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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

Andy and Mike, thank you for your informative comments, We do have in NZ what we term distributed generation where installations with PV are not only able to supply their own appliances at light load periods but are to feed back into the local reticulation and be paid for the electrical energy exported. I'm sure the electricity retailers like this as they just sell the export at a considerable profit to the neighbouring installations rather than have to import it from a much more remote transmission grid export point. The distribution network companies are less thrilled since they have to keep an eye on the voltage of the reticulation when injection to it occurs- I believe that this is a problem in sunny Australia where many houses have PV panels on their roofs.

It's fairly standard to shut down the inverter whenever the grid connection is lost because of the risks of back feed into a fault or livening reticulation or HV that is being worked on.

Currently, the distributed generating installations will all be MEN and there may be no strong argument that they should be TT instead, other than the "standard" reasons of remote supply, risk of EPR imposition on the LV, or broken PEN conductors.

There are many consumers with their own generation who wish to disconnect from the grid altogether to avoid the daily rate charged by distributors - so the distributors need to avoid being too greedy with their daily rates!

I can see that islanded systems would need to have their own earthing system such as a TN-S so there would be no need for a TT system to safeguard against EPR imposed on a PEN conductor or a broken PEN conductor.

I can also see that for a "transformerless" converter, as Mike describes, the DC system would need to float free of earth. I note that BS 7671 requires the use of a Type B RCD on the AC sub-circuit linking the AC busbars to the converter and that appears to be sensible in view of DC current content- one might argue that a RDC-DD plus a Type A or Type F might suffice in its place.

Regards

Peter Browne
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0 Former Community Member over 4 years ago

Andy and Mike, thank you for your informative comments, We do have in NZ what we term distributed generation where installations with PV are not only able to supply their own appliances at light load periods but are to feed back into the local reticulation and be paid for the electrical energy exported. I'm sure the electricity retailers like this as they just sell the export at a considerable profit to the neighbouring installations rather than have to import it from a much more remote transmission grid export point. The distribution network companies are less thrilled since they have to keep an eye on the voltage of the reticulation when injection to it occurs- I believe that this is a problem in sunny Australia where many houses have PV panels on their roofs.

It's fairly standard to shut down the inverter whenever the grid connection is lost because of the risks of back feed into a fault or livening reticulation or HV that is being worked on.

Currently, the distributed generating installations will all be MEN and there may be no strong argument that they should be TT instead, other than the "standard" reasons of remote supply, risk of EPR imposition on the LV, or broken PEN conductors.

There are many consumers with their own generation who wish to disconnect from the grid altogether to avoid the daily rate charged by distributors - so the distributors need to avoid being too greedy with their daily rates!

I can see that islanded systems would need to have their own earthing system such as a TN-S so there would be no need for a TT system to safeguard against EPR imposed on a PEN conductor or a broken PEN conductor.

I can also see that for a "transformerless" converter, as Mike describes, the DC system would need to float free of earth. I note that BS 7671 requires the use of a Type B RCD on the AC sub-circuit linking the AC busbars to the converter and that appears to be sensible in view of DC current content- one might argue that a RDC-DD plus a Type A or Type F might suffice in its place.

Regards

Peter Browne
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TT Earthing Systems - Interest by New Zealand

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