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