Peter Browne:

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.  

 

The requirements in the UK have recently been updated, to permit devices that help detect open-PEN events. This is, in part, due to issues with providing TT for electric vehicles in a MEN (we call this PME in the UK) environment. Find below links to a couple of articles on this:

• https://electrical.theiet.org/wiring-matters/years/2020/80-may-2020/the-iet-code-of-practice-for-electric-vehicle-charging-equipment-installation-4th-edition/

• http://www.gkenyontech.com/2020/03/18/fourth-edition-of-the-ev-code-of-practice-now-available/

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.

 

In the UK, a TT earth electrode with resistance above 200 Ω is seen as unstable. Yes, it's not always easy to get resistances down to double figures in some parts of the UK. Depending on the residual current rating of the RCD, earth electrode resistances above 100 Ω would still permit the RCD to operate - IEC 60364-7-722 (and the UK's BS 7671) requires 30 mA RCDs for electric vehicle charging equipment installations, so values slightly above 100 Ω should not be a problem. Of course, installers should worry if the earth electrode resistance test in wet soil conditions is too high, because it certainly won't get any better on a dry day !


There's are other considerations, including:

• Risk of striking buried services.

• Ensuring adequate separation below ground from buried metalwork (extraneous-conductive-parts etc.) connected to the MEN (PME) system

• Ensuring adequate separation above ground to prevent simultaneous contact between conductive parts connected to the MEN (PME) earthing system and conductive parts connected to the separated TT earthing system.

• Consideration of "return of the MEN (PME) broken neutral touch voltage" if someone happens to be standing above buried conductive parts connected to the MEN (PME) system, with the vehicle on a separated TT system, if the soil resistivity below the feet is low enough, or the ground is wet, etc.

 

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.  

There is no way, in dwellings, of ensuring testing is carried out. The UK's wiring code, BS 7671, requires appropriate user instruction labels to be provided in a prominent position at or near each RCD, advising of the recommended cycling of the device each 6 months. There is no data to show how often householders cycle their RCDs - but I would guess most households don't bother (I would be pleasantly surprised, and happy, for this to be incorrect).



 

I should be grateful for any comments or suggestions.

 

P M R Browne BE(Elect) FIET FENZ

I would be happy to engage with you further - please privately message me and I will provide contact details.

Peter Browne:

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.  

 

The requirements in the UK have recently been updated, to permit devices that help detect open-PEN events. This is, in part, due to issues with providing TT for electric vehicles in a MEN (we call this PME in the UK) environment. Find below links to a couple of articles on this:

• https://electrical.theiet.org/wiring-matters/years/2020/80-may-2020/the-iet-code-of-practice-for-electric-vehicle-charging-equipment-installation-4th-edition/

• http://www.gkenyontech.com/2020/03/18/fourth-edition-of-the-ev-code-of-practice-now-available/

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.

 

In the UK, a TT earth electrode with resistance above 200 Ω is seen as unstable. Yes, it's not always easy to get resistances down to double figures in some parts of the UK. Depending on the residual current rating of the RCD, earth electrode resistances above 100 Ω would still permit the RCD to operate - IEC 60364-7-722 (and the UK's BS 7671) requires 30 mA RCDs for electric vehicle charging equipment installations, so values slightly above 100 Ω should not be a problem. Of course, installers should worry if the earth electrode resistance test in wet soil conditions is too high, because it certainly won't get any better on a dry day !


There's are other considerations, including:

• Risk of striking buried services.

• Ensuring adequate separation below ground from buried metalwork (extraneous-conductive-parts etc.) connected to the MEN (PME) system

• Ensuring adequate separation above ground to prevent simultaneous contact between conductive parts connected to the MEN (PME) earthing system and conductive parts connected to the separated TT earthing system.

• Consideration of "return of the MEN (PME) broken neutral touch voltage" if someone happens to be standing above buried conductive parts connected to the MEN (PME) system, with the vehicle on a separated TT system, if the soil resistivity below the feet is low enough, or the ground is wet, etc.

 

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.  

There is no way, in dwellings, of ensuring testing is carried out. The UK's wiring code, BS 7671, requires appropriate user instruction labels to be provided in a prominent position at or near each RCD, advising of the recommended cycling of the device each 6 months. There is no data to show how often householders cycle their RCDs - but I would guess most households don't bother (I would be pleasantly surprised, and happy, for this to be incorrect).



 

I should be grateful for any comments or suggestions.

 

P M R Browne BE(Elect) FIET FENZ

I would be happy to engage with you further - please privately message me and I will provide contact details.