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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

Parents
  • Former Community Member
    0 Former Community Member
    Andy, thanks for the explanation of the neutral earthing resistor.  However, the MEN system and no doubt the PME system provides safety from line to earth faults by the high fault current that returns back to the transformer via the PEN conductor that then operates the MCB or blows the sub-circuit or service fuse to interrupt the phase supply.   


    I don't think that we have ever had a TN-S LV system in NZ - if so, it's been lost in the mists of antiquity, even for NZ's comparatively short history.   A few of our EDBs use resonant neutral earthing at zone substations to permit 11 kV feeders to remain in service with a permanent earth fault on one phase - that is a well established distribution practice in some parts of the world.   Expensive to install so it's not widespread.   


    As for the UK, the government here in NZ (pushed by our Greens) is also hell for leather in respect to promoting the adoption of EVs and the phasing out of ICEVs altogether in the next 10 to 20 years..  But I'm sure it remains largely ignorant of the problem of EV charging equipment being supplied from MEN systems.    Our NZ EV charging guidelines documents require the use of supplies from MEN installations as there is no available alternative at present.   Our regulator is well aware - even if the ESI here has not been hammering the need for an alternative because of the risks of broken PEN conductors - mainly in services rather than in the LV reticulation. 


    We would not look for a wholesale change from MEN, just the ability to use TT for, say, a selected distribution board supplied from a MEN main switchboard, if that is a safe solution to the problem (having due regard to the need to have good separation between the MEN earth electrode and the TT earth earth electrode) .   


    So there is bound to be interest in devices that will reliably detect broken PENs and interrupt all live conductors in such an event.   Might be a much cheaper solution!   


    I'm aware that, with a three phase supply, it would be possible to derive the TN-C neutral voltage as a reference against the voltage on the PEN conductor but, as has been said, that won't apply for single or two phase supplies to domestic premises.  Hence I would think that an earth reference electrode would be required, located well away physically from the MEN earth electrode.   


    It has been a good discussion so far.   It will all help as I'm preparing to write a report on TT for my committee to submit to the regulator.   


    Regards


    Peter Browne
Reply
  • Former Community Member
    0 Former Community Member
    Andy, thanks for the explanation of the neutral earthing resistor.  However, the MEN system and no doubt the PME system provides safety from line to earth faults by the high fault current that returns back to the transformer via the PEN conductor that then operates the MCB or blows the sub-circuit or service fuse to interrupt the phase supply.   


    I don't think that we have ever had a TN-S LV system in NZ - if so, it's been lost in the mists of antiquity, even for NZ's comparatively short history.   A few of our EDBs use resonant neutral earthing at zone substations to permit 11 kV feeders to remain in service with a permanent earth fault on one phase - that is a well established distribution practice in some parts of the world.   Expensive to install so it's not widespread.   


    As for the UK, the government here in NZ (pushed by our Greens) is also hell for leather in respect to promoting the adoption of EVs and the phasing out of ICEVs altogether in the next 10 to 20 years..  But I'm sure it remains largely ignorant of the problem of EV charging equipment being supplied from MEN systems.    Our NZ EV charging guidelines documents require the use of supplies from MEN installations as there is no available alternative at present.   Our regulator is well aware - even if the ESI here has not been hammering the need for an alternative because of the risks of broken PEN conductors - mainly in services rather than in the LV reticulation. 


    We would not look for a wholesale change from MEN, just the ability to use TT for, say, a selected distribution board supplied from a MEN main switchboard, if that is a safe solution to the problem (having due regard to the need to have good separation between the MEN earth electrode and the TT earth earth electrode) .   


    So there is bound to be interest in devices that will reliably detect broken PENs and interrupt all live conductors in such an event.   Might be a much cheaper solution!   


    I'm aware that, with a three phase supply, it would be possible to derive the TN-C neutral voltage as a reference against the voltage on the PEN conductor but, as has been said, that won't apply for single or two phase supplies to domestic premises.  Hence I would think that an earth reference electrode would be required, located well away physically from the MEN earth electrode.   


    It has been a good discussion so far.   It will all help as I'm preparing to write a report on TT for my committee to submit to the regulator.   


    Regards


    Peter Browne
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