TT can I "export" to garage?

[academy]

Hi all, done a cu change and periodic for a family member recently and found they had no earth supplied other than what they were getting off the gas and water main. Put a rod in for them, 84ohms, with all bonding on it drops to 0.25 ohms, even better!
They wanted the garage at the end of the garden powered up and there was an old swa 2core 4mm running to the garage. Now I know we are not supposed to export pme, so I thought even though its a tt i may as well put another rod down at the garage end, and only use the armour at one end to protect the cable, put a rod in and it is 450 ohms, put another rod on top and it drops to 430 ohms. The garage is no more than 25-30 metres from the house.
What's your thoughts on connecting both ends of the armour and doing away with the rod at the garage?
The garage is one of these concrete pre fab ones with the steel beams for the roof, I have bonded to the steel and there is no other services down there. Garage is on a 30 ma rcbo.
Also, a 2 core 4mm swa xlpe, the armouring is sufficient to use as the earth right? I have looked in gn8 at the tables for it but as I remember there is a conversion you have to do to convert it from steel to copper but as I have just finished a night shift and am typing this from my phone, I thought I would just check!

Sorry for any spelling mistakes but like I say, sending from phone, thanks in advance

 

[DurhamSparky]

Now I know we are not supposed to export pme

why not...??

put a rod in and it is 450 ohms, put another rod on top and it drops to 430 ohms

get it deeper!! buy a version that screws together!

ideally if you had 3 core cable it would be better, i dont trust using sheath as earth! prone to corrosion and can become detached if incorrectly terminated!

 

[malcolmsanford]

THe 84omhs you achieved at the house is an acceptabel result and as you say with bonding you get it a results of 0.25, though I hope you use the 84 ohms for yuor EIC.

Though you do get varying results because of soil conditions, IMO the difference your getting in a 20 metre area is quite stark. Have you tried different places on the outbuilding rod, you may find that there is something stopping you getting a decent result in the place your trying now, there may be rubble there where your trying now.

 

[andyb]

Now I know we are not supposed to export pme,

You haven't got pme, you have a tt system.

 

[telectrix]

nothing to stop you using the armour. and rodding at both ends.

 

[academy]

You haven't got pme, you have a tt system.

That's why I put "even though it's tt". I was unsure if the same applied as if it were a pme system.

Yes I put the 84 ohms reading on the eic.

I used the screw together rods when I put the second one on top, thats why I was so surprised to still have such a high reading when I got 84 ohms 25 metres away. I was going to try a bit further away from the garage but that's when I thought I would ask about earthing both ends of the swa.

As for why not export pme, I thought that to be the case. Was sure I read it, possibly even in the brb but maybe gn8, happily stand corrected if that's not the case. Maybe it's one of those things you hear so frequently you believe it to be the truth!! Lol

Thanks for the replies so far

Edit also swa was existing, I would have used a three core as well but it tests fine and they didn't want to pay for a new cable, mind you it looks like it could cost them a few quid in earth rods at this rate

 

[Deleted member 26818]

There is nothing in BS7671 prohibiting the exportation of a PME system.
There are instances where it cannot be used, such as for caravans and construction sites etc.
There are also times when the local distributor may have exceptions to the exportation, or they may have additional requirements before they will allow it.

 

[Chr!s]

Put a rod in for them, 84ohms, with all bonding on it drops to 0.25 ohms, even better!

Sounds as though it maybe fortuitously connected to the neighbors pme

Now I know we are not supposed to export pme, so I thought even though its a tt i may as well put another rod down at the garage end, and only use the armour at one end to protect the cable, put a rod in and it is 450 ohms, put another rod on top and it drops to 430 ohms. The garage is no more than 25-30 metres from the house.
What's your thoughts on connecting both ends of the armour and doing away with the rod at the garage?

If you leave the rods in at the garage, you will now have several in parallel.

The garage is one of these concrete pre fab ones with the steel beams for the roof, I have bonded to the steel and there is no other services down there.

Are the steel beams extraneous?

I

have looked in gn8 at the tables for it but as I remember there is a conversion you have to do to convert it from steel to copper but as I have just finished a night shift and am typing this from my phone, I thought I would just check!

You may be thinking with regards to bonding.Swa armoring in in most instance adequate as a cpc, though bonding may require the use of a core.

 

[Engineer54]

Academy,

Very pleased to see that your trying your best to get a decent Ra on your TT system where others would have just bunged a rod in the ground and if it had a Ra of 200 ohms or less they would have left it as a good-un!! Then totally rely on an RCD for the earth fault protection....

Now you have over 400 ohms on the garage rod after extending. Now try adding another rod but at least the length of the extended rod distant from it. If you can double the distance, even better..lol!!! I would also add another rod to the house rod with 84 ohms, should be aiming for around 20 ohms to make it functional, without the presence of the bonded pipework!! By the way, ...what size rods and what length are the rods your using?? Personally i never use anything under 5/8'' and normally 3/4'' by 1.8mt

Have your parents enquired with the DNO, if it is possible to have there supply PME'd ? Which is a far better option than having a TT system supplying there home.

 

[academy]

Thanks for the replies everyone, with regard to the rods they were the 3/4 ones, two screwed together. I think I am going to move the rod a couple of metres away towards the house and try again ( just hope I can get them out!).
It's my sister in laws place and it was a mission to her her to agree to the periodic and board change, she isn't interested in certificates (getting them anyway). I told her ages ago to get the boars out to come and Chechens earthing arrangements, after one of her inlays got a shock bad enough to send him to hospital she decided to act on it. Board come out and announced " you have an earth" ( this is either through a neighbours tn system or the gas and ware main though) and went on there way. I just thought it easier to tt it what with doing the garage as well.
This is obviously what happens when you try and help someone!!
Any other ideas always appreciated!

 

[Engineer54]

Academy,

You gotta be joking?? The DNO actually said that you have an earth thru the Gas and Water supply pipes?? Write her a short letter stating that, by there own and BS7671 regulations, that incoming metallic service pipes are not, and cannot be classed as the main form of installation earthing.... and then get her to write the DNO a letter of complaint, and asking what they are going to do about it?? She asked if her supply could be PME'd, ...not for them checking that the service pipework are giving an earth path!!!

 

[academy]

Academy,

You gotta be joking?? The DNO actually said that you have an earth thru the Gas and Water supply pipes?? Write her a short letter stating that, by there own and BS7671 regulations, that incoming metallic service pipes are not, and cannot be classed as the main form of installation earthing.... and then get her to write the DNO a letter of complaint, and asking what they are going to do about it?? She asked if her supply could be PME'd, ...not for them checking that the service pipework are giving an earth path!!!

Sorry, no, what they said was "the earth was fine". I am assuming the just took a reading at the cu (with all bonding in place) without actually checking anything, there is no physical suppliers earth cable, only the two bonds to the gas, water and from the met to the cu. The supply is two seperate cable from overhead. The service head does look like it could be converted to pme (by DNO obviously).

 

[Engineer54]

Sorry, no, what they said was "the earth was fine". I am assuming the just took a reading at the cu (with all bonding in place) without actually checking anything, there is no physical suppliers earth cable, only the two bonds to the gas, water and from the met to the cu. The supply is two seperate cable from overhead. The service head does look like it could be converted to pme (by DNO obviously).

All overhead supplies can be converted to PME, if the DNO have done the necessary neutral grounding work on that system. They have been doing such conversions for over 40 years, throughout the UK!!

What you describe, is exactly what they are saying, that your Bonding cables are fine, but that's NOT what you (client) asked them to check!!! My comments above still stand, they are approving bonding of service pipes as a main earthing source.... whether that be by default or not, they didn't bother check where the source of that ''earths fine'', they at best assumed!!!

 

[academy]

Cheers, do you think they will charge for the conversion? While I have there attention I could ask their permission about exporting the pme to the garage, although that would mean using the armour as the cpc which I guess would not happen.

Tried moving the earth rod closer to the house (for the garage supply) today and still got a crappy reading, going to wholesalers tomorrow to get another half a dozen

 

[Engineer54]

They don't normally charge for converting TT and TN-S earth systems to PME, if it is available on your supplying system. The latter would generally be, because of the lead sheathing on the original cable no longer being able to support a low enough Ze to the transformer due to corrosion etc.

Get them back in, leave a letter or something that explains the present system of that ''fine earth'' they quoted previously. At this stage i wouldn't mention your TT system. Get the house supply sorted first.

You can use the SWA armouring for PME distribution, but you will need a suitably sized bonding cable if there is any service pipes in the garage back to the MET. You are then Extending the equipotential zone of the PME system in the house, not exporting it....

Your TT system, As you are going to purchase 6 more rods, I would be trying to create a system using a configuration similar to the letter ''E''.. You will need a rod at each point along the long line of the E, where the short lines meets the long line, and a rod at the end of each of the short lines (Total 6 rods) Firstly drive the rods down approx half the length of the rod, at a distance, at least to the rods length and if possible twice the length, if you have the room. Take readings at each of the rods, and then combined. If you get a half decent reading at this point, drive all the rods home to the full depth. (If not try another location and try again)

You only need to provide one point of this arrangement with an accessible test point box at ground level. The others can be below ground level and permanently connected with the connections protected against corrosion. This can be either by welding (ie braising, exothermic, etc) or by bolted connection that is either thick heat shrink protected, or by means of a good amalgamating tape.