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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
    For rods, I should have thought that a spacing of over 1.2 x rod length should be applicable to obtain more benefit from the additional rods.

    Can depend on the history of the site - the 2nd rod might be intended for a replacement for the first - perhaps being longer (often one deep rod works better and more reliably than the same total length in many shallower rods) - but then the original rod was left connected as it does no harm and might even help a little.


    As for testing there's often no need to have two (or more) separate electrode systems - at least for typical domestic/commercial where the installation can be disconnected for testing - most often an ordinary L-PE loop test is done (using an instrument with a suitable range) - isolate the installation, disconnect the electrode from the earth bar and test between supply L and the loose end of the conductor to the electrode. Admittedly that gives a whole loop impedance rather than just the resistance of the consumer's electrode - so includes the supply/MEN electrodes (and the L supply conductors) but these are usually small compared to Ra - and errs on the side of safety anyway, so if Zs is low enough to meet Ra requirement that can be no doubt that Ra will be fine.


    Separate electrode systems can be useful if you need to keep the installation live at all times - so each part can be tested independently (perhaps even without disconnection if you use one of the new fangled clamp-on earth testers) - but you'd really need each rod (or group of rods) to be fully independent of each other for the tests to be valid - which might mean tens of metres apart.


        - Andy.
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  • 0
    For rods, I should have thought that a spacing of over 1.2 x rod length should be applicable to obtain more benefit from the additional rods.

    Can depend on the history of the site - the 2nd rod might be intended for a replacement for the first - perhaps being longer (often one deep rod works better and more reliably than the same total length in many shallower rods) - but then the original rod was left connected as it does no harm and might even help a little.


    As for testing there's often no need to have two (or more) separate electrode systems - at least for typical domestic/commercial where the installation can be disconnected for testing - most often an ordinary L-PE loop test is done (using an instrument with a suitable range) - isolate the installation, disconnect the electrode from the earth bar and test between supply L and the loose end of the conductor to the electrode. Admittedly that gives a whole loop impedance rather than just the resistance of the consumer's electrode - so includes the supply/MEN electrodes (and the L supply conductors) but these are usually small compared to Ra - and errs on the side of safety anyway, so if Zs is low enough to meet Ra requirement that can be no doubt that Ra will be fine.


    Separate electrode systems can be useful if you need to keep the installation live at all times - so each part can be tested independently (perhaps even without disconnection if you use one of the new fangled clamp-on earth testers) - but you'd really need each rod (or group of rods) to be fully independent of each other for the tests to be valid - which might mean tens of metres apart.


        - Andy.
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