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TT Earthing Systems - Interest by New Zealand

Former Community Member over 4 years ago

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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0 Former Community Member over 4 years ago

Graham Kenyon kindly pointed out to me the safety risks associated with the siting of a TT earthing electrode system in the vicinity of extraneous conducting parts that might be connected to the PME earthing system either at the installation itself or at a neighbouring installation so care needs to be taken where there are incoming water or gas pipes and service cables. Their whereabouts are not usually well known although toby taps, water meters and gas meters should give a clue as to their starting point in the street. Looks as though having a tracing cable / pipe device would be a good investment for any electrical contractor. So that is good advice and I'll include it in our committee's report to the regulator as something to be included in any future guidance to TT installation designers / installers.

I have never purchased a Type B RCD so am blissfully ignorant of its bankrupting qualities - one would not wish to have one just sitting in a van in case the need to replace one in service turns up. However, keeping in mind the advice provided in IEC 60364-7-722 that I mentioned in my last post, why not look at a RDC-DD plus a Type A or Type F instead of a Type B if the cost of the latter is horrendous? I must admit I would not wish to have to install a Type B RCD to protect my washing machine or induction top and would be looking closely at cheaper options that would nevertheless be effective.

Regards

Peter Browne
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0 Former Community Member over 4 years ago

Graham Kenyon kindly pointed out to me the safety risks associated with the siting of a TT earthing electrode system in the vicinity of extraneous conducting parts that might be connected to the PME earthing system either at the installation itself or at a neighbouring installation so care needs to be taken where there are incoming water or gas pipes and service cables. Their whereabouts are not usually well known although toby taps, water meters and gas meters should give a clue as to their starting point in the street. Looks as though having a tracing cable / pipe device would be a good investment for any electrical contractor. So that is good advice and I'll include it in our committee's report to the regulator as something to be included in any future guidance to TT installation designers / installers.

I have never purchased a Type B RCD so am blissfully ignorant of its bankrupting qualities - one would not wish to have one just sitting in a van in case the need to replace one in service turns up. However, keeping in mind the advice provided in IEC 60364-7-722 that I mentioned in my last post, why not look at a RDC-DD plus a Type A or Type F instead of a Type B if the cost of the latter is horrendous? I must admit I would not wish to have to install a Type B RCD to protect my washing machine or induction top and would be looking closely at cheaper options that would nevertheless be effective.

Regards

Peter Browne
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TT Earthing Systems - Interest by New Zealand

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