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

Parents
  • OlympusMons:

    Double insulated cars only allowed to be imported to NZ?
    https://communities.theiet.org/discussions/viewtopic/1037/25909#p137872 


    Thought I had linked to a post, not the thread, anyway thinking of Davezawadi's post amonst others:
    You have hit the nail on the head there Graham. In fact there is no completely safe way to charge class 1 electric vehicles when the supply system is TNC-S. The whole system is "unsafe" in all usage once we start on the "what if" scenarios. For every other use we deem the TNC-S system "safe enough" for normal consumers, and the number of accidents is tiny, and usually stems from stupidity of some kind (taking the heater or radio in the bath for example). The current obsession with RCD protection and AFDDs is another part of this attempt to stop any conceivable risk at any cost, something which is  obviously an impossible goal. At the moment there are not many Ecars, so the way to make charging safe is simply to set the goal as making all new ones class 2. They can make RH drive cars, so why not class 2 ones (although this is probably not needed in Japan with 100V nominal mains often less than 90V). Now is the time for leadership, there will be no sales for months so now is the time made available to tweek a bit of electronics in a fairly simple way. In fact for a sensible fee I will do the work for them! Then PROBLEM GONE forever! All over the world, now that really is an improvement in a time of crisis. The remaining older class 1 cars will not present much risk, and nothing needs to be done to existing charging points, it is just that new ones will be simpler and safer in use. We can make virus tests, drugs, ventilators etc very quickly once there is a good reason to do so, there seems to me to be a very good reason to fix EVs. We can also throw 722 away to the annals of history to everyone's great delight. ""

     


Reply
  • OlympusMons:

    Double insulated cars only allowed to be imported to NZ?
    https://communities.theiet.org/discussions/viewtopic/1037/25909#p137872 


    Thought I had linked to a post, not the thread, anyway thinking of Davezawadi's post amonst others:
    You have hit the nail on the head there Graham. In fact there is no completely safe way to charge class 1 electric vehicles when the supply system is TNC-S. The whole system is "unsafe" in all usage once we start on the "what if" scenarios. For every other use we deem the TNC-S system "safe enough" for normal consumers, and the number of accidents is tiny, and usually stems from stupidity of some kind (taking the heater or radio in the bath for example). The current obsession with RCD protection and AFDDs is another part of this attempt to stop any conceivable risk at any cost, something which is  obviously an impossible goal. At the moment there are not many Ecars, so the way to make charging safe is simply to set the goal as making all new ones class 2. They can make RH drive cars, so why not class 2 ones (although this is probably not needed in Japan with 100V nominal mains often less than 90V). Now is the time for leadership, there will be no sales for months so now is the time made available to tweek a bit of electronics in a fairly simple way. In fact for a sensible fee I will do the work for them! Then PROBLEM GONE forever! All over the world, now that really is an improvement in a time of crisis. The remaining older class 1 cars will not present much risk, and nothing needs to be done to existing charging points, it is just that new ones will be simpler and safer in use. We can make virus tests, drugs, ventilators etc very quickly once there is a good reason to do so, there seems to me to be a very good reason to fix EVs. We can also throw 722 away to the annals of history to everyone's great delight. ""

     


Children
No Data