Extending TT to remote outbuilding ??

[Bennewman]

Hi all.
As I understand it, exported earth is only an issue for PME situations? Therefore, when a domestic installation is a TT and power is to be brought out to a remote outbuilding, is there a requirement to fit a second earth electrode and if so, should this be independent of the house TT setup? I've added a crude picture of the installation scenario wiring. As you can see, if a second earth electrode is installed, I've questioned by use of the red zigzag whether there should be continuity in the earth from the house to the cabin CU, or it should be terminated without connection?

Many thanks.

 

[dazza1981]

Thanks mdj

 

[MDJ]

Thanks mdj

Just having some fun with dave, no malice intended

 

[davethsparky]

Blimey, people apparently qualified and employed in a technical trade without a grasp of the basics of the language.

The point is that extending a PME supply is possible and there are no issues, exporting it however is fraught with issues.

Maybe a dictionary would help?

 

[dazza1981]

You are just a genius mate..... so clever and yet still cant answer a simple question

 

[davethsparky]

Actually I am not a genius, and intelligence is largely irrelevant to the point.

This is a forum for tradesmen in the electrical installation industry, this a technical trade and anyone who is incapable of maintaining a good degree of technical accuracy really need to re-consider their career choice. You have been chopping and changing between exporting and extending, do you expect to be taken seriously?

As mentioned previously there is no problem extending a PME and earthing system if it is done properly.

Exporting PME is a little harder as it would require selling it to another country, this could raise a significant philosophical issue as to how exactly you would define the physical manifestation of the theory which you are trying to export.

- - - Updated - - -

And yes I realise there is a significant chance of me falling victim to Muphry's law sometime soon.

 

[dazza1981]

dear 'o' dear

 

[Squid_uk]

I recently have done a similar installation to an out building. I installed a new submain to the building with a new DB and earthing stake. I used a 16mm 3 core SWA but did not connect the 3 core (used as earth) to the house earth, therefore keeping it seperate but still protecting the submain via a 3rd core and the steel armour. Also my Ra reading was 60ohms after installing 2 stakes coupled, so as stated in earlier posts, depth is good!

 

[EastAnglian]

How to you think DNO get TNC-S values of < 0.35---- by rodding all the way down the line.

Nope - its primarily because the Neutral is connected to Earth at the sub-station!

 

[davethsparky]

Nope - its primarily because the Neutral is connected to Earth at the sub-station!

Nope, its because they are one and the same conductor in the distributors network, disconnect the link to earth at the substation and you will get pretty much the same loop impedance, just the minor problem of far too many volts!

 

[Engineer54]

Nope - its primarily because the Neutral is connected to Earth at the sub-station!

So is TN-S!! Just that the further away from the sub-station you go, the higher the Ze value is going to be. Multiple low value N-E connections along a cable supply run, help keep low values throughout a PME run, the more the better....

A true PME system on an underground DNO LV distribution network cable will have roded N-E connections at every joint. Needless to say, their are relatively few ''converted'' true underground PME systems around, only the newer DNO PME distribution installs will meet this criteria. Overhead converted supplies will generally be True PME.

 

[davethsparky]

I recently have done a similar installation to an out building. I installed a new submain to the building with a new DB and earthing stake. I used a 16mm 3 core SWA but did not connect the 3 core (used as earth) to the house earth, therefore keeping it seperate but still protecting the submain via a 3rd core and the steel armour. Also my Ra reading was 60ohms after installing 2 stakes coupled, so as stated in earlier posts, depth is good!

Should the sub-main cable not have had its earth connected to the appropriate earth for the associated OCPD though rather than the new earth you have created at its load end?

 

[Bennewman]

So.......a second earth stake has to be fitted. BUT, should this be independent of the original??

 

[davethsparky]

No, but you may have to give a little thought in relation to the size of the CPC in the sub main as a result of this.

I may be completely wrong about this, but I think earth fault current will be share between the two rods in proportion to their Ra values ( as resistances in parallel)

 

[Engineer54]

See post 24 of this thread praising an Ra of 15 ohms..........http://www.electriciansforums.co.uk...cal-forum/77449-tt-installing-5-8-rods-2.html

"Nothing more needs to be said than ...Well Done!! Though for the life of me, with all those power tools you say that you own, .....Why didn't you use a decent SDS hammer to drive them rods in?? A few minutes and you'd of been done and wouldn't leave a bloody big entry hole from all the swinging around of the rod!! lol!!"

Cant possibly be relying on one of those nasty o'll RCD's though can it?

Tut Tut!! Still trying to convince begineers to be lazy so and so's then!!

 

[Engineer54]

Should the sub-main cable not have had its earth connected to the appropriate earth for the associated OCPD though rather than the new earth you have created at its load end?

Why on earth (excuse the pun) would you not link the two earth rod positions via the 3rd core between the MET and EMT??