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

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 


 

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.

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

Generally we treat it as two distinct situations - call them grid-tied and islanded if you will.


In grid-tied setups the inverters will shutdown on loss of mains supply - in in effect they behave like a 'negative load' and don't require anything special at least as far as earthing is concerned.


Islanded systems will usually be configured as their own little TN-S arrangements so will need an earth electrode and a N-PE link in the same was as a LV supply transformer.


It is possible to have one system that switches between the two modes - typically when switching into islanded mode all live conductors (L & N) are disconnected from the grid and a N-PE link switched in. Typically the local earth electrode is left connected even in grid-tie mode - being treated as just another extraneous-conductive-part with no particular function but does not harm (or might even be beneficial in your case as a extra MEN electrode). Likewise the supplier's Earth connection is often left connected in islanded mode (for the sake of not having potentially unreliable switching in it) - so remains tied to Earth via the installation's electrode (in the same way bonding to gas or water pipes would) but doesn't form part of normal earth fault loops.


Graham might even be able to point you to a recent publication that covers this very subject in quite some detail ?


  - Andy.

Andy and Mike, thank you for your informative comments,   We do have in NZ what we term distributed generation where installations with PV are not only able to supply their own appliances at light load periods but are to feed back into the local reticulation and be paid for the electrical energy exported.  I'm sure the electricity retailers like this as they just sell the export at a considerable profit to the neighbouring installations rather than have to import it from a much more remote transmission grid export point.  The distribution network companies are less thrilled since they have to keep an eye on the voltage of the reticulation when injection to it occurs- I believe that this is a problem in sunny Australia where many houses have PV panels on their roofs.   


It's fairly standard to shut down the inverter whenever the grid connection is lost because of the risks of back feed into a fault or livening reticulation or HV that is being worked on. 


Currently, the distributed generating installations will all be MEN and there may be no strong argument that they should be TT instead, other than the "standard" reasons of remote supply, risk of EPR imposition on the LV, or broken PEN conductors.   


There are many consumers with their own generation who wish to disconnect from the grid altogether to avoid the daily rate charged by distributors - so the distributors need to avoid being too greedy with their daily rates!     

I can see that islanded systems would need to have their own earthing system such as a TN-S so there would be no need for a TT system to safeguard against EPR imposed on a PEN conductor or a broken PEN conductor. 


I can also see that for a "transformerless" converter, as Mike describes, the DC system would need to float free of earth.  I note that BS 7671 requires the use of a Type B RCD on the AC sub-circuit linking the AC busbars to the converter and that appears to be sensible in view of DC current content- one might argue that a RDC-DD plus a Type A or Type F might suffice in its place.   


Regards


Peter Browne

If you want to see how everything can go wrong with operating an electric generation and distribution company when politicians get involved you need to have a look at the recent history of the Sark Electric company on the UK Channel Islands.


If too many consumers disconnect and go off grid the financial burden increases significantly on those who are left. It would probably make more sense to have a higher standing charge and virtually unlimited usage in a country where people don’t need space heaters.

https://www.itv.com/news/channel/2019-12-05/hotel-in-sark-is-to-produce-its-own-electricity-after-price-rise/

It should be remembered that Sark is highly exceptional.

It is an island with a small local population (about 600) - to put that  into scale that is less  than the number attending  most secondary schools in England (about 1200). Total consumption is ~  1MVA , and there are two 600kVA transformers to step up from the 230V generators to drive the 6.6kV HV distribution to local transformers in the community. 

1MVA is the size of transformer normally found in the basement of a large block of flats, and the total capacity is not much more than the size of genset normally seen in a container in a field at a pop festival.


In the absence of a 10km extension lead to plug it into it's nearest neighbour (Guernsey), itself connected to the French HV grid via Jersey, all generation is local and from diesel fuel - with corresponding costs dependant on the world oil price.


But I agree, the politics is very silly- a fixed standing charge and 'all you can eat' may be a better way to go to get an income to maintain the system..

It is indeed a concern when politicians get into the act for "social" reasons and apply "solutions" that are completely against the economics of the generation, transmission and distribution of electricity to consumers.  The ESI has a large proportion of fixed costs, which ideally should be recovered by a fixed daily charge that is as high as the market will bear (without consumers disconnecting from the grid in droves and forcing the remainder of consumers to bear the costs).   However, in NZ, consumers have the option of paying a low daily charge and a higher / kWh charge, with a break-even point of 8000 kWh.  You have a choice of one or the other.   Those with their own generation like the low fixed charge option it as their generation is effectively valued at the higher avoidance cost - but of course they are not contributing.adequately to the infrastructure costs that provides their make-up energy import.   The ESI is hoping that the Government will remove the lower fixed charge option but there is resistance from the distributed generation owners.  


Don't get me started on capacity and peak charges v energy charges, which remain a bone of contention.   At least,many distribution companies are now offering time of use charges, which is at least a recognition that network capacity is expensive to provide and it is correct to signal to consumers that they should avoid increasing the peak demands placed on the network and so delay the need for its reinforcement.


By the way, NZ is a long narrow land in the antipodes of Spain and its climate runs from the "winterless north" to the deep south.   We do use space heaters!   Dunedin had -10 degrees C the other day!   


At least we have a lot of renewable generation.    


I'm getting well off the subject of TT.!


Regards


Peter Browne.      

Three rods in a cellar,  two into the floor and one out into the front garden,  I measured 224 ohms combined with a loop test.


I guess they they are all four foot rods and the one with the white box lid was supplemented by the two with the red and white lids.


As you can see TT earthing in UK homes is not over engineered.


Andy B.

Three earth rods at a local church probably installed over the last sixty years or so. 


The standard of installation has not varied much over that time. 


There is one thing that begs a question.