I should like to heartily thank all the many contributors to my post of yesterday.    The responses were all gratefully received and I was not surprised to receive the general impression that the regular testing of RCDs in UK domestic installations is what we thought it might be if we had TT domestic installations here, close to zero!   .The idea of linking reminders to six monthly changes to clocks is probably worth pursuing as over here our NZ Fire and Emergency organisation puts adverts on TV reminding to check fire alarm batteries and some extension to RCDs in the name of greater safety would probably be worthwhile.  Access to switchboards and the need to reset all clocks without battery back-up would might discourage some but they probably won't appreciate the importance of regular checking unless your electricity distribution businesses (EDBs) or other organisations reinforce it.  The necessity of having operative smoke alarms receives much TV publicity here as we do lose several lives from house fires.   We had not thought of the requirement under insurance contracts.  That would indeed be a driver at least once a year when the premium demand arrives!   


In response to the query re our required earth electrode at each installation under our MEN earthing system, AS/NZS 3000 (Australia and NZ Wiring Rules) requires the driving of a single 12mm Dia copper clad rod 1.8 metres into the ground,   Other electrodes are allowable but the rod is the most commonly used.    There is no requirement to measure its resistance to the general mass of earth.   There is a requirement to measure earth loop return impedance but, of course, for a TT-C-S system, the return circuit is via the PEN conductor and is always acceptably low.   The driven rod is connected to the earth bar, which has a removable (for testing) link to the neutral bar and helps to hold down the voltage of the PEN conductor and the neutral conductor (also a PEN) of the supplying LV reticulation.   Ideally, the reticulation PEN should also be connected to earth rods along its length but for the same purpose but I think this would be rare.   


In response to the query regarding the loss of lives in NZ through the breaking of a PEN conductor, I can only say that fortunately we have very few electrocutions here.   There was a spate of them some years ago but they were caused by installing underfloor insulation consisting of material with a conducting heat reflecting surface that became alive when someone stapled the material through a TPS cable - not to be recommended when lying on damp ground under a house!     With MEN as with PME, a broken neutral can lead to the livening of all earthed and bonded surfaces including the garden tap.   if the occupants are inside a house a shock only is likely but with an EV being charged or just plugged in, a livened car chassis on possibly wet ground could present a lethal mix.   The RCD protecting the charging circuit won't see it, even if someone touches the chassis.   Hence I guess the prohibition in the CoP (and other publications) of EV charging using a PME earthing facility.  In short, the risk appears high in overhead supply situations where broken neutrals are known to occur.in storms and high winds.  NZ is a windy country!     We have world records for wind farm outputs!   


In response to the query regarding smart meters, NZ is well along with that.    I can't comment from personal knowledge on the detection of the loss of the neutral by a smart meter but take the point that if the voltage reference to the meter is lost, it should signal that fact to the reading interrogation system.   A very good thought!       

   

I take the point that the achievement of a 100 ohm maximum TT earth electrode system is not easy in the UK either and I appreciate the hint that the reinforcing steel in barn floors would be useful.   I was involved with the writing of Standards NZ HB 6117:2104 Electrical Installations in Dairy Sheds, which was all about the avoidance of stray voltages in milking sheds that were upsetting cows and reducing milk production,   We concluded that TT  would be better than MEN in avoiding the return of load current through the earth and through imperfect bonding of metalwork, etc.   So if TT becomes permitted in NZ we should see TT electrical installations in new dairy sheds,   I think that for any TT domestic installations that might be desirable in rural areas to avoid the transfer of EPRs on PEN conductors, some experimentation with rods and buried electrodes will be necessary until the electrical industry here gains experience.  As mentioned above, it will be a new thing for the industry but the EDBs here can probably offer advice from their knowledge of local substation earthing and the required five yearly measurements of substation resistances.   


Having gained such a great response, I should ask if some PME installations are being used to feed a TT part installation or a TT sub-circuit for the purposes of supplying EV charging equipment.   I have a copy of the IET;s CoP for EV Charging Equipment Installation (2nd Edition) and noted the advice in it on converting a EV charging sub-circuit to TT operation.  That appeared to me to be sensible for domestic installations fed from overhead reticulation via an overhead service where the risks of a broken PEN conductor would be higher compared to underground fed installations.   There would need to be some precautions, especially if nearby circuits in a garage or workshop were supplied from a PME system with the risk of differing voltages on different exposed and bonded surfaces - it might be safer to feed a local distribution board converted to TT operation.   


Regards


Peter Browne

TT installations and their earth rods are rarely installed to a high standard in the UK.

Andy Betteridge

It is a bit sweeping to say that all  TT installations in the UK are poor !

I think it depends where you live and the general experience of the local 'sparks'. In areas with a lot of TT folk are more used to it. For example round here (Hants) there are plenty of stables where the stock is so valuable that anything less  than an absolutely 1st class installation is not even considered. This sort of sets the bar quite high for all electricians who have seen it, even when they are only doing  the more down market "value engineered"  jobs where a detached garage or caravan point on the side of a garden summer house is being set up as a TT island supplied from a PME house.

Equally from the no. of times it comes up on this forum, there are clearly parts of the country where there is almost no TT experience at all, and on first encounters  folk get easily confused by it - I suspect that some very clear guidance will be needed for the first times this is introduced in a new country as is proposed here.


In that case an NZ version of this sort of http://IET wiring Matters tutorial example  may be worth considering, although it could be improved by adding further clarifications  about how to achieve electrical separation from the house services, and the desirability of plastic pipes for outside taps near charge points and so on.  Even a length of a meter or so of plastic pipe in a metal section  is a very significant fault current limiter.


Another article here


(isolation of water pipes unofficially demonstrated  here  - the actual tests from about half way in)

In response to the query re our required earth electrode at each installation under our MEN earthing system, AS/NZS 3000 (Australia and NZ Wiring Rules) requires the driving of a single 12mm Dia copper clad rod 1.8 metres into the ground, Other electrodes are allowable but the rod is the most commonly used. There is no requirement to measure its resistance to the general mass of earth. There is a requirement to measure earth loop return impedance but, of course, for a TT-C-S system, the return circuit is via the PEN conductor and is always acceptably low. The driven rod is connected to the earth bar, which has a removable (for testing) link to the neutral bar and helps to hold down the voltage of the PEN conductor and the neutral conductor (also a PEN) of the supplying LV reticulation. Ideally, the reticulation PEN should also be connected to earth rods along its length but for the same purpose but I think this would be rare.

The problem we had with the proposed (and dropped) requirement for additional electrodes in consumer's installations on PME systems was the resistance to Earth it would need to achieve in order to be effective. For example if we had a 20Ω electrode, a broken PEN that affected just our one installation and the installation had loads connected that attempted to draw say 10A - then we'd have 10V * 20Ω = 200V developed between the installation's earthing system and true Earth - so hardly providing safety. OK you can refine the numbers - with some voltage lost across the electrode, so a reduced voltage appearing across loads, many loads (e.g. those having a fixed resistance) would reduce their current draw somewhat but then there's the possibility of larger loads happening to be connected. If the PEN break was further upstream then you might have a contribution from several consumer's electrodes in parallel, which should help - but on the flipside there would be several consumer's N currents to sink. In the UK even domestic installations can have up to a 100A supply - so to be completely sure of keeping things below the usual limit of 50V we'd have to be looking at ridiculously low values like 0.5Ω for each electrode. So we're interested in how it's approached elsewhere - or whether it's just a case of PEN conductors being more reliable where they're not built from rotting Edwardian paper & lead cables or corroding 1970s aluminium ones like much of ours seems to be.

 

By the way, there are devices in the process of patent approval that deal with the loss of neutral issue for EV application and they don’t require an electrode.

BS 7671 allows a couple of different 'voltage monitoring' approaches - some use a local electrode (or an artificial N star point for 3-phase systems) as a reference, but others (as mentioned) just monitor the L-N voltage. That second approach as a number of disadvantages - not least where supplied from a 3-phase distribution system that it's entirely possible for the L-N voltage to remain within limits (e.g. 230V+10%-6%) but the N-Earth voltage to greatly exceed 50V (or indeed 70V) - depending on the relative loading of the three phases. It also can't disconnect promptly without risking a lot of nuisance tripping since the L-N (or L-PE) voltage can be taken out of limits by "normal" faults elsewhere in the installation, distribution system or other installations, which can take anything up to 5s to clear (sometimes longer for some faults on the distribution system). Thus it's only really intended where there are no other practical alternatives - thus BS 7671 doesn't permit that approach in 3-phase installations where an artificial N point would provide a more reliable reference.


   - Andy.

The responsibility of maintaining the integrity of the PEN on a public supply should rest squarely with the supply provider. It seems to me that the attitude taken by the supplier is “well here is an earthing terminal, use it if you want”. If you choose not to and employ your own TT arrangement in say an estate that is all PME, there is still a credible risk. So what to do? Nail the supplier for compensation for any injury that might arise! I am sure that they are delighted that the electrical installation industry is picking up the tab for the perceived risk in loss of PEN, especially where EV charging is concerned.

mapj1:

It is a bit sweeping to say that all  TT installations in the UK are poor !

 

I actually said "TT installations and their earth rods are rarely installed to a high standard in the UK."


Which is a factual statement.


Andy Betteridge

If you are lucky a house with a TT installation gets a £ 2.99 earth rod with a clamp to attach the cable and a plastic box to cover it, the whole set up rarely costs a ten pounds in materials.


This is one of mine from a few years ago for a hot tub.




Here is another one  I did with a decent plastic pit that was then set in concrete.




I do usually allow for enough materials for a reasonable job and take some extendable rods with me so I can pull a four foot rod back out then install a longer one if I cannot get a good test result.


There was around forty pounds worth of materials for the set up with the pit, which is several times more than is typically spent, the earth rod pit is around thirty pounds.

 

Andy Betteridge.

In fact I pulled the basic four foot rod out that can be seen in the picture of the pit and replaced it with three sections of extendable rods, which took the cost of materials up to around sixty pounds. But being twelve feet long the rod has a considerably lower resistance.


With labour and materials the total cost went up to over hundred pounds, which is far in excess of what generally gets spent. 


And yes, that is a earth loop impedance tester, the electric supply was on as it was an existing installation that was being upgraded.

Thank you all again for your informative responses on the construction of practical earth electrode systems.   If you have been able to get your resistance to earth down to 5 ohm with a single long rod, you have done well but you may been fortunate with your soil resistivity at the site.   For what it's worth the earthing rod looks very similar to to these used in NZ and Australia but a single driven rod here can yield a resistance to earth of hundreds of ohms in some places.  The MEN system relies very much on the "multiple" from which its name derives!.    I think that it will be a matter of trial and error involving several driven rods (with good separation among them) to get the connection down to 100 ohms and achieve reliable operation of the 300mA or 100mA property protection RCD.   In some situations, the use of bare wire in trenches or buried electrodes may be necessary.   Canterbury in the South Island has wide areas of old ex-river shingle beds where earthing is particularly difficult.   As has been suggested, the reinforcing in concrete floors may be useful but members of my committee have pointed out that concrete slabs are often insulated by plastic sheeting from the ground against moisture ingress so they may not help much.  


I am aware there has been pursuit of a reliable broken PEN conductor detector but ideally it should not just give an alarm but operate to interrupt all active conductors as soon as such an event occurs.   That would certainly lessen the risk of livening an EV chassis if that were a concern.   Otherwise the TT system should be effective in preventing it.    However, I can't see wholesale conversion of MEN distribution board sub-circuits here for a while.   Even if the TT earthing system were to be permitted in NZ, it will be some time before our electrical industry here becomes familiar and comfortable with it.   It has been said that many of our electricians don't adequately understand the MEN system as it's the only one permitted and its advantages and disadvantages are not taught to the trade unfortunately - it will take some effort here over some years to have the TT system taught and understood as well.    However, you have to start somewhere! 


Cheers


Peter Browne