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TT Earthing Systems - Interest by New Zealand

Former Community Member
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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

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    Soil resistivity mapping is available for the UK from the Ordnance Survey.


    That is way way beyond the level of planning used by most electricians, they just look for somewhere no one will trip over the box on top of the rod.


    If you look at YouTube videos of UK electricians installing an earth rod they are usually knocking a 4’ rod in with a 20 oz carpenters claw hammer around the back of a shed, you don’t seem to see them standing on a hop up driving it with a 2lb lump hammer or a small sledgehammer which you generally need to do with a 12’ rod.


    I live on the second terrace River Severn alluvial flood plain, which is actually about the present day flood levels, I can see the river from my bedroom window, but the flood water only gets to the bottom of the road. We are over sand and gravel, which is still quarried just up from us though some of the pits are now flooded as nature reserves and for water sports. Over the sand and gravel is class one horticultural soil, so driving rods actually around where I live is not difficult, but not so far away the bedrock comes to the surface and driving rods is a nonstarter.


    But around here if you have not got a low reading at a depth of 12’ it’s not going to make a huge difference going any further, I put a job up for assessment with a rod reading of over 400 ohms with the rod twelve feet into the ground and the assessor agreed that at that depth it’s a stable reading and unlikely to change.


    You have to bear in mind that here in the UK we are putting a lot of faith in RCD protection rather than getting the earth resistance down in TT installations. Being “old school” I like a 100 mA S-Type RCD main switch upfront of the 30 mA RCDs, but with the change from Type AC to Type A RCDs that is not possible, so the upfront RCD needs to be a 300 mA RCD, which means a lower Ra is required, however 300 mA Type A RCDs are made for the European market so are not readily available rated at more than 63 amps and UK installation practice is to have a 100 amp main switch, so availability of suitable RCDs can be an issue.


    So a lot of installers use a 100 amp main switch and 30 mA RCBOs , which is deemed acceptable. However some use a consumer unit with a 100 amp main switch and a split arrangement with two 30 mA RCDs supplied by internal tails , now bear in mind that we have to use consumer units with steel enclosures in domestic installations, so that is not deemed acceptable. It was something my NAPIT assessor quizzed me on this year as it is a recurring problem he is seeing.


    So personally I see issues with the general standard of TT installations in the UK, many are still protected by voltage operated earth leakage breakers, despite them having been obsolete for over forty years, others have RCDs that do not trip, that have not been tested since the day they were installed and the are some very questionable rod installations.


    Andy Betteridge
Reply
  • 0
    Soil resistivity mapping is available for the UK from the Ordnance Survey.


    That is way way beyond the level of planning used by most electricians, they just look for somewhere no one will trip over the box on top of the rod.


    If you look at YouTube videos of UK electricians installing an earth rod they are usually knocking a 4’ rod in with a 20 oz carpenters claw hammer around the back of a shed, you don’t seem to see them standing on a hop up driving it with a 2lb lump hammer or a small sledgehammer which you generally need to do with a 12’ rod.


    I live on the second terrace River Severn alluvial flood plain, which is actually about the present day flood levels, I can see the river from my bedroom window, but the flood water only gets to the bottom of the road. We are over sand and gravel, which is still quarried just up from us though some of the pits are now flooded as nature reserves and for water sports. Over the sand and gravel is class one horticultural soil, so driving rods actually around where I live is not difficult, but not so far away the bedrock comes to the surface and driving rods is a nonstarter.


    But around here if you have not got a low reading at a depth of 12’ it’s not going to make a huge difference going any further, I put a job up for assessment with a rod reading of over 400 ohms with the rod twelve feet into the ground and the assessor agreed that at that depth it’s a stable reading and unlikely to change.


    You have to bear in mind that here in the UK we are putting a lot of faith in RCD protection rather than getting the earth resistance down in TT installations. Being “old school” I like a 100 mA S-Type RCD main switch upfront of the 30 mA RCDs, but with the change from Type AC to Type A RCDs that is not possible, so the upfront RCD needs to be a 300 mA RCD, which means a lower Ra is required, however 300 mA Type A RCDs are made for the European market so are not readily available rated at more than 63 amps and UK installation practice is to have a 100 amp main switch, so availability of suitable RCDs can be an issue.


    So a lot of installers use a 100 amp main switch and 30 mA RCBOs , which is deemed acceptable. However some use a consumer unit with a 100 amp main switch and a split arrangement with two 30 mA RCDs supplied by internal tails , now bear in mind that we have to use consumer units with steel enclosures in domestic installations, so that is not deemed acceptable. It was something my NAPIT assessor quizzed me on this year as it is a recurring problem he is seeing.


    So personally I see issues with the general standard of TT installations in the UK, many are still protected by voltage operated earth leakage breakers, despite them having been obsolete for over forty years, others have RCDs that do not trip, that have not been tested since the day they were installed and the are some very questionable rod installations.


    Andy Betteridge
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