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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

Parents

0 Former Community Member over 4 years ago

Thank you for the continuing comments on the above theme. Three phase supply to a domestic installation would be very rare and would be justified only by a mansion very heavy electrical loading or else a three phase industrial load in the garage or workshop. The norm is a 60A single phase supply and unless in new subdivisions is likely to be overhead by open wires (less common these days) or a neutral screen cable. At least the overhead neutral screen cable is inherently safe but its neutral tail connections may not be. Ideally a double clamp on the screen would be wise but does not always apply. i

The concept of a Class 2 EV is interesting and, who knows, we may all be driving around in EVs with fibreglass or plastic chassis in a few years' time. Not sure about carbon fibre - might be conducting!

Regarding RCDs, the need for a Type B RCD when supplying power electronic equipment is appreciated and is certainly advised here. I had not contemplated the use of a Type B RCCB for front end property protection purposes. My thoughts are that we could have a 300mA or 100mA Type S RCCB as the main switch on a TT main switchboard (or on a TT distribution board fed from a MEN main switchboard) with a 30mA Type B RCBO supplying the EV charging equipment and 30mA Type A RCBOs supplying the close by electrical equipment so there is an equipotential zone in the vicinity of the EV. But there would be several possible solutions and we have plenty of time to think about it while we change the regulations - never an easy matter!

By the way, we require all RCDs in MEN installations to interrupt all live conductors as a matter of course, which should avoid any problem with an earth faulted neutral conductor. Same will apply to any TT installation.

Regards

Peter Browne
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0 Former Community Member over 4 years ago

Thank you for the continuing comments on the above theme. Three phase supply to a domestic installation would be very rare and would be justified only by a mansion very heavy electrical loading or else a three phase industrial load in the garage or workshop. The norm is a 60A single phase supply and unless in new subdivisions is likely to be overhead by open wires (less common these days) or a neutral screen cable. At least the overhead neutral screen cable is inherently safe but its neutral tail connections may not be. Ideally a double clamp on the screen would be wise but does not always apply. i

The concept of a Class 2 EV is interesting and, who knows, we may all be driving around in EVs with fibreglass or plastic chassis in a few years' time. Not sure about carbon fibre - might be conducting!

Regarding RCDs, the need for a Type B RCD when supplying power electronic equipment is appreciated and is certainly advised here. I had not contemplated the use of a Type B RCCB for front end property protection purposes. My thoughts are that we could have a 300mA or 100mA Type S RCCB as the main switch on a TT main switchboard (or on a TT distribution board fed from a MEN main switchboard) with a 30mA Type B RCBO supplying the EV charging equipment and 30mA Type A RCBOs supplying the close by electrical equipment so there is an equipotential zone in the vicinity of the EV. But there would be several possible solutions and we have plenty of time to think about it while we change the regulations - never an easy matter!

By the way, we require all RCDs in MEN installations to interrupt all live conductors as a matter of course, which should avoid any problem with an earth faulted neutral conductor. Same will apply to any TT installation.

Regards

Peter Browne
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Vote Up 0 Vote Down

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TT Earthing Systems - Interest by New Zealand

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