Another Earth Rod or Export from MET?

[MarcT]

The trouble with TN-C-S systems is that although they provide a good Ze reading, their integrity is not guaranteed by the DNO. As far as i'm aware, the issue with the TT outbuildings started when a guy got electrocuted when his swimming pool metalwork (which was connected to a TN-C-S earth) was a different potential to the earth outside. We are therefore advised (by tutors at least) not to bring TN-C-S outside, and to earth a covered metal sheath to the MET, and an exposed metal sheath to the rod in the outbuilding. In this case there is extraneous metalwork common to both buildings. I think Des's solution is correct in this case. Its hard to find regs to corroborate your beliefs as well. However when I get stuck, I look for regs in a different situation but the same problem. For instance, caravan parks don't export TN-C-S for the same reason, but on the other hand if the the metalwork in the garden happened to be to do with a swimming pool instead of service pipes, then it is advised to bond them to an electrode as well as the TN-C-S
There is a small note on page 171.
The thing to remember with TN-C-S is that it's 0v nearly all of the time

 

[Guest 004]

Thanks for the replies. I just have trouble getting my head around the fact that you are being advised to do away with the lovely, low reading earth that the supplier has given you, and fit an earth rod, which you may struggle to get below 100ohms What about exporting the supply earth, AND using a rod? Is this better or worse (presuming worse) than just exporting the supply earth?

Edit- just re-read Des's post, I like the idea of installing a rod to the PME.

Cheers, Dave.

Agreed

 

[ian3012]

just a point mr enigma... the 100ma rcd you mention as a main switch, is not required in a 17th edition tt installation.. ie. a tt installation where all circuits are additionally protected by 30ma rcd....

 

[Guest 004]

just a point mr enigma... the 100ma rcd you mention as a main switch, is not required in a 17th edition tt installation.. ie. a tt installation where all circuits are additionally protected by 30ma rcd....

same difference whether whole install is protected by a single 100 ma main switch, or all individual are protected by RCBO's. Simple answer is cost

 

[electric-ian]

Hi guys,

Interesting thread.

I assume the gas and water services from the house are in plastic...?

 

[walec]

No they are copper and are now bonded back to the MET at the TT.

 

[electric-ian]

Ok, I stand to be corrected if I'm wrong but surely if pipes are copper TT is pointless ??

The resistance of the copper would be way lower than the stake and therefore any fault path would be down the pipes not through the stake if they are bonded at both ends..

Definitely a case for extending the bonding in 10mm from the house.

I suspect that Zs at the sockets is way below your Ze figure for the stake due to the bonding.....

 

[Jimmy Boy]

Sorry to drag this oldie up but it came up on a goggle search, my Bungalow is on TNCS fed from a pole, deffo not TT, the garage has an exported earth via an SWA cpc csa would be 6mm would it wise to run another 10mm cpc to be belt and braces ? and I was also wondering how can I reduce the risk of shock if the PME N drops out in the road ? can I install a E rod to use in conjunction with the TNCS ? as a back up ? can't find owt in regs

 

[Guitarist]

You only need a 10mm main bond if you have services in the garage.

 

[Jimmy Boy]

You only need a 10mm main bond if you have services in the garage.

Water fed from a plastic main mate so I couldn't bond it so would there be any point in running a 10 ?

Any comment on my dual earthing thoughts in my post ?

 

[Deleted member 9648]

Water fed from a plastic main mate so I couldn't bond it so would there be any point in running a 10 ?

Any comment on my dual earthing thoughts in my post ?

I think it's been suggested on here before that an earth rod to the MET is beneficial....personally I've never seen it done. Lost neutrals on the supply are so rare as to be not worth worrying about,and the neutral is normally connected to earth at regular intervals anyway,thats why its called PME.
I wouldnt bother with a 10mm to the garage unless there is likely to be an ECP in the future.

 

[Jimmy Boy]

Thanks all much appreciated

 

[Engineer54]

This thread is over 2 years old, ...there are far more informed threads on this issue within the forums now. Save to say there is NOTHING wrong with extending a PME/TNC-S earthing system from the main house to an out building/garage etc!!

Why would you settle for a numbty TT system with high Ra values, over a proven TN system?? It's about time we kicked these die-hard scaremongers into touch!! lol!!

Steve Breeds.... If the installation has been converted from a TT system to a PME earthing system at some stage, then it would be prudent to leave the existing earth rod(s) connected to the MET, especially if Ra is half decent!! It'll act as another point of the multiple neutral earthing system. I wouldn't however go out of my way to provide an earth rod to an installed PME system!!!

It is by the way, an actual requirement to provide a local earth rod connection at every customers PME service cut-out, in many of our Western European cousins countries... lol!!

 

[oldAL]

E54,
this is the exact point you fail to grasp, just because the DNO calls it PME doesnt mean it is, there are far more TNCS systems out there than proper PME,.

 

[Jimmy Boy]

Page 44 of the BGB offers no difference between PME and TNCS and the definitions back that up, not saying your wrong fella but if it ain't in the bible lol, and it says a earth electrode may be required

 

[oldAL]

yep, but the 'bible' as you call it isnt really law, is it?
yes, I know we all work to it, but that doesnt make it ok to do something that 'may' be dangerous.
I see no issue with providing your own rod/s and really making sure it is almost a PME system, personally I measure all of the 'PME' systems I work on and add a rod if I think it is needed.

 

[Jimmy Boy]

yep, but the 'bible' as you call it isnt really law, is it?
yes, I know we all work to it, but that doesnt make it ok to do something that 'may' be dangerous.
I see no issue with providing your own rod/s and really making sure it is almost a PME system, personally I measure all of the 'PME' systems I work on and add a rod if I think it is needed.

Nope indeed, but it is what a lawyer would use to defend you with if you got in hot water, try the defence of knowing better than the regs book, you will soon be kicking the soap around the shower room

 

[oldAL]

you could always try EAWR16, and that is the law.

 

[Engineer54]

E54,
this is the exact point you fail to grasp, just because the DNO calls it PME doesnt mean it is, there are far more TNCS systems out there than proper PME,.

I Don't know about the area where you are from, but that's not the case as far as a just retired DNO area manager....

Most TNC-S systems are ''In Progress'' TN-S systems being converted to PME, often supplying both TN-S and PME service cut-outs. These in progress systems can also have a slightly higher Ze value, as i'm told anything around 0.50 to 0.60 ohms. Still a dammed sight better value than relying on a 1m twig at 200 ohms and a possible dodgey RCD device!! lol!!

 

[Jimmy Boy]

I Don't know about the area where you are from, but that's not the case as far as a just retired DNO area manager....

Most TNC-S systems are ''In Progress'' TN-S systems being converted to PME, often supplying both TN-S and PME service cut-outs. These in progress systems can also have a slightly higher Ze value, as i'm told anything around 0.50 to 0.60 ohms. Still a dammed sight better value than relying on a 1m twig at 200 ohms and a possible dodgey RCD device!! lol!!

As I said in an earlier thread I have overhead TNCS I assume but am not sure that it is true PME I will measure my ze alter in the week would a 'true' PME/TNCS give me a reading considerably lower than 0.35 ?

 

[Engineer54]

As I said in an earlier thread I have overhead TNCS I assume but am not sure that it is true PME I will measure my ze alter in the week would a 'true' PME/TNCS give me a reading considerably lower than 0.35 ?

You'll find that most, if not all overhead PME supplies are just that, ...True PME!!
Overhead supplies are the easiest distribution systems to PME. To confirm, you should be able to see an earth/neutral connection at every third pole!!!

 

[Jimmy Boy]

I will go and have a look and take a pic !

 

[Engineer54]

Rural and semi rural overhead distribution supplies were the first systems to be PME'd, to give the then many TT households a TN earthing system that most towns and cities enjoyed. The then electrical boards spent millions converting thousands upon thousands of such systems...

Now we have new and/or brainwashed electricians creating new TT systems to replace a perfectly good PME earthing a the drop of a hat, because they believe all the crap and scaremongery that abounds!! It wouldn't be so bad, if they knew what they were doing in creating a TT system, but the truth is, more often than not they DONT!!

 

[Guitarist]

Anyone know if there is a charge to TNC-S an old TT system? Quite a few around my part of the world with all kinds of things used as earth electrodes.

 

[Engineer54]

As far as i'm aware, if the supply to which your presently connected is a working PME system then there should be no charge. But i'm afraid there are many of the new DNO operators that see this as another money making service for them, and forget all about the standing charges that are supposed to cover such upgrades etc!!

So the correct answer is that some DNO's charge and some Don't!! lol!!

 

[Guitarist]

As far as i'm aware, if the supply to which your presently connected is a working PME system then there should be no charge. But i'm afraid there are many of the new DNO operators that see this as another money making service for them, and forget all about the standing charges that are supposed to cover such upgrades etc!!

So the correct answer is that some DNO's charge and some Don't!! lol!!

Thanks for answering Eng. I will give UKPN a ring and see what they say.

 

[jreg]

My thoughts are that if the garage 20m away is connected to the MET from the dwelling surely this extends the equipotential zone to the garage
I have a situation where the garage/workshop is fed from the meter box on the dwelling via 16mm2 SWA & 30 ma RCD with 1 core of the SWA plus armour connected to the MET. This extends the earth to the garage.
In a TNCS installation, if there is a neutral break then the RCD to the garage would trip as it would see an unbalance between L & N and dis power from the garage.
I feel that if you were to install a TT system in the garage, you would still have to bond the heating boiler pipes in the garage to the garage earth. Now these heating pipes are already bonded to the main buildind MET which is still exporting the TNCS earth in spite of the TT installation in the garage. I would be interested in what others think of this idea

 

[Engineer54]

Where utility services serve or pass through more than one building, the same earthing and bonding arrangements should be applied to all buildings.

You can compare that ruling, with say a row of houses all served by the same water and gas mains, or in multi storey blocks of flats/apartments etc!! Those common service pipes will still be required to be bonded at every premises... lol!!

The only time i would leave a PME earthing system behind, is where it becomes uneconomical to extend the existing equipotential zone. (eg, distance over cost of providing minimum bonding conductor)

 

[Guitarist]

A little update on the TT situation....

Phoned DNO to ask about the conversion last Thursday.... "Well, it is free, but the power guys will have to actually carry out the work".
Phoned power guys....."Yes, we can do that for you. I will send someone out this afternoon."

I explain again that it isn't an emergency, just a conversion. "No problem." says the guy.

That afternoon power guy turns up and I explain the situation.... "Sorry, we don't do the conversion. Your DNO have to do that. I will check the EFLI while here though".

Power guy pops in a new neutral block which will accommodate the 16mm earth and gets a nice 0.36 ohm reading. Really nice bloke, very polite and helpful, wishes me all the best.

Phoned DNO yesterday to get it connected, and they insist that it's the power guys who are responsible. Power guys closed due to emergency call-outs....

So, no further forward at this point

 

[Engineer54]

Orchestrated Chaos reigns supreme then!! Nothing much changed there then... lol!!

At least their not charging you for the privilege!! lol!!