pme debate

[sparkatus1]

Can you export pme on a farm if a separate bonding conductor is run with the supply. Done periodic and found pme exported to various barns all of which have extraneous parts. Ie steel structure etc. All DBs were earthed via armouring of supply cable. All had there own earth rod and all bonded to steel structure. Now to rectify problem I was under the impression you could run separate bonding conductor satisfying table 54G. However section 7 of bs7671 says you cannot use tnc systems on farms. Any ideas much appreciated and also what code would everybody put on report.

 

[steptoe]

well, if they all had their own local rods is it actually exporting the PME?

are you certain it is actually PME at the source?

yes, the regs do prohibit it, but what does your DNO say about it?

 

[NozSpark]

For a first post you've chosed a right one, haven't you

You are not allowed TNC-S on farms,,,, end of!

---------- Post Auto-Merged at 23:12 ---------- Previous post was made at 23:11 ----------

TBH I thought that the DNOs wouldn't even supply TNC-S to farms

 

[steptoe]

For a first post you've chosed a right one, haven't you You are not allowed TNC-S on farms,,,, end of!

---------- Post Auto-Merged at 23:12 ---------- Previous post was made at 23:11 ----------

TBH I thought that the DNOs wouldn't even supply TNC-S to farms

well, I thought that too, until I came across one, and on talking to a very helpful bod from here I was assured they did permit it, so rang them, and they had no issue with it.!

I think it may be the difference between TNCS and PME that is the limiting factor in this scenario

 

[batty]

I think a lot of the DNO are getting more lapse and seem to allow TNCS on farms these days.

 

[kme]

I think a lot of the DNO are getting more lapse and seem to allow TNCS on farms all sorts of nonsense these days.

Fixed that for you mate

 

[sparkatus1]

Farm is all fed from submain board at house with a pme cut out. All barn DBs are wired in armoureds from this board. Armouring as the earth. Also if each board has 300mA rcds and the neutral does happen to go down I take it the rcd wouldn't trip because wouldn't see difference between L and N. Is this correct. Finally if neutral did go down and there were extraneous parts about would the potential become very high between the two because you have still got your bonds acting as earth rods and also the neutral is tapped to earth every so many poles isn't it. Thanks for input.

 

[steptoe]

Farm is all fed from submain board at house with a pme cut out. All barn DBs are wired in armoureds from this board. Armouring as the earth. Also if each board has 300mA rcds and the neutral does happen to go down I take it the rcd wouldn't trip because wouldn't see difference between L and N. Is this correct. Finally if neutral did go down and there were extraneous parts about would the potential become very high between the two because you have still got your bonds acting as earth rods and also the neutral is tapped to earth every so many poles isn't it. Thanks for input.

do you really mean a TNCS cut-out?

 

[sparkatus1]

Pme sticker and 16 mm earth coming from neutral in head. Am I getting confused. Isn't a pme the same as tncs anyway. Or does a pme refer to tnc.

 

[steptoe]

Pme sticker and 16 mm earth coming from neutral in head. Am I getting confused. Isn't a pme the same as tncs anyway. Or does a pme refer to tnc.

:shakehead

no, common error,,,,,,,,,,

 

[sparkatus1]

Please explain. Regs. Say you can't export tnc system. So doesn't this mean tncs aswell.

I take it as soon as earth is connected to neutral it changes from pme to tncs. However can you export tncs?. Sorry only been doing electrics 5 years. Still learning every day.

 

[Manator]

To fully grasp the idea behind earthing, we have to look at the way the power is distributed to our homes.

By now we are all aware of the TN types and how the neutral dictates which type it is, but to recap, TNS is when the distributed power is delivered down line and neutral the earth being provided by a separate conductor, historically the sheath of the supply cable.

TNC-S is when the power is delivered down the line and neutral, the neutral being connected to the earth at the supply side. Earthing within the property is then split from the neutral.

PME is the same type of distribution as TNC-S but with some vital and often overlooked differences.

In true PME supplies the neutral will also be connected to earth along its length and more importantly close to the final connection.

The DNO

 

[sparkatus1]

Sorry to be a pain but basically you are saying the earth shouldn't be connected to the neutral at the head on farms if its pme. (making it a tncs) and even if it is the tncs should not be exported to other buildinds. Thanks.

---------- Post Auto-Merged at 14:11 ---------- Previous post was made at 14:06 ----------

Also I take it on other installations ie sheds you can export pme as long as you run a bonding conductor with it and extraneous parts are bonded.

---------- Post Auto-Merged at 14:11 ---------- Previous post was made at 14:11 ----------

Also I take it on other installations ie sheds you can export pme as long as you run a bonding conductor with it and extraneous parts are bonded.

 

[steptoe]

Sorry to be a pain but basically you are saying the earth shouldn't be connected to the neutral at the head on farms if its pme. (making it a tncs) and even if it is the tncs should not be exported to other buildinds. Thanks. being correct you shouldnt use PME on agricultural premises---------- Post Auto-Merged at 14:11 ---------- Previous post was made at 14:06 ----------

Also I take it on other installations ie sheds you can export pme as long as you run a bonding conductor with it and extraneous parts are bonded. No

---------- Post Auto-Merged at 14:11 ---------- Previous post was made at 14:11 ----------

Also I take it on other installations ie sheds you can export pme as long as you run a bonding conductor with it and extraneous parts are bonded. No

sparkatus1 , you are looking at this in a very simplistic manner,

its not so plain and simple as you are making it,

to be very, very, basic, you should NOT be using TNCS on any agricultural install,

and you should NOT be taking an earth from ANY TNCS system outside the normal Equipotential Zone.

that is a very VERY BASIC way of looking at it, there are a multitude of other factors dependant on any individual site.

---------- Post Auto-Merged at 15:18 ---------- Previous post was made at 15:17 ----------

Im still not sure if you have grasped the differences between PME and TNCS

---------- Post Auto-Merged at 15:19 ---------- Previous post was made at 15:18 ----------

look here

 

[sparkatus1]

Steptoe unfortunately I am not as experienced as you and joined this forum for some friendly advice. Which I have been given by the majority.

---------- Post Auto-Merged at 19:13 ---------- Previous post was made at 19:04 ----------

I am still relatively new to the industry and try to do things correctly. After all none of us know everything!

 

[Manator]

I think that what Steptoe is trying to convey is that everything can have an exception, earthing is not simplistic in the sense that you are trying to justify it.

You need to fully understand the concept of earthing, the relation with both touch and step voltages, and design the installation accordingly.

We all have to start somewhere, and is is harder for those who have more experience to understand why some can not grasp simple principles, unless of course they are tutors. Whilst some answers may appear to be abrupt they are not intended to be so.

Every electrical installation in this country must be installed to protect both buildings and persons from harm. One method used to achieve this is to have a path for any fault to be quickly dissipated back to earth.

I for example did not approve of the 17th edition and the subsequent lack of required earthing and bonding, I also find it hard to beleive that some installers do not think it is required because it is not in the regulations any more.

I have used earthing and bonding to reduce both touch voltages and to ensure disconnection times to under 0.2 seconds, without the use of any RCD.

Earthing can be very scientific, and I have some very useful data on soil types, earthing types and how the great scientists from around the world have debated and concluded on how earth works and in which form it works better. However for the most part we mere mortals can only follow the guidance given in BS7671, if you are confident that your installation complies with BS 7671 then everything will be OK.

 

[sparkatus1]

Much appreciated.

 

[Evans Electric]

Steptoe unfortunately I am not as experienced as you and joined this forum for some friendly advice. Which I have been given by the majority.---------- Post Auto-Merged at 19:13 ---------- Previous post was made at 19:04 ----------

I am still relatively new to the industry and try to do things correctly. After all none of us know everything!

Sparkatus , you have released the Forum Genie from the bottle , mention exporting earth and you will be Steptoed !!!

You are not alone with the confusion over PME by the way . Because of the way we were shown the types of supply many sparks are unaware there is a difference between TNC-S and PME

---------- Post Auto-Merged at 23:53 ---------- Previous post was made at 23:30 ----------

Steptoe unfortunately I am not as experienced as you and joined this forum for some friendly advice. Which I have been given by the majority.---------- Post Auto-Merged at 19:13 ---------- Previous post was made at 19:04 ----------

I am still relatively new to the industry and try to do things correctly. After all none of us know everything!

Spartacus , you have asked a sensible question and Stepps is one of the best to answer that particular one, don't be afraid to use the Forum . And you have explained your position within the trade , ie; "In the trade for 5 years" This helps guys to respond as sometimes you can't tell who is asking , a trainee,, an apprentice , a DIYer etc.

 

[apprentice87]

Hi all,

Just to add to the debate!!!!!! I cannot understand all the fuss about "exporting" tncs..... The DNO have installed a substation thingy down the road from me. It is all fenced in with metal railings and all bonded and earthed, and everyone is happy.

Now though, they have only gone and run a 500 yard long cable right down the middle of the road, and worse, then they have exported "PME" to every house in the entire street, and down a crappy little spur from their big cable too!!!!! This is 500 yards outside the EQ zone of their building....How can this be right.....

I know there is all this fuss about "losing the neutral" but, you might think you are on nice safe TNS, but, would it not be the case that, "one on tncs, ALL on tncs" ?????

If the neutral and earth are linked outside a house up the road from me, then so is mine, as i am, after all, connected to the same cable as he is....

If neutral disappears, making HIS earth "live" then my MET is connected to his "live" earth whether i am technically on tncs or not....

The DNO told me that every time they repair an LV main, they use concentric anyway, so we are ALL on tncs really, that is how it seems to me anyway....

Soooooo, the best and safest method would be TT, or at least it would be if the RCD's were to be trusted, which, as we all know, they are not... [7% failure rate i think it is]

Anyway, dangerous stuff this electricity, i am going over to gas!!!!!!!!

john...

 

[steptoe]

Steptoe unfortunately I am not as experienced as you and joined this forum for some friendly advice. Which I have been given by the majority.---------- Post Auto-Merged at 19:13 ---------- Previous post was made at 19:04 ----------

I am still relatively new to the industry and try to do things correctly. After all none of us know everything!

sparkatus, my apologises, I obviously didnt come across the way it was intended,

I wasnt trying to berate you in any way, just saying that you were looking at earthing in a simple manner, as most people do.

again, sorry if I came across in a condescending manner, it certainly wasnt intended like that.