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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

  • 0
    I have actually been doing a bit of background reading and watched a couple of relevant videos whilst this discussion has been progressing.


    This is probably the most relevant The New Zealand Government Worksafe electric vehicle charging guide  the description of what a TT installation comprises of is rather light, a installation with a 100 mA to 300 mA RCD instead of a MEN link.


    Andy Betteridge
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    What RCD main switch would you put upfront of a 30 mA Type B RCD located in an outbuilding to protect the distribution circuit from the house?


    Andy Betteridge
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    Without intending to annoy anyone, I will just say it seems that electricians in New Zealand have little or no experience of working with RCDs and have a lot of catching up to do if they are going to be working with EV chargers, induction hobs, direct drive washing machines, Solar PV with battery storage and RCDs, these and other equipment can cause all sorts of issues with RCDs.


    Andy Betteridge
  • 0
    My thoughts are that we could have a 300mA or 100mA Type S RCCB as the main switch on a TT main switchboard (or on a TT distribution board fed from a MEN main switchboard) with a 30mA Type B RCBO supplying the EV charging equipment

    RCDs and d.c. leakage is yet another can of worms.


    As I understand it, B-type RCDs aren't blinded by d.c. residual currents, but may not trip for less than 60mA d.c. (compared with 30mA a.c.) - whereas A and AC type RCDs can be affected (blinded, or in some way made less sensitive) by d.c. residual currents as low as 6mA. So putting  B-type downstream won't necessarily protect an upstream A (or AC) type (whether time delayed/selective  (s-type) or not). So the rule of thumb seems to be, if you need a B-type anywhere, then everything upstream also has to be B-type.


    An alternative approach is to arrange for the downstream protection to trip at (max) 6mA d.c. - sometimes that's built into the charge point itself, or is sometimes available as a separate unit (sometimes called an RDC-DD (Residual Direct Current Disconnection Device)), and some RCD manufacturers offer them built into what otherwise would be an A-type RCD and call it something like an EV type.


    (The next question is whether the d.c. leakage is a feature of normal operation, or just faults, so in cases where you have more than one charge point (or other similar electronic or d.c. containing loads) in the same installation, whether you need to consider the cumulative effect of d.c. residual currents...

        - Andy.
  • 0
    If domestic properties generally have 60-amp supplies that allows the use of RCDs rated at 63-amp made for the European markets, so immediately avoids one of the problems we have in the UK with having 80 and 100-amp domestic supplies.


    Andy Betteridge
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    Following on from the comments about EVs being Class II. Is not the effort we are making to devise solutions to charging EVs from TNC-S supplies a case of treating the symptoms rather than the disease? With all the different scenarios required to fit charging points into existing buildings, generally using existing infrastructure, we should be making it simpler and not more complex. What could be simpler than making the EVs Class II, which can still have metal chassis and bodies, and while we are at it isolate the control systems from the supply systems so we are not at risk of DC voltages blinding RCDs.


    As with the use of VFDs and third harmonics we appear to be designing the supply infrastructure to cope with inadequacies in the appliance rather than designing out the inadequacies. I appreciate this may be a bigger problem than making EVs class II and enforcing it even harder given the amount of existing electrical equipment that cannot meet the low requirements  of current EMC standards.


  • Former Community Member
    0 Former Community Member
    Thank you for the continuing comments on the above theme.    Three phase supply to a domestic installation would be very rare and would be justified only by a mansion very heavy electrical loading or else a three phase industrial load in the garage or workshop.   The norm is a 60A single phase supply and unless in new subdivisions is likely to be overhead by open wires (less common these days) or a neutral screen cable.   At least the overhead neutral screen cable is inherently safe but its neutral tail connections may not be.   Ideally a double clamp on the screen would be wise but does not always apply.   i


    The concept of a Class 2 EV is interesting and, who knows, we may all be driving around in EVs with fibreglass or plastic chassis in a few years' time.  Not sure about carbon fibre - might be conducting!   


    Regarding RCDs, the need for a Type B RCD when supplying power electronic equipment is appreciated and is certainly advised here.   I had not contemplated the use of a Type B RCCB for front end property protection purposes.   My thoughts are that we could have a 300mA or 100mA Type S RCCB as the main switch on a TT main switchboard (or on a TT distribution board fed from a MEN main switchboard) with a 30mA Type B RCBO supplying the EV charging equipment and 30mA Type A RCBOs supplying the close by electrical equipment so there is an equipotential zone in the vicinity of the EV.    But there would  be several possible solutions and we have plenty of time to think about it while we change the regulations - never an easy matter!      


    By the way, we require all RCDs in MEN installations to interrupt all live conductors as a matter of course, which should avoid any problem with an earth faulted neutral conductor.  Same will apply to any TT installation.  


    Regards 


    Peter Browne
  • 0
    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. ""

     


  • 0
    Double insulated cars only allowed to be imported to NZ?
    https://communities.theiet.org/discussions/viewtopic/1037/25909#p137872
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    It’s a bit of a sod if you decide that TT installations should be protected by a RCD that is no available or even made.


    An upfront 100 amp 100 mA Type B S-type time delayed RCD could be considered as a suitable main switch, but as far as I know no such device is made.


    If you need a 30 mA Type B RCD to protect an EV charger is there a RCD that you can install upfront of it that will afford discrimination?


    Andy Betteridge

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