if it were a private generator, and of course a non standard inverter.

On the basis of the information that I had accumulated plus al the valued advice and comment from contributors to my original request to this forum, I have completed a draft report and circulated it to my committee members to obtain their comments before it is sent to our regulator for consideration.    The next matter to be considered are to provide advice on where the TT system should be preferentially used as providing a lower safety related risks to personnel and property so that there are reasons to seek regulatory amendments to permit the supply of electricity directly to TT installations as opposed to just MEN installations now.   


Identified sites to date are rural areas with weak HV systems, especially where there is non-separation of HV and LV earths, the charging of EVs (although that might be addressed by the part conversion of an installation to TT), marinas where direct supply is provided to boats without benefit of any isolating transformers, dairy sheds and the like.   


An interesting situation on which I should welcome UK comment as to which is the safest system, PME or TT, is where a domestic installation has PV panels, a battery for energy storage and a (two way) converter that is run in parallel with the local grid supply or, at times, free of the grid supply and relying on its converter and battery only to supply its AC appliances,    That is not an uncommon situation here in NZ and the installation of PV panels is increasing - despite NZ being known as "the land of the long white cloud."   


I can envisage all sorts of operational issues arising in various circumstances, including whether or not there is any reference to earth on the installation's DC system.   It would be important to provide effective protection against short circuits and earth faults in all operational situations to safeguard both persons and property.    Faced with such a situation, what would an electrical installer do in the UK and why?     Inherently, I think that TT may be the safer system in that it provides a reliable earth, albeit up to 100 ohms, for earthed surfaces and extraneous conducting parts but, where the PME system is underground and provides a reliable PEN conductor for earthing, there may not be much in it.


Regards


Peter Browne 


 

Graham Kenyon kindly pointed out to me the safety risks associated with the siting of a TT earthing electrode system in the vicinity of extraneous conducting parts that might be connected to the PME earthing system either at the installation itself or at a neighbouring installation so care needs to be taken where there are incoming water or gas pipes and service cables.   Their whereabouts are not usually well known although toby taps, water meters and gas meters should give a clue as to their starting point in the street.   Looks as though having a tracing cable / pipe device would be a good investment for any electrical contractor.   So that is good advice and I'll include it in our committee's report to the regulator as something to be included in any future guidance to TT installation designers / installers.   


I have never purchased a Type B RCD so am blissfully ignorant of its bankrupting qualities - one would not wish to have one just sitting in a van in case the need to replace one in service turns up.    However, keeping in mind the advice provided in IEC 60364-7-722 that I mentioned in my last post, why not look at a RDC-DD plus a Type A or Type F instead of a Type B if the cost of the latter is horrendous?    I must admit I would not wish to have to install a Type B RCD to protect my washing machine or induction top and would be looking closely at cheaper options that would nevertheless be effective.


Regards


Peter Browne

I am finding this all quite interesting.


On a hopefull note, RS often is not the cheapest of suppliers and things often go down anyway as popularity increases.

I remember back to mid/late 70s as an apprentice, the first readily available calculator, Sinclar Cambridge about twice a skilled weeks wage, very simple by todays standards, nowadays you can get a far more powerfull one for a fiver or even a quid. Such is life as the French would say

Na, only about a hundred?

   - Andy.

Graham Kenyon has written an article for the IET Wiring Matters Magazine highlighting risks associated with creating “”TT islands” within TN earthed electrical installations.


Having watched a few YouTube videos of electricians tapping rods in with their hammers some of them need to give it a bit more consideration.


Andy Betteridge

Thanks again for your latest contributions, which are all appreciated by one who has no practical experience with TT earthing systems or TT installations.    There would be a few UK ex-pats here in NZ who would have seen some TT installations in their time but if TT were to be permitted here our electrical industry would have a lot to learn about some of the practical details.    I don't have a problem with my induction hob but as my installation is MEN (and old), it is currently protected by a circuit breaker only.   I'm not sure what would happen if it were supplied from a TT installation but I would not have thought of using a Type B RCD, when it might well require it.  Same for my washing machine!


I have found Chapters of the Schneider Electrical Installation Guide 2018 very informative on TT and RCDs.   Chapter F deals with RCDs and sets out acceptable configurations for the use of Type B RCDs with upstream Type A RCDs.    I might have mentioned previously that we don't permit the use of Type AC RCDs in NZ, although they are permitted across the ditch in Australia.   I note that IEC 60364-7-722 permits the protection of EV chargers by a Type B RCD or a Type A RCD plus a RDC-DD or a Type F RCD plus a RDC-DD.    I guess it might boil down to which is the cheaper of the three options to install!     


There is reference in the WorkSafe EV Charging Safety Guidelines to TT but only to say that its use might be mentioned in a later edition since there is currently no recognised guidance on TT in NZ.  AS/NZS 3000 Part 2 sets out installation details but only for MEN installations.    I suspect that the Guidelines are a prime reason why my committee was formed since a downstream workstream may be to replace the content of AS/NZS 3000 Part 2 that is not compatible with TT.    I am relieved, however, that the great majority of Part 2 is fully applicable to either MEN or TT installations.   So the job should not be over onerous.   


Regards


Peter Browne