Someone clear this up asap!

[domestic spark]

"Generally speaking the exportation of a TN-C-S supply is the decision of the local DNO and only done with their permission. In the absence of such permission it would be recommended to use a TT system and RCD protection. However most DNO's will allow such exportation to an outbuilding provided it is 2 meters or less in distance from the main
building.

I advise that you check with your local DNO as to their requirements."







2 meters or less in distance from the main
building.

OPEN TO CORRECTIONS BUT, this is stating that, if you want wiring done in garage, conversion etc further than 2 meters from your house, it has to be a TT system??????

 

[Deleted member 26818]

By rights, you should be liasing with the DNO for any major work to an installation, if only to check the type of earthing and if there are any requirements for earthing/bonding conductors.
Some DNO's require 16mm² or larger for main earthing/bonding conductors where PME conditions apply. They will also be able to inform you whether exportation of the PME earth is acceptable, and whether they have any additional requirements.
As for TTing outbuildings, often it is more cost effective than installing appropriately sized bonding conductors if there are extraneous-conductive-parts that require bonding.

 

[Engineer54]

By rights, you should be liasing with the DNO for any major work to an installation, if only to check the type of earthing and if there are any requirements for earthing/bonding conductors.
Some DNO's require 16mm² or larger for main earthing/bonding conductors where PME conditions apply. They will also be able to inform you whether exportation of the PME earth is acceptable, and whether they have any additional requirements.
As for TTing outbuildings, often it is more cost effective than installing appropriately sized bonding conductors if there are extraneous-conductive-parts that require bonding.

First off, ...most DNOs are not bothered about domestic situations involving extending PME. Maybe in London they are more cautious, because of the sometimes high fault levels that they have, especially in central London.

Secondly, that very much depends on the type of ground your going to be creating that TT system in, and whether it's going to be installed correctly or not. Along with what your going to accept as a decent Ra figure. Please don't say 200 ohm's, as that isn't going to protect anyone, almost as bad as not having an earthing system at all!!!

It starts getting less cost effective to extend a PME system, the further you go from the source, and the point of supply requires extraneous-conductive-parts that will require bonding. ....then Yes, it may then be more suitable to install a TT system....

 

[Deleted member 9648]

This has all been covered a thousand times before on here...the only thing in engineers post I would question is the "200 ohms is almost as bad as no earth at all"...the fact is that RCD's will provide fault protection with very high earth resistances...up to 1667 ohms for a 30ma RCD. My preference is to always use the TNCS earth where practical ,and I never contact the DNO who will most likely not have a clue what you are asking them.Others prefer to TT,both are acceptable,you takes your choice.
What I did take issue with when I first joined this forum was the number of people who were insisting a TNCS earth CANT be exported...that is simply not the case.

 

[Sintra]

The only problem i personally have with the use of RCD's is that we can all rely on them too much for protection. These when working correctly are fine but remember when they fail they do not failsafe and could very possible leave persons in danger especially on a TT system with a high Ra.

 

[Engineer54]

Sintra,

That is my very reasoning behind this crazy 200ohm figure. The electricians are then totally relying on a functioning RCD device to protect the installation and it's occupants against earth faults. If they fail and left with that sort of Ra, then the chances of any protective device operating in any meaningful time period is nigh on Nil!!!

 

[Deleted member 9648]

Quite right Sintra....I guess there is a case for having an upfront 100ma T/D RCD on TT's to allow for this scenario.
Generally though a TT is used where the DNO dont provide a means of earthing.There's not really much that can be done in that case to provide a low resistance earth fault path.

 

[Sintra]

It has all come down to sticking RCD's onto circuits which is all fine and dandy when they work, but they are never tested regularly once fitted. Gone are the days of bringing the Ra down by adding additional electrodes, so long as the 200 ohm figure is met.

Again the omittance of supplementary bonding in bathrooms just because there is an RCD or RCD's on the circuits of the location but who knows when they will fail. The only way that these devices could be relied upon is if they were manufactured to be FAILSAFE.

 

[somersetsparks]

Our lot (DNO) would insist the size of the earth is the same as that of the supply earth if pme is exported - i.e. even if you only took a 4mm 32a supply out you would have to have a 16mm2 earth, obviously not to much of a problem for the domestic lot, but when you have a 70mm2 main earth can become impractical!

Forgot to say as far as im aware (in our area) 2m is a myth.

 

[Guest123]

tested secondry board where i ran i to and got a reading of 36ohms on ZE. ???????????????????????????????????

thats not good enough? now what?

max is 0.35 right!

An installation has ONLY ONE Ze, which is obtained at THE ORIGIN.

What you measured there was the Zs of the distribution circuit or Zdb as it is also known.

The 0.35 max is for the Ze measurement at the origin and has no bearing on the result you got.

 

[Engineer54]

Still confused here, what is the current Bonding cable size for a PME domestic installation, 16mm or 10mm??

 

[Sintra]

Table 54.8

 

[Engineer54]

Afraid your numbers are no good to me here, still waiting for my copy of the 17th to arrive, .....and that seems to be already out of date before i get it !! haha!!!

Been working for the past 4.5 years with NEC and DIN standards, coupled with the 16th edition...

 

[tony mc]

Neutral conductor up to 35mm bonding 10mm

 

[Engineer54]

Thanks Tony..... i've just been searching myself on the web, and come up with the same result...

 

[Elidor]

Throwing a spanner in the works here but an RCD is additional protection only right? Primary protection must come from the main circuit protection device which if exported to a TT scenario must now have a disconnection time of 0.2 seconds?