I should like to heartily thank all the many contributors to my post of yesterday.    The responses were all gratefully received and I was not surprised to receive the general impression that the regular testing of RCDs in UK domestic installations is what we thought it might be if we had TT domestic installations here, close to zero!   .The idea of linking reminders to six monthly changes to clocks is probably worth pursuing as over here our NZ Fire and Emergency organisation puts adverts on TV reminding to check fire alarm batteries and some extension to RCDs in the name of greater safety would probably be worthwhile.  Access to switchboards and the need to reset all clocks without battery back-up would might discourage some but they probably won't appreciate the importance of regular checking unless your electricity distribution businesses (EDBs) or other organisations reinforce it.  The necessity of having operative smoke alarms receives much TV publicity here as we do lose several lives from house fires.   We had not thought of the requirement under insurance contracts.  That would indeed be a driver at least once a year when the premium demand arrives!   


In response to the query re our required earth electrode at each installation under our MEN earthing system, AS/NZS 3000 (Australia and NZ Wiring Rules) requires the driving of a single 12mm Dia copper clad rod 1.8 metres into the ground,   Other electrodes are allowable but the rod is the most commonly used.    There is no requirement to measure its resistance to the general mass of earth.   There is a requirement to measure earth loop return impedance but, of course, for a TT-C-S system, the return circuit is via the PEN conductor and is always acceptably low.   The driven rod is connected to the earth bar, which has a removable (for testing) link to the neutral bar and helps to hold down the voltage of the PEN conductor and the neutral conductor (also a PEN) of the supplying LV reticulation.   Ideally, the reticulation PEN should also be connected to earth rods along its length but for the same purpose but I think this would be rare.   


In response to the query regarding the loss of lives in NZ through the breaking of a PEN conductor, I can only say that fortunately we have very few electrocutions here.   There was a spate of them some years ago but they were caused by installing underfloor insulation consisting of material with a conducting heat reflecting surface that became alive when someone stapled the material through a TPS cable - not to be recommended when lying on damp ground under a house!     With MEN as with PME, a broken neutral can lead to the livening of all earthed and bonded surfaces including the garden tap.   if the occupants are inside a house a shock only is likely but with an EV being charged or just plugged in, a livened car chassis on possibly wet ground could present a lethal mix.   The RCD protecting the charging circuit won't see it, even if someone touches the chassis.   Hence I guess the prohibition in the CoP (and other publications) of EV charging using a PME earthing facility.  In short, the risk appears high in overhead supply situations where broken neutrals are known to occur.in storms and high winds.  NZ is a windy country!     We have world records for wind farm outputs!   


In response to the query regarding smart meters, NZ is well along with that.    I can't comment from personal knowledge on the detection of the loss of the neutral by a smart meter but take the point that if the voltage reference to the meter is lost, it should signal that fact to the reading interrogation system.   A very good thought!       

   

I take the point that the achievement of a 100 ohm maximum TT earth electrode system is not easy in the UK either and I appreciate the hint that the reinforcing steel in barn floors would be useful.   I was involved with the writing of Standards NZ HB 6117:2104 Electrical Installations in Dairy Sheds, which was all about the avoidance of stray voltages in milking sheds that were upsetting cows and reducing milk production,   We concluded that TT  would be better than MEN in avoiding the return of load current through the earth and through imperfect bonding of metalwork, etc.   So if TT becomes permitted in NZ we should see TT electrical installations in new dairy sheds,   I think that for any TT domestic installations that might be desirable in rural areas to avoid the transfer of EPRs on PEN conductors, some experimentation with rods and buried electrodes will be necessary until the electrical industry here gains experience.  As mentioned above, it will be a new thing for the industry but the EDBs here can probably offer advice from their knowledge of local substation earthing and the required five yearly measurements of substation resistances.   


Having gained such a great response, I should ask if some PME installations are being used to feed a TT part installation or a TT sub-circuit for the purposes of supplying EV charging equipment.   I have a copy of the IET;s CoP for EV Charging Equipment Installation (2nd Edition) and noted the advice in it on converting a EV charging sub-circuit to TT operation.  That appeared to me to be sensible for domestic installations fed from overhead reticulation via an overhead service where the risks of a broken PEN conductor would be higher compared to underground fed installations.   There would need to be some precautions, especially if nearby circuits in a garage or workshop were supplied from a PME system with the risk of differing voltages on different exposed and bonded surfaces - it might be safer to feed a local distribution board converted to TT operation.   


Regards


Peter Browne

I should like to heartily thank all the many contributors to my post of yesterday.    The responses were all gratefully received and I was not surprised to receive the general impression that the regular testing of RCDs in UK domestic installations is what we thought it might be if we had TT domestic installations here, close to zero!   .The idea of linking reminders to six monthly changes to clocks is probably worth pursuing as over here our NZ Fire and Emergency organisation puts adverts on TV reminding to check fire alarm batteries and some extension to RCDs in the name of greater safety would probably be worthwhile.  Access to switchboards and the need to reset all clocks without battery back-up would might discourage some but they probably won't appreciate the importance of regular checking unless your electricity distribution businesses (EDBs) or other organisations reinforce it.  The necessity of having operative smoke alarms receives much TV publicity here as we do lose several lives from house fires.   We had not thought of the requirement under insurance contracts.  That would indeed be a driver at least once a year when the premium demand arrives!   


In response to the query re our required earth electrode at each installation under our MEN earthing system, AS/NZS 3000 (Australia and NZ Wiring Rules) requires the driving of a single 12mm Dia copper clad rod 1.8 metres into the ground,   Other electrodes are allowable but the rod is the most commonly used.    There is no requirement to measure its resistance to the general mass of earth.   There is a requirement to measure earth loop return impedance but, of course, for a TT-C-S system, the return circuit is via the PEN conductor and is always acceptably low.   The driven rod is connected to the earth bar, which has a removable (for testing) link to the neutral bar and helps to hold down the voltage of the PEN conductor and the neutral conductor (also a PEN) of the supplying LV reticulation.   Ideally, the reticulation PEN should also be connected to earth rods along its length but for the same purpose but I think this would be rare.   


In response to the query regarding the loss of lives in NZ through the breaking of a PEN conductor, I can only say that fortunately we have very few electrocutions here.   There was a spate of them some years ago but they were caused by installing underfloor insulation consisting of material with a conducting heat reflecting surface that became alive when someone stapled the material through a TPS cable - not to be recommended when lying on damp ground under a house!     With MEN as with PME, a broken neutral can lead to the livening of all earthed and bonded surfaces including the garden tap.   if the occupants are inside a house a shock only is likely but with an EV being charged or just plugged in, a livened car chassis on possibly wet ground could present a lethal mix.   The RCD protecting the charging circuit won't see it, even if someone touches the chassis.   Hence I guess the prohibition in the CoP (and other publications) of EV charging using a PME earthing facility.  In short, the risk appears high in overhead supply situations where broken neutrals are known to occur.in storms and high winds.  NZ is a windy country!     We have world records for wind farm outputs!   


In response to the query regarding smart meters, NZ is well along with that.    I can't comment from personal knowledge on the detection of the loss of the neutral by a smart meter but take the point that if the voltage reference to the meter is lost, it should signal that fact to the reading interrogation system.   A very good thought!       

   

I take the point that the achievement of a 100 ohm maximum TT earth electrode system is not easy in the UK either and I appreciate the hint that the reinforcing steel in barn floors would be useful.   I was involved with the writing of Standards NZ HB 6117:2104 Electrical Installations in Dairy Sheds, which was all about the avoidance of stray voltages in milking sheds that were upsetting cows and reducing milk production,   We concluded that TT  would be better than MEN in avoiding the return of load current through the earth and through imperfect bonding of metalwork, etc.   So if TT becomes permitted in NZ we should see TT electrical installations in new dairy sheds,   I think that for any TT domestic installations that might be desirable in rural areas to avoid the transfer of EPRs on PEN conductors, some experimentation with rods and buried electrodes will be necessary until the electrical industry here gains experience.  As mentioned above, it will be a new thing for the industry but the EDBs here can probably offer advice from their knowledge of local substation earthing and the required five yearly measurements of substation resistances.   


Having gained such a great response, I should ask if some PME installations are being used to feed a TT part installation or a TT sub-circuit for the purposes of supplying EV charging equipment.   I have a copy of the IET;s CoP for EV Charging Equipment Installation (2nd Edition) and noted the advice in it on converting a EV charging sub-circuit to TT operation.  That appeared to me to be sensible for domestic installations fed from overhead reticulation via an overhead service where the risks of a broken PEN conductor would be higher compared to underground fed installations.   There would need to be some precautions, especially if nearby circuits in a garage or workshop were supplied from a PME system with the risk of differing voltages on different exposed and bonded surfaces - it might be safer to feed a local distribution board converted to TT operation.   


Regards


Peter Browne