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??????

 

[DaveyD]

Does this help? http://www.electriciansforums.net/e...l-electrical-forum/28414-exporting-earth.html

 

[domestic spark]

Under the Electricity safety, quality and continuity regulations 2002 you cannot use a suppliers earth to an out building


so every out building has to be tt???

see what i mean, first it fine, then its ok to do as long as garage is no more than 2 meters from house? now its, not allowed at all and must be TT!




NO CLEARER SOMEONE PUT ME STRAIGHT!

 

[Engineer54]

Read this....

A good guide For 'exported PME' and outbuildings!


Where does this ''2 metre'' myth come from, that many keep quoting??
​

 

[DaveyD]

Or this pdf http://www.electriciansforums.net/e...9-good-guide-exported-pme-outbuildings-2.html

 

[domestic spark]

Read this....

A good guide For 'exported PME' and outbuildings!


Where does this ''2 metre'' myth come from, that many keep quoting??
​

BROWSING THE INET and found this quote in many places!

so is exporting tn-cs acceptable? over a distance of 25m? max

 

[Engineer54]

We need Official quotes, not assumptions by those that don't really know!!

It has more of an economical bearing in a domestic situation, when it ceases to become acceptable. Otherwise it has everything to do with cable calculations... Personally i've seen PME taken almost 48 metres, and that was some years ago in Essex. And the owner of the property was and still is a Regional DNO manager !!!!!

 

[domestic spark]

Re: A good guide For 'exported PME' and outbuildings!

A lot of these myths stem from the numerous ''Scaremongers'' of the PME system!! In that ''IF'' (and it's a very big IF) a break in the neutral occurs all connected metal parts will have a potentially dangerous voltage. In reality, the chances of that happening is remote to say the least. I would take a PME system earth out, to any out-building, wouldn't give it a second thought either

Again In reality, what would you rather have as your earth connection, a system that gives you 0.35ohms or a rod at maybe 200ohms, ...i know what i would rather have in my home.

There is no reason why you can't employ a rod on a PME system, in fact many countries insist on it. Many local authorities used to rod TNS supplies to there council houses in the 50s.


i think this clears my questions up!!!!
​

 

[domestic spark]

engineer, presumably a much older wiser man than myself, ran armoured off a pme supply yesterday, 30meters max, 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!

 

[Engineer54]

Do you really think 0.01ohms is going to make a difference to your earth fault protection??? I take it you mean't 0.36 ohms ...not 36 ohms??

 

[domestic spark]

0.36 yes, and no i dont, only done 2392 testing course so new to testing, been informed it doent matter anyway as it only applies to orign, i thought that but was admittidly unsure!

 

[Deleted member 9648]

The vast majority of installations in the UK come under Bs 7671.....the only document you need to comply with when choosing the earthing system for an outbuilding is Bs7671. There are no special requirments for ordinary outbuildings in Bs 7671.

 

[malcolmsanford]

The crazy thing about this argument is, as the earth on a TNC-S is the responsibilty of the DNO, it is still their call if they will allow you to export it to an outbuilding, I'm not saying that today they won't allow you but it's best to ask IMO, so just call them. If you don't call them, and export, and something happens you are the one that is going to carry the can.

What is this hatred of TT systems on here.

 

[Deleted member 9648]

Hi Malcolm,
It's not a hatred of TT for me it is just that it is totally unnecessary.As long as the install complies with Bs 7671 and is certified as such,if anything happens the installer is not going to carry the can...it's no different to any other situation.
You can go on and on...what if the earth on a TT goes missing?,they are always vunerable to damage and changing ground conditions,is the installer going to be held to account over that?
The thing I cant fathom is why outbuildings are the only thing people always use to highlight the horrors of TNCS...if you apply that logic it has to apply to all electrical installs outside of the main building,people never seem to ask..."I'm installing an outside light,do I have to rod it?"......yet the same person comes over all unnecessary as soon as an outbuilding is mentioned.

 

[malcolmsanford]

In agreement, but a lot of us more mature guys remember the old electricity supply regulations which IMO were so much more concise as they had a part in them for PME supplies.

I have no trouble exporting the earth, as long as I get assurances from the DNO that they are happy for me to do so. I can say that 99 times out of 100 it would either be "Why are you asking" or "yes".

I still like to make sure that they are happy with this. I would more shocked if the DNO said no, but they may do just that for some reason,

It's not that your doing the install to the regulations, a fault can still develop in that installation, and if you exported the earth without the DNO knowing, and you did have a situation where you had a difference in potential because exporting it was no viable, for whatever reason, then you are held responsible.

It's not a case for me TNC-S then outbuildings TT every time, it's a case of calling the DNO and enquirying if I can export it. IMO though a building that is over 25 metres away from the origin of the supply is more economical to be a TT

 

[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?

 

[telectrix]

depends on what you are using for the " main circuit protection device".

 

[malcolmsanford]

Look at table 41.1 note 2 of the Regs. Then go to appendix 3 and look there. That is why a 30mA RCD complies with the 0.2 second rule on a TT circuit that is less than 32 amps.

It's all down the IdN x2 times.

 

[telectrix]

but what elidor seems to be saying is that you should achieve 0.2 disconnection without reliance on RCD. with TT systems this is impossible

 

[malcolmsanford]

To be honest reread it twice now and it's got me a little fuddled, I thought he was aking how a RCD conforms to the 0.2 disconnection time as IdN x1 is 0.3 seconds. But at IdN x2 it is 0.15 seconds which brings it into compliance ...............

 

[Elidor]

Im very new to this trade so i wouldnt take anything i say as reliable information hehe, I was following the thread with interest and thought it was implying that RCD protection alone was a suitable circuit protection device. I was under the impresion that any electrical circuit (TN or TT) must provide fault protection by automatic disconnection of the supply within the 0.4s for TN and 0.2s for TT for all circuits up to and inclusing 32A. As the RCD offers no protection for overcurrent or short circuit protection between live conductors so it would still be necessary to provide conventional circuit protection with disconnection times to table 41.1 (P46). If the disconnection times cant be met then I was told the circuit must not be installed. Is this correct?

I'm new to the trade so please go easy on me hehe, im another product of the intensive training programs designed specificaly to confuse the hell out of everyone who signs up lol.

 

[Sintra]

If You have a look at the graphs on pages 244 - 251 it shows the characteristics for the different forms of overcurrent protective devices. These have to be installed in all system types for short circuit and overcurrent protection.

Eg a BS 60898 32A type B MCB will trip with an overcurrent of 160A in 0.1 seconds and will therfore comply to both a TN 0.4 & a TT 0.2.

 

[Elidor]

The original post was regarding an outbuilding where it was suggested that it may be necessary to provide its own earth electrode as it may not be permitted to share the PME from the original supply. In either case if the disconnection times cant be met then is it forbiden to install the circuit? or are there any other ways of installing the circuit and still be complying with the regulations?

 

[malcolmsanford]

The disconnection time of a protection device is determined by the Zs of that circuit. Not sure how far you have gone in your training with this but the Zs is the maximum value of resistance a circuit can have to allow that protection device to function. So what it is saying, say we take a BS 60898 type B 6amp MCB, for that device to trip in 0.1-5 seconds it will take 30amps to trip it. that is fig 3.4 in the regs page 249. To allow that amperage to be achieved in a circuit you go to table 41.3 on page 49 and it tells you that the maximum ohmeric value the Zs is 7.67 ohm. again I don't want to bog you down with rule of thumbs and such so let's say your circuit when you test it must be below 7.67 ohms for that 30amps to realised on that circuit to trip that device.

Now if as your saying you can't obtain that 7.67 ohm for whatever reason there are several ways that can be over come.

1. increase the size of your cable or run a seperate CPC that will help you achieve that Zs
2. Another is supplementry bonding as you do in bathrooms this will help as well
3 RCD protection.

There should never be a situation where you are forbidden to install a circuit becasue of a high Zs value, there are always options to achieve your disconnection times

 

[Deleted member 26818]

The original post was regarding an outbuilding where it was suggested that it may be necessary to provide its own earth electrode as it may not be permitted to share the PME from the original supply. In either case if the disconnection times cant be met then is it forbiden to install the circuit? or are there any other ways of installing the circuit and still be complying with the regulations?

It would be very rare to find a case where disconnection times cannot be met, especially at the design stage.
More likely where the earthing sytem on an existing installation has been changed, or it is found after installation that Ze is higher than recommended.
In such instances, supplementary bonding would be required.

 

[Elidor]

Thanks for the information, appologies if i have steared the thread off track a little. Does the thread starter have the informaiton he originaly requested?