TT Earthing Systems - Interest by New Zealand

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

Andy Betteridge

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

Not sure if NZ is in the process of introducing "smart" meters as we are in the UK, and how far along you have got. Consideration should be given to including loss of neutral detection within the meters, and possibly a means of isolation. We have a bit of a mess with it in this country, hopefully NZ can do better.

Hello Peter and welcome.

As said I think most of us here are used to TN systems in practice with just the odd TT install or part of an install now and again. A few of us are more into TT where in very rural areas. As far as RCD testing we as an industry tend to test them with our test meters to give time to trip data rather than just the test button trip. Having said that, even though we label it, even though we tell our clients at time of install then the test button just gets forgotten about I`m afraid. I wonder if say an insurance requirement for households on renewal might entice people to tick a box and therefore, in theory, entice them to try the trip button might be an extra way to help jog memories too. I believe that in Oz anyone wanting to renew "REGO" for cars need to have a little insurance if not full whack, so a little of the same idea might exist in NZ.

Good luck and it is nice to hear from you. Indeed it would be nice to know what you finally resolve as the way forward too.

Peter


You may wish to see my post on PME here. https://communities.theiet.org/discussions/viewtopic/1037/26035


If you email myself or Graham Kenyon who has already responded to this post we may be able to put you in contact with our National Committee who will be able to help you with more information.


Regards


John Peckham email     info@astutetechnicalservices.co.uk

While I can provide no numbers, I would say that the majority (probably all)  of houses I have inspected  or worked on have never had the RCD test button pressed except by an electrician when installed or working on the premises. I usually have to explain what it is to the resident and tell them to test it when they change the clocks (twice per year). I am not sure how many ever do. I do find the rare failure when testing, either of the test button or not tripping  with fault currents, rarely do the test times fail. Many properties old and new have the consumer units hidden in cupboards full of stuff or in inaccessible positions so they are difficult to access which increases the probability of them never being touched.


Due to the requirement to use metal consumer units in domestic premises there are issues about protecting supply tails from earth faults on TT installations especially if the consumer unit is some way from the supply. Many of us install upfront 100mA time delayed RCDs to protect the tails, don't tell anyone but we often put these in plastic enclosures as it can be argued it is not a consumer unit,  and provide a backup in the event of a downstream 30mA RCD failing to operate on a fault. There were some very long threads on the old forum about this. Some like to use a 100mA S type RCD as the main switch on dual RCD or RCBO based consumer units as a backup on TT systems. 


TT has been relatively rare here except in the countryside but the introduction of electric car charging points and the requirement for TT has identified the problems of retro fitting TT onto premises with PME supplies and the issues about trying to maintain separation from two earthing systems in domestic and some commercial situations.

Peter Browne:

I am the chair of a Standards NZ committee charged with the production of a technical report to the NZ regulator regarding the possible use of the TT earthing system in NZ.   Currently, NZ uses the MEN earthing system (as does Australia), being similar to the PME earthing system used in the UK but with an earth electrode being required in each electrical installation to assist in keeping the voltage to earth of the neutral conductor of the LV reticulation close to zero.   Otherwise the MEN system is TN-C-S and relies on the PEN conductor as a return path to clear earth faults by the operation of OCPDs.   The use of RCDs is now required for most sub-circuits to provide additional shock protection.  


As is well known, TN systems are not perfect and a broken or high impedance PEN conductor causes the livening of earthed and bonded surfaces, including the chassis of EVs when they are plugged in to EV charging equipment.   It is noted that the IET Wiring Rules do not permit the use of PME systems to supply EV charging equipment unless the voltage on earthed surfaces is held to a non-lethal value.  

 

The requirements in the UK have recently been updated, to permit devices that help detect open-PEN events. This is, in part, due to issues with providing TT for electric vehicles in a MEN (we call this PME in the UK) environment. Find below links to a couple of articles on this:

• https://electrical.theiet.org/wiring-matters/years/2020/80-may-2020/the-iet-code-of-practice-for-electric-vehicle-charging-equipment-installation-4th-edition/

• http://www.gkenyontech.com/2020/03/18/fourth-edition-of-the-ev-code-of-practice-now-available/

Without going into further detail, the committee, in preparing a report, remains concerned about and seeks information on two possible problems.   

The first is how to attain at reasonable cost a TT earthing electrode system that does not exceed 100 ohms to earth in many NZ locations where the soil resistivity and the seasonal variation of this is high.   Does it cost a fortune to do this in the UK?    We have difficulty at many sites in reducing substation earthing mat and rod systems to less than 10 ohms and sometimes that is not achievable.

 

In the UK, a TT earth electrode with resistance above 200 Ω is seen as unstable. Yes, it's not always easy to get resistances down to double figures in some parts of the UK. Depending on the residual current rating of the RCD, earth electrode resistances above 100 Ω would still permit the RCD to operate - IEC 60364-7-722 (and the UK's BS 7671) requires 30 mA RCDs for electric vehicle charging equipment installations, so values slightly above 100 Ω should not be a problem. Of course, installers should worry if the earth electrode resistance test in wet soil conditions is too high, because it certainly won't get any better on a dry day !


There's are other considerations, including:

• Risk of striking buried services.

• Ensuring adequate separation below ground from buried metalwork (extraneous-conductive-parts etc.) connected to the MEN (PME) system

• Ensuring adequate separation above ground to prevent simultaneous contact between conductive parts connected to the MEN (PME) earthing system and conductive parts connected to the separated TT earthing system.

• Consideration of "return of the MEN (PME) broken neutral touch voltage" if someone happens to be standing above buried conductive parts connected to the MEN (PME) system, with the vehicle on a separated TT system, if the soil resistivity below the feet is low enough, or the ground is wet, etc.

 

The second is how to be reasonably sure that the RCDs in any TT installation will be regularly tested every six months or so by the users of the installation?  RCDs are not perfect but are much more important safety devices when used in a TT installation than in a TN installation.   Therefore regular testing appears to be important to maintain safety.    With non-domestic installations this should not be a problem as their regular testing (by pushbutton) can be linked to annual building inspections or included in maintenance schedules.  However, how does the UK ensure - if it does - that the occupants of domestic TT installations regularly check the operation of their RCDs?   One sensible suggestion made by a committee member was that the regular RCD checking could be linked to the six-monthly call by our Fire and Emergency Service to check the batteries in fire alarms installed in houses.   That might prompt a few people to check their RCDs.    

 

Since I was intending to ask about the practicability of 100 ohm earth electrode systems in the UK, I thought that I should also enquire about the regular testing of RCDs in domestic installations.  

There is no way, in dwellings, of ensuring testing is carried out. The UK's wiring code, BS 7671, requires appropriate user instruction labels to be provided in a prominent position at or near each RCD, advising of the recommended cycling of the device each 6 months. There is no data to show how often householders cycle their RCDs - but I would guess most households don't bother (I would be pleasantly surprised, and happy, for this to be incorrect).



 

I should be grateful for any comments or suggestions.

 

P M R Browne BE(Elect) FIET FENZ

I would be happy to engage with you further - please privately message me and I will provide contact details.

I have yet to speak to a lay person who is aware that test buttons exist!

Peter, have you any statistics from NZ with respect to injuries suffered because the earthing system was MEN? An engineer can demonstrate the potential hazard with PME but the probability of an incident needs to be established before the actual risk can be evaluated. 

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.