PME EARTHING FOR DUMMIES

[VoltzElectrical]

just checking that i have got this right.

In a PME system, the current is returned via the neutral, so if this connection (outside the premises) is lost for whatever reason, the current is then returned via the earth. If the premises are bonded correctly, no raise in potential can happen.

If you export the PME connection to an un-bonded outhouse with extraneous conductive parts, there would be a potential difference between these parts and earth, rendering it dangerous?

Please, I don't want to turn this into another thread on exported PME, hence why i have not and will not look back through the old threads, as I have read them all already, and still don't understand!

I was just reading an article on earthing in Wiring Matters and wanted to clarify the science behind what i was reading.

Thanks, and please correct me if I'm wrong. Let's make this matter simple, hence the title, PME for Dummies.

 

[Andy™]

if you loose neutral outside the property (i.e before the N-E split), then depending on resistance (could well be hundreds of ohms), the the neutral, and more importantly, the earth will become live

inside a building, shouldnt be a major issue since everything should then be the same voltage with regard to each other. except something exported, which would then be live, and with a potential between that an true earth

and if you go get a shock from touching something (i.e a pipe), the RCD is guaranteed NOT to trip

 

[steptoe]

NO,

PME and TNCS are two different earthing systems, often confused, but NOT the same,

try having a look here in the student and learning zone as this is a rather basic question IMHO

 

[VoltzElectrical]

TN earthing systems in the UK are always earthed at the supply transformer, a TN-S system utilizes a separated PEN conductor throughout its length, The TNC-S system utilizes the neutral conductor, PEN , which is earthed at the supply transformer, and then separated at the consumers point of supply.

I lifted this excerpt from the excellent recommended post by Steptoe. Just for clarification, it seems to read that TN-S earthing conductor is called PEN, and in TNC-S, it's also called PEN?

I understand a bit more about stray neutral current though.

 

[steptoe]

sorry about that,

obviously a typo, but I think even with that it is quite obvious by the word seperated what was meant.

sorted now.

 

[VoltzElectrical]

sorry about that,obviously a typo, but I think even with that it is quite obvious by the word seperated what was meant.

sorted now.

Ok, clearer now! Of course I knew about the difference between TN-S and TNC-S, but when I read that I started to question my own understanding. Good to have a better idea of the difference between TNC-s and PME.

Do dangerous voltages build up in the neutral in fault conditions because premises are supplied on different phases then?

 

[Andy™]

Ok, clearer now! Of course I knew about the difference between TN-S and TNC-S, but when I read that I started to question my own understanding. Good to have a better idea of the difference between TNC-s and PME. Do dangerous voltages build up in the neutral in fault conditions because premises are supplied on different phases then?

loose a neutral and you could have upto 400v depending on loads etc

 

[Sidewinder]

The voltage rise on lost N's is due to the way the 3ph supply works, and yes as Andy says you can see an effective 400V ph-"N" of the N is open cct.

I think the typo that Steps has admitted to is akin to the reason I have lambasted others for incorrect quotes and references and then been criticised myselt.

Steps you incompetent fool!

Can you not even get simple descriptions right!!!

I'm sure he will re-read the thread/post and correct, and post an apology.

This is an issue, trust me, when you write, proof read and correct your own documents, you know what you mean, you write it, you read it, of course it makes sense!!!

This is why I am so critical, when I see incorrect quotes, phrases, or terms (r1/R1 etc.) as these can totally confuse learners.

Voltz has passed a lot of his exams, but not yet finished, he is now moving on to the more advanced topics, where one begins to question in depth that which one is told & taught.

Then he finds this...

I rest my case M'Laud!

Oh, & I'm off up the wooden hill.

 

[Technician]

I have made a few notes off this thread...fascinating.

 

[plumberjohn]

thanks all - i`ve read many posts in the past about exporting TNCS, and never fully understood why you can`t do it.

I think the mist is starting to clear !

It`s also made it clear how dangerous not having water/gas bonding can be - and I have found this so many times in the course of my work, even one where an old cert was left by the cu, bonding box ticked!

 

[Evans Electric]

May I just add that I'm not sure if all the DNO people know the difference and that definitely a PME sticker on a cut-out may not mean what it says .

( Main reason being that the link can be missing between N and the earth terminal.)

And I assume that the DNOs are no longer installing PME as it doesn't appear in the BS7671 list of types of supply.

I think that in this area anyway, they may be upgrading supplies from TN-S with poor Ze to TN-C-S and stickering it PME .

 

[Sidewinder]

Deke,

PME is no longer a recognised supply in 7671 you are obviously correct.

However, TN-C-S is, and the DNO's have shortly to complete converting their TN-C-S networks to also be PME.

 

[VoltzElectrical]

Thanks for all the replies, I'm glad I started this thread and feel happy that others have benefitted too.

Cheers all. x

 

[Evans Electric]

Deke,PME is no longer a recognised supply in 7671 you are obviously correct.

However, TN-C-S is, and the DNO's have shortly to complete converting their TN-C-S networks to also be PME.

Ah I see what you mean now ........ I thought PME was being abandoned and the new Rock n Roll was TNCS.

PME must be a safer system .

I'm not aware of any great programme of digging and rodding going on TBH .

But surely KING RODDERS of Manchester and NI should know all about this ............. Was it the final scene of "It's a Wonderful Life" where James Stewart's daughter says " Every time you knock a rod in..... an angel gets his wings " " Good on you, Clarence!!" Perhaps not !!

 

[steptoe]

^^ :slap ^^

Deke, PME is indeed a very good system, the problem arises when TNCS is wrongly provided as being PME when it is in fact not a true PME system, and more likely than not has no (or very limited) rods,

this is why the DNO will usually refuse you permission to export the earth from their TNCS system beyond the EZ.