Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 73 replies
  • Subscribers 27 subscribers
  • Views 5164 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

TT Earthing Systems - Interest by New Zealand

Former Community Member
Former Community Member
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.  
 


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.




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.  


I should be grateful for any comments or suggestions.

 

P M R Browne BE(Elect) FIET FENZ

  • 0
    I am not aware of any official figures for the fraction of RCDs that get tested as frequently as they should, anecdotally, in many cases they often do not, but it does not seem to be a problem. There are figures from surveys for fire alarm tests, which are probably similar, and they too show a general apathy. Govt safety campaigns to encourage users to press "test" when the clocks change to summer time seem to have had some success.  page 18 shows 25% of housholds have never tested their fire/smoke alarms


    For situations where RCD failure is a more serious risk, such as caravans and shore supplies to moored up boats, then the usual recommendation is that two RCDs are cascaded - so there is one at the source end, the caravan pitch or the mooring post,  and another in the load - i.e. within the caravan or boat. If the failure rate of any one is say 1%, then the chance of 2 independent RCD failures, and an un-noticed dangerous fault condition, is considered rare enough to be safe.

    On the same lines one or more 100mA delay RCD(s) at the head of an installation, and 30mA instant devices (RCBO or RCD )for final circuits are a 'belt and braces' solution, and is the common agricultural configuration (or a 300mA delay type covering  big sites with many out buildings)

    Domestic users in built up areas in the UK are probably mostly not  TT - new housing tends to be PME, and older housing stock is often TN-S (more than a third of  the 15 million or so houses in the UK predate the second world war, (and so does their hemp wrapped lead sheathed supply wiring!) UK housing stock data  Note that PME only came in in earnest from about 1970 onwards, before that TT and TN-S were the only normal options.

    Where TT is common is in rural areas where there are few buildings per transformer, and especially farms with livestock, but at the same time there is more land with each curtilage in such locations. For private earth electrodes, rods are common, though the regs permit more or less any shape of metal. rods are often 4ft (1.2m long) and we'd normally also bond to barns an so on with steel foundations, and this is often a significant bonus electrode.

    The supplier electrodes are supposed to be not more than 20 ohms, even for the lone pole-pig transformer in a field, and these are more likely to be bare wires or tapes buried at the foot of the supply poles, as unlike the typical home owner with a hammer or SDS drill,  the installers have access to a digger.

    Others with more experience will doubtless chip in shortly.


    regards

    Mike.
  • 0

     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.



    Nobody regularly tests RCDs in domestic situations.  I know mine has been tested twice in the last 10 years.  Once when I bought the house and I pressed the button before unpacking all my electronic gadgets, and once again by the electrician who re-wired my kitchen.  Other than that, we have so many gadgets in our homes that are intended to be powered on 24/7, that people don't want the inconvenience of going around resetting everything after turning off the power.


    We don't have regular visits from the fire services either.


  • 0
    Further to electrode resistances - locally to me the ground is quite gravelly, and more than 100 ohms per 4 ft rod is common. The quick fix is to try 2 or 3 rods a rod lenth apart, wired in parallel, ideally sited somewhere shady and likely to be a bit damp.

    However, a few years ago I was involved in a Scout Jamboree with a great many earth rods, in Essex, where the soil is clay, and there we had surprisingly good earthing ( original post )

    I can imagine dry and sandy places having a far greater resistance (and a far lower step voltage barefoot shock risk)
  • 0
    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.
  • 0
    I have yet to speak to a lay person who is aware that test buttons exist!
  • 0
    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:




    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.


  • 0
    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.
  • 0
    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
  • 0
    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.
  • 0
    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.

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines