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Posted By Topic: TT system cowsheds

Opunake2017
Aug 29 2017 17:21

will it ever happen?
   

AlecK
Aug 29 2017 17:39

I'm sure it will......_eventually.
If only because the networks & Energy Safety want it.
   

Opunake2017
Aug 29 2017 18:07

I was reading the NZS standard and they say to add a dummy MEN link. What the heck!?
   

Opunake2017
Aug 29 2017 18:14

I was researching the cowshed NZS standards 6117 , and it seems to be to do with MEN recirculating currents, poor bonding. Im ex power board/sub station tech, and feel sorry for cows. must be hell all that earth, nuetral return stuff!! Like my old uncles house when i was eight... (shocks) in the shower of his villa.we just put up with it!!
probably no bonding, no main earth.

   

Sarmajor
Aug 29 2017 19:14

Most of the problems in cow sheds are caused by incorrect installation of the electrical system and items like VSD's.

Changing to a TT system will not be a magic bullet I'm afraid, unless the people doing the electrical installations up skill and have greater involvement in the build of the shed to get bonding wires installed for all the metalwork. To late after the concrete is poured.

And retrofitting a TT system into an older shed may prove to be very expensive.


   

zl2aj
Aug 30 2017 07:56

Answer to opening question - I hope so.

MEN varient of TNC-S earthing was a brillient system when every property was connected to their neighbour via the metalic plumbing running down the street. Unfortunately those days have gone yet we still are expecting MEN to work with the same level of safety, relying on a spike in the dirt.

TT system is perfectly safe if done properly. Arguably safety than TNC-S. And with the new version of AS/NZS3000 madating RCDs practically everywhere most of the cost of TT is already spent.

Retrofitting TT will still be expensive yes. And its not a solution to earthing issues. All it is doing is hiding bad earthing systems. TT is no excuse for poor earthing to survive. But it does have its place.

So that is why "I hope so".
   

AlecK
Aug 30 2017 09:07

True we no longer have multiple installations effectively sharing a common earth electrode - but then outside of cities / towns we never did.
Not quite true to say "relying on a spike in the dirt"; because what we're actually relying on is a whole lot of "spikes in the dirt" all in parallel - so providing effective contact with mass of earth. Especially since some of those "spikes" are on the network and (unlike installation electrodes) have to meet minimum effectiveness. So worst case for MEN is a country situation with SWER supply to a single installation; ie only 2 electrodes (one network, one installation)

Compare TT, where the fault protection depends entirely on the effectiveness of the installation electrode. Which will vary from fair (at best) to bloody awful; even if done to a much higher standard than we can get away with on MEN. Hence the need to require a front-end RCD; it's the only way to get fault protection by automatic disconnection working. But only downstream of the RCD, and it's unclear how networks will provide fault protection up to and including main switchboard.

What TT will do is reduce problems from imported earth potential rise; eg faults on network. Which - as Sarmajor observes - is NOT the main source of cowshed problems.

The plastic "false link" is actually a very good idea. without it you can guarantee a high proportion of sparkies (of those few that notice it isn't there) will put a link in and stuff up the system. Most of our trade has a poor understanding of the MEN earthing system; and having two systems in use is likely to lead to more problems.

As for cost, not huge really. There's additional cost for a better earth electrode system, plus the front-end RCD. But there will be NO saving due to any other RCD provisions, because those 30 mA subcircuit RCDs will still be needed (the front-end one being 300 mA). Then there's the cost of blacking out the entire installation when the front-end RCD trips - there's NO discrimination; unless the 300 has a time delay - which means it can't provide fault protection.

I believe adopting TT will cause at least as many problems as it solves. The main difference being the costs will fall on installation owners rather than on networks.
   

zl2aj
Aug 30 2017 09:32

How will networks protect from faults?

The same way they do now I suspect. With a fuse that will never blow in a phase to earth fault.

TNC-S doesnt exist until the first switchboard - before then it is TNC. So the only fault path is via the main neutral. That will not change (in a practical sense) with TT.

AGree re plastic MEN link, the need for training and education, and effective earthing.

Using the 300 mA RCD for the main switch - V/I gives us a resistance of 767 Ohms. From my experience in testing earthing systems this is extremely high. I agree it does need to be tested to ensure effectiveness of the earthing system (and thus the protection). And that test benchmark should accommodate for varying soil conditions during dry seasons etc.

Yes its a cost that installation owners may choose to bear. Remember TT will always be an option - I would not like to think it is forced upon an installation owner. As far as lighting network costs - not really. Maybe lightening the risk of network faults causing installation damage that they may be liable for (post event). For those consumers that own shares in their network this is just a cost shift not a cost removal. Semantics I know.
   

Andrew
Aug 30 2017 10:27

Aleck: Why would you say a time delay on the 300mA RCD can't provide fault protection? As long as the delay allows it to still switch within 0.4s it should be fine. Most 30mA RCDs act far faster than the 0.3s upper limit they're allowed so there is still leeway for discrimination.
   

AlecK
Aug 30 2017 11:18

ESR 32 requires networks to provide fault protection for mains.
Yes it currently only works due to the MEN link, so using the N rather than the installation's earthing system to let enough current to flow.

How it would work for a TT system has yet to be determined, but i suspect it will be by connecting all "required to be earthed" items upstream of RCD to mains N instead of to installation earthing system. That will include meter boxes and main switchboard enclosures.

Also not yet decided is the extent of the "option" to use TT. It could be imposed on the installation by a network decision. Or it may be an option the installation owner can choose - though it may only be available for particular circumstances.

I presume these aspects will be discussed & dealt with by the Committee that will need to write amendments to "3000" before TT can be used. After which there will be "public comment" on the proposed rules.

Andrew:
The time delay built into Type S RCDs exceeds 0.4 s; so a Type S can't provide fault protection by automatic disconnection.
And that's the only realistic option, since the permitted alternatives allowed by 1.5.5.2 are not practicable for an entire installation.

So the front-end RCD is going to have to be Type A (or maybe Type B or type F); and there won't be discrimination with the subcircuit RCDs. All there will be is the fact that the actual current will be higher in relation to rated residual current for the lower-rated RCD, so - at some current levels - it is likely to trip sooner. Also fault current through a typical human body is in the order of 230 mA, so may not trip a 300 mA RCD. But that's a dry-skin value; and TT is being proposed with particular reference to cowsheds, which tend to be pretty damp. Touch current is therefore likely to be higher, well above 300 mA. And if a live wire touches earthed metal, higher still; with good chance of tripping both RCDs.


Point being: RCDs do not limit the value of the current, they just react to it.
   

dbuckley
Aug 30 2017 15:24

The "problem" with TNCS as seen in cowsheds and some and other situations is not actually the fault of TNCS, but the way that TNCS is implemented *in practice*.

If one looks at the Wikipedia illustrations of the various earthing systems, TNCS is different to MEN in an important way, and that is the "M" bit of MEN. With "pure" TNCS the "cowshed problem" doesn't occur other than in fault conditions. With MEN, the body of earth is placed in parallel with the neutral conductor, and thus there are both step potential problems and soil-to-earth-wire voltage differences.

The other problem that makes MEN difficult is when the equipotential zone is imperfect, or worse, is perforated.

TT is a cracking system __under certain specific situations__ when the benefits of TT outweigh the benefits of MEN. For most "normal" situations, MEN is the system of choice, even in an age of the RCD.

As Alec notes, once TT is permitted the question then will be one of whose decision it is to allow or choose TT over MEN, because there is ample scope to select TT for all the wrong reasons, and thus end up with a less safe system than may otherwise be possible.

This is not a new debate; I'm quite fond of some words (behind paywall unfortunately) written in 1937 on the issue....
   

Andrew
Aug 30 2017 17:03

So is the Type S time delay required to be more than 0.4s by the RCD standard or is that just the state of existing devices? I would expect that by the time we're getting TT installations happening there would be RCDs suitable for protecting the installation that can discriminate with RCDs for personal protection.
   

pluto
Aug 30 2017 17:26

As one of the committee who wrote the report recommending TT system of supply, the education of ALL tradespersons to fully understand the Existing TN-C-S (MEN stort of) system and then the need to educate and fully understand the TT system of supply was strongly recommended before everyone will be let loose in electrical installation that will be a requirement.

It must be remembered that the MEN system used in AU and NZ is NOT the same as TN-C-S used by the International Standards.

ESR 2010 will need amendment to allow use of TT and you have to use a Part 1 solution for the electrical installation, so lots of study will be required by ALL in the near future.