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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

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  • Former Community Member
    0 Former Community Member
    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
Reply
  • Former Community Member
    0 Former Community Member
    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
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