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TT Earthing Systems - Interest by New Zealand

Former Community Member
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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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  • 0
    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.
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  • 0
    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.
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