Pme

[Cj24]

Hi can someone explain PME for me please, is every house suppose to have it, I worked on building sites and just recently started doing domestic, Iv done few jobs but was told some earthing needs upgrading so obv assumed 10mm for gas and water but was told to look for a pme block????

 

[malcolmsanford]

Protective Multiple earthing it is in section 2 of the BS 7671-2008.

After you read that you will see it is a TNC-S system and again you can see section 2 for an explanation which will point you to a further figure 2.4 which is also in section 2 of the BS 7671-2008.

How it is done in the industry is that the DNO bring into their head a Line and Neutral for let's say a single phase supply. Within their cut out they will link the Neutral to an Earth bar and so you have the combined Neutral and Earth at the origin of the installation.

In order to make the system safe at various intervals along that Neutral it will be staked into the ground with Earth spikes all the way back to the Transformer, where it originates.

 

[malcolmsanford]

Just out of interest cj, is there an outbuilding with a sub board here? if so then theres issues with "exported earths" and pme......

I'm definitely going to need my pills for this one .............................in the words of Julius Ceasar I'm came I'm saw and I'm buggering off now

 

[Guest123]

Just out of interest cj, is there an outbuilding with a sub board here? if so then theres issues with "exported earths" and pme......

Not bad....4 posts until it was mentioned!!!!

Sorry mate no offence intended or anything, it's just we've had some epic threads on PME & exportation/extending etc, and I can see the ususal suspects coming over the hill now.......I'm off!!!!

:tank:

 

[Guest123]

Sorry guys.

No worries.

:66:


They usually last for 15 - 20 pages and end up getting closed.

 

[Guest123]

Hi can someone explain PME for me please, is every house suppose to have it, I worked on building sites and just recently started doing domestic, Iv done few jobs but was told some earthing needs upgrading so obv assumed 10mm for gas and water but was told to look for a pme block????

This is what the fault loop path looks like, hope it helps a bit.

 

[telectrix]

insiteful???

 

[IQ Electrical]

Thanks lenny for the understanding but i just couldn`t help my mischevious self....TEE HEE! On a serious note though i hope it doesn`t ruin what would have been an insiteful thread.....

Have a read of this post: http://www.electriciansforums.net/e...159-good-guide-exported-pme-outbuildings.html

The problem is that the 'you can't export an earth from a PME supply' is in all but some rare cases, just not true.

There may be economic reasons because of bonding conductor sizes and costs.

 

[ezzzekiel]

YOU ARE NOT ALLOWED TO EXPORT AN EARTH ON A PME!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

oh yes you can
oh no you cant
ooh yes you can
ooh no you cant
oooh yes you can
oooh no you cant
ooooh yes you can
ooooh no you cant
oooooh yes you can
oooooh no you cant
ooooooh yes you can
ooooooh no you cant

 

[Engineer54]

Just looked at it IQ and what they are refering to is earth bonding of any gas and water pipes which would have to go back to the met at the main dis board (not to the met at the sub board). And also the seperation between exposed conductive parts being protected by 2 different types of earthing arrangements ( to prevent different potentials).

All you are doing is ''Expanding'' the equipotential zone, ...Your NOT exporting it, when you connect a bonding cable to any extranious pipework at the new location.

 

[IQ Electrical]

All you are doing is ''Expanding'' the equipotential zone, ...Your NOT exporting it, when you connect a bonding cable to any extranious pipework at the new location.

That sums it up perfectly, if you have a decent understanding of protective bonding with a view to bringing an area into the equipotential zone then any PME mysteries are gone!

Obviously with the exception of the specific instances ie. caravan supplies where this scenario is forbidden.

 

[IQ Electrical]

So in effect 54 you can export a pme earth to lighting and points etc in an outbuilding but when it comes to pipework etc this has to be on an extention of the equipotential zone so would still require a separate earth pulling in going back to the main met....

It can be within a multicore cable or an armoured cable SWA earth as long as the CSA meets the requirements.

 

[IQ Electrical]

So what you are saying IQ is as long as your R2 is good (0.05 ohms) using the armoured then its good.........

No, it's down to size (the continuity would still need to be proven but not recorded).

The 0.05 Ohms is more to do with resistance between extraneous-conductive parts to establish the effectiveness of supplementary bonding.

Check out 544.1.1 and Table 54.8

 

[Engineer54]

No, ....If you were to use the cables SWA as your bonding conductor, it would need to have the equivalent CSA of 10mm Copper, not 10mm CSA of steel!!!


So really, you have a choice of installing a correctly sized SWA cable for your supply needs and a separate 10mm single bonding conductor, ...OR 10mm or greater SWA cable with a separate core to be used as your bonding conductor, and using the SWA as your CPC....

Look up the K ratings of Copper and Steel in the BRB, or other publications and you'll see what i mean...

 

[Deleted member 26818]

The conductivity of copper in relation to steel, is about 8 or 9 to 1.
As such, you would require an armour CSA of about 90mm² in order to utilise the armour as a bonding conductor.