mbrooke
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Why is TT earthing so common outside of North America? I mean, why would an RCD/GFCI main and ground rod be so advantageous over a main bonding jumper (TN-C)? :dunce:
TT earthing... Why?
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iceworm
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Here are two papers - possibly somewhat interesting
The firstis more generic. The second goes into the faults and the the math.
http://www2.schneider-electric.com/...=JlYUy26PgQtflpywqGqfHcYI5KP-5KwWM1eQF_GNF4Y,
http://www2.schneider-electric.com/...ility-safety/low-voltage-minus-1kv/ect172.pdf
ice
mbrooke
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Thanks
Ive seen those, but they don't necessarily go into the advantages of TT. I guess Im not seeing the whole picture, but my understanding is based on that a 5 cent bonding jumper is cheaper over having an RCD main, or going to the greater extent of having an RCD delayed trip main with RCD branch breakers in the event one RCD fails. Also the disadvantage that the neutral must essentially be treated as live since it will be in excesses potential to the structure's equal potential system.
To me the cost to overcome the disadvantages as well as the risks would have made TT exceptionally rare but some advantage must be great enough to keep it in use.
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One consideration in the evolution of the national system is that a cost saving for the utility may be offset by a greater cost for the user's side. But the engineering decisions may have been made based on the first consideration only, especially given that at the time the systems were first being designed the RCD requirement, etc. did not exist.
Momentum.....
Objects that have been arrested tend to remain arrested.
mbrooke
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One consideration in the evolution of the national system is that a cost saving for the utility may be offset by a greater cost for the user's side. But the engineering decisions may have been made based on the first consideration only, especially given that at the time the systems were first being designed the RCD requirement, etc. did not exist.
Momentum.....
Objects that have been arrested tend to remain arrested.
You have a point.
There is one example were TT is mandatory over TN_C, that being a trailer. Should the PEN break, the shell will be live to remote earth where in TT it will not be.
mbrooke
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FWIW this is might be of help... I think...
http://www.eea.co.nz/Attachment?Action=View&Attachment_id=1436
http://www.eea.co.nz/Attachment?Action=View&Attachment_id=1436
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Assuming, of course, that the earth electrode of the trailer has a low enough resistance to provide an effective fault current path to trip OCPD.
Another potential [sic] reason for the difference might be the higher voltage in many of these other systems. Not that earth makes a reliable fault path for those either.
But one thing that does allow the earth to be an effective fault path is that these same countries often have requirements in the vicinity of one ohm or less for the building earth electrode resistance. Very necessary when it carries normal current too.
Sounds expensive.
iceworm
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iceworm
Curmudgeon still using printed IEEE Color Books
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- North of the 65 parallel
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- EE (Field - as little design as possible)
Possibly you are not seeing the whole picture.
Recommend reading pages:
11, 12, 13 of ect172.
and pages 11, 12 of ect173
I beleive they accurately show the differences between TT, TN (and IT).
ice
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mbrooke
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I will re-read them. But I do see one advantage (I think)... MV crossing in LV. If such a rise happened, it would transfer to the home ground system. As long as it doesn't go over a few hundred volts to ground, beyond that point would mean flash over.
The greatest advantage of TT over TN is far less arc flash and fire hazards.
mbrooke
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Good point
That did cross my mind but left it, until you mentioned it. The high soil resistance will limit current, perhaps only to a few amps. Much safer over several thousand or more amps of fault current.
Tony S
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Good point
And a rise in potential touch voltage.
I?ve only done two domestic TT installs being in the main industrial for the past 40 years, in each case achieved <1ohm.
Unfortunately with the advent of the RCD (GFCI) the younger electricians in the UK seem happy with a Ze of <200ohm. This is in the main attributable to the ambiguous reference in IET BS7671 to >200ohm as being unstable, this has been translated to the minimum requirement. Readings for Ze are for the earth nest alone, bonding to services are not to be included due to the possibility of replacement with alkathene pipe.
These young electricians now see the RCD as a ?get out of jail free card? and simply don?t see the need to put in the work to create a good stable earth nest.
I have a text book from the 1930?s that states <10ohm as being borderline acceptable <1ohm preferable.
How things have progressed (backwards.)
As for MV/LV earth fault crossover, it is normal UK practice for the first PME (CNE) earth rod to be at least 8m from the nearest MV earth rod.
Through my work I installed several substations. All of them TN-S on private networks each substation with solid bonds between the MV/LV MET?s. (It made my life easy.)
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mbrooke
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How would one receive one ohm with TT? Unless metal waterlines existed? What is alkathene pipe?
I don't blame newer electricians for being less concerned over earth resistance in TT or any network for that matter. Maintaining something like 1ohm or 10 ohms is a gamble and cant be done in all soil conditions. RCD is the real work horse where earth is the return conductor.
IMO, Id be more concerned about bonding than grounding in general. Grounding may be what clears a fault in TT, but bonding is what provides all the safety during a fault. I think outside of TT earthing grounding does little. In some soil conditions low ohms is pointless. Making sure everything is bonded to the earth bar in the consumer unit is what makes for safety.
There is one advantage of TT over TN-C. If one has poor bonding, should you loose the neutral TT will protect you, however TN-C will become a nightmare. Anything not bonded to the earth bar like the slab will have an opposing potential.
Personally, both TT and TN-C have short falls under different contingencies. TN-S should be mandatory in all networks and DNOs as this would solve many problems associated with both systems.
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Tony S
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Alkathene is a form of heavy gauge plastic pipe now used for both water and gas distribution.
<1ohm is achievable with deep driven rods with a spacing of 1.5xdepth between rods. For my own house I used 25x3mm copper tape between the rods to form a peripheral ring.
Being friends with a senior DNO engineer I asked if he would test the nest around the house, it was better that the local substation. I did say in my post I?ve only ever worked on two domestic installs. For myself and my sister, otherwise you wouldn?t see me for dust.
TN-C is more or less the standard system for new distribution systems.
TN-C-S All the UK DNO?s now use it to repair TN-S.
To be honest the UK distribution system is in a total mess! All for the sake of economy.
As you say TN-S is the preferred system and as I said, I?ve always worked on private industrial systems where TN-S is standard.
That said, in my time I?ve also worked on IT MV distribution systems becoming an SAP.
One type of system still makes me break out in a cold sweat, we used 660V earth free single phase for induction furnaces. I spent three months looking for an intermittent earth fault on one system.
<1ohm is achievable with deep driven rods with a spacing of 1.5xdepth between rods. For my own house I used 25x3mm copper tape between the rods to form a peripheral ring.
Being friends with a senior DNO engineer I asked if he would test the nest around the house, it was better that the local substation. I did say in my post I?ve only ever worked on two domestic installs. For myself and my sister, otherwise you wouldn?t see me for dust.
TN-C is more or less the standard system for new distribution systems.
TN-C-S All the UK DNO?s now use it to repair TN-S.
To be honest the UK distribution system is in a total mess! All for the sake of economy.
As you say TN-S is the preferred system and as I said, I?ve always worked on private industrial systems where TN-S is standard.
That said, in my time I?ve also worked on IT MV distribution systems becoming an SAP.
One type of system still makes me break out in a cold sweat, we used 660V earth free single phase for induction furnaces. I spent three months looking for an intermittent earth fault on one system.
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mbrooke
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How is the reading derived? Sorry for the doubtfulness. In the US its is exceptionally difficult to obtain a true 1ohms via ground rod.
If you ask me Belgium has got the right idea with there 133/230Y system, no neutral to worry about:angel: If you really want an odd system you should see Norway's older ITs But economy is what triumphs over all else, unfortunately.
Tony S
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For my own house by fall of potential. How else could we do a comparison to the local substation?
?In the US its is exceptionally difficult to obtain a true 1ohms via ground rod?. In my post I stated multiple rods at a spacing of 1.5 x rod length, not a single rod.
Derbyshire tends to have a somewhat damp climate. A beautiful and scenic place to live if you like the rain.
As for the voltage in Belgium, I had to install a 11/.22/.38KV Dyn11 transformer to run a factory in the UK using Belgian made equipment as they wouldn?t guarantee their equipment otherwise.
?In the US its is exceptionally difficult to obtain a true 1ohms via ground rod?. In my post I stated multiple rods at a spacing of 1.5 x rod length, not a single rod.
Derbyshire tends to have a somewhat damp climate. A beautiful and scenic place to live if you like the rain.
As for the voltage in Belgium, I had to install a 11/.22/.38KV Dyn11 transformer to run a factory in the UK using Belgian made equipment as they wouldn?t guarantee their equipment otherwise.
mbrooke
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What do you mean by fall of potential? If you are referring to neutral voltage drop equal potential is done by bonding, not grounding (I will post something I typed up to explain).
Even a hundred rods, I just cant see 1ohm all year round. In my opinion, it gets to a point where multiple rods are a waste of money, but in TT that might not be to bad. But still, ground rods over the code minimum are a waste of money in TN-C and TN-S.
220/380 does exist in Belgium but I have heard of DIYs and Engineers bring up 3x220 no neutral. If you google 3x220 or "3x230 sans neutre" "prise 3x220" or the like you get posts on this system.
Home purchased in Belgium: http://fr.sci.electrotechnique.narkive.com/2WHrMQjo/triphase-belge-bizarre
6th post in: https://www.flukso.net/content/how-many-clamps-do-i-need
DIY example: http://www.bricozone.be/fr/electricite/t-3x220-sans-neutre-66671.html
http://www.bricozone.be/fr/electric...iments-du-courant-triphase-3-x-220-72375.html
http://www.bricozone.be/fr/electricite/t-raccordement-taque-en-3x220v-sans-neutre-53866.html
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