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.

 

[Des 56]

What could be considered as solutions to satisfy a TT earthing system that had unsatisfactory high Ra readings ?


What solutions would you attempt to reduce this Ra?

Having answered this question , would it help you resolve your question in the opening post

 

[Aindowsimon]

Pending on Ra reading ( if they were in excess of 200ohm ) don't see a need for additional earth rod ?

Nice pic !

 

[Andy78]

I very much like the picture. I particularly like the neatness of the straight lines, and the earth colouring on the rods, nice touch ! I feel the picture could have been improved however with some scantily clad girls frolicking in the hot tub, and maybe a flying bear with a unicorn's horn growling out a few rainbows for the girls to look at.
This is all I have to offer.

 

[EastAnglian]

That's the way I would do it.

 

[Engineer54]

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.

View attachment 20419

Let's get something straight you don't export earthing Zones you extend them, and providing you apply a couple of simple rules, there is no problem in extending a PME equipotential zone either!!

As we are talking true earth with TT systems then the more deep earth electrodes the better. They can be linked together preferably via a 3rd core of a SWA cable or via the armouring of the SWA cable How many CU's and RCD's do you have on this sub-main cable?? Remembering that in-line/series RCD's need to discriminate between each other

Don't take any notice whatsoever of Post #3, the author obviously knows little to Jack about installing TT systems. He's a reader rather than a do'er!! lol!!

 

[Aindowsimon]

Explain wot I've say'd that's wrong ?

 

[Engineer54]

You'd be quite happy with a 200 ohm Ra would you?? I'll say no more!! lol!!

 

[Aindowsimon]

If Ra is 200ohm or above in bs7671 that earth is unreliable as a TT. I know it isn't a good earth at that but with the correct rcd it would be compliant , and providing the Ra is below 200ohm ?

 

[GLENNSPARK]

and again we have the lack of understanding about good, low, stable, Ra ...and how to get it...

depth is the way here

depth is key with TT...

 

[Engineer54]

If Ra is 200ohm or above in bs7671 that earth is unreliable as a TT. I know it isn't a good earth at that but with the correct rcd it would be compliant , and providing the Ra is below 200ohm ?

And that's why i stated earlier your a reader, rather than a do'er!!

 

[Deleted member 9648]

Another recruit to the cause......:clap:



:wink_smile:

 

[Aindowsimon]

Tel me why it isn't , rather than "becouse it is " answer's. I wasn't born with the knowledge you know I learn , and yet to this day I'm learning so give me you're side and il check it out , and at the end of it might come out wiser

 

[telectrix]

rods, rods, and more rods: the more the merrier. and go deep.

 

[Engineer54]

Tel me why it isn't , rather than "becouse it is " answer's. I wasn't born with the knowledge you know I learn , and yet to this day I'm learning so give me you're side and il check it out , and at the end of it might come out wiser

There are more than enough Threads on this subject here on the forum, check them out!!

But the last thing you want on any TT system is a ridiculously high 100/200 ohm Ra value, where your totally relying on a single unreliable RCD device for your earth fault protection covering several circuits. Just because it states a figure of 200 ohms in BS 7671, DOESN'T mean that's the figure you aim for, or should be happy with.... Oh, and never use those thin short 3/8'' twigs (so-called rods) you will never achieve anything like a suitable or stable value....

 

[Bennewman]

Whilst I appreciate your replies, I'm not sure my questions have been answered. Let's say the current rod ra is 60 ohms, Do I need to fit another? If so, should this be independent of the current one?

 

[Deleted member 9648]

There are more than enough Threads on this subject here on the forum, check them out!!

But the last thing you want on any TT system is a ridiculously high 100/200 ohm Ra value, where your totally relying on a single unreliable RCD device for your earth fault protection covering several circuits. Just because it states a figure of 200 ohms in BS 7671, DOESN'T mean that's the figure you aim for, or should be happy with.... Oh, and never use those thin short 3/8'' twigs (so-called rods) you will never achieve anything like a suitable or stable value....

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?

 

[telectrix]

Whilst I appreciate your replies, I'm not sure my questions have been answered. Let's say the current rod ra is 60 ohms, Do I need to fit another? If so, should this be independent of the current one?

my input is: take the TT earth through the SWA and bang in a decent extra rod. how to you think DNO get TNC-S values of < 0.35---- by rodding all the way down the line.

 

[sparky351355]

A new earth rod should be installed in each outbuilding . They may well be linked together which will give you parallel earth paths thus lowering the Ra . Hope this makes things clear. Each outbuilding should have its own earth zone because if the main earth is lost at the mains u will still have a reliable earth in the outbuilding .

 

[telectrix]

he'll not get a reliable earth with a 3ft. coathanger. big rods, drivven deep is the only way.