mbrooke
Batteries Included
- Location
- United States
- Occupation
- Technician
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:
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:
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
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.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.
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.
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.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.
Sounds expensive.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.
...Ive seen those, but they don't necessarily go into the advantages of TT. I guess Im not seeing the whole picture, ....
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
The greatest advantage of TT over TN is far less arc flash and fire hazards.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.
The greatest advantage of TT over TN is far less arc flash and fire hazards.
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.
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.)
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.
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.