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Topic Title: porta-cabin and PME earthing
Topic Summary: connecting up a porta-cabin where supply has PME earthing
Created On: 29 January 2013 11:31 PM
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 30 January 2013 06:56 PM
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UKPN

Posts: 547
Joined: 17 January 2012

porta-cabin and PME networks.

we dont offer a PME terminal for this type of installation.

BS7671?

Regards
 30 January 2013 07:10 PM
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OMS

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Originally posted by: UKPN

porta-cabin and PME networks.

we dont offer a PME terminal for this type of installation.

BS7671?

Regards


On what basis don't you offer it - I could find several in a small area where we've been involved in stacked modular buildings with overcladding being supplied via PME by UKPN or your predecessors.

Regards

OMS

-------------------------
Failure is always an option
 30 January 2013 09:15 PM
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psychicwarrior

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@OMS

ahh - thank you for that... yes i can see a bit clearer now

i had it explained to me previously that TNCS was more an issue if it was supplied without PME in a 'lost' neutral scenario; think i've since read that elsewhere too but not sure where now.

thats the crack then in terms of those fault issues, in that its because TNCS its more likely used in short runs and single ownership (rather than general multi-installation supply) that its less a problem than PME which is more likely to affect many in a fault occurrence.

or in other words i would suspect that TNCS with PME is 'better' (if i can use the poor description) than TNCS no PME with all other things being equal ?

and thanks a lot for the input. its been appreciated.
 31 January 2013 10:24 AM
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AJJewsbury

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i had it explained to me previously that TNCS was more an issue if it was supplied without PME in a 'lost' neutral scenario; think i've since read that elsewhere too but not sure where now.

In the UK you can't have an extensive TN-C-S system without PME (legislation prohibits it). While multiple electrodes in theory keep even a broken PEN close to earth potential, in practice the resistance of the soil means that there's usually quite a few Ohms between the electrode and the general mass of the earth - 230V through 10 Ohms doesn't deal with very much of a several hundred amp supply. You might have several electrodes on the consumer's side of the break (which would bring the resistance down) ... or you might not have any at all - it all depends on where the break is

In PNB (protective neutral bonding), the earth electrode and N-PE link are at pretty much the same position - usually at the only consumer's cut-out - if you like it's where TN-S and TN-C-S systems become indistinguishable from each other (the PEN conductor is pretty much zero length). A break in the supply conductors (upstream of the N-PE link and hence the one point where the live conductors (N) are referenced to earth) doesn't jeopardise the consumer's reference to earth and a single break downstream of that point with either break N (disconnecting the installation but not energising bonded metalwork) or PE (leaving bonded metalwork unearthed, but not energised). Some argue (quite convincingly) that PNB would be better categorised as TN-S rather than TN-C-S, but current standards do seem to designate it TN-C-S.

Probably better with pictures - I'll see what I can do when I get a bit of time.

- Andy.
 31 January 2013 11:15 AM
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Avatar for OMS.
OMS

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Originally posted by: psychicwarrior

@OMS

ahh - thank you for that... yes i can see a bit clearer now

i had it explained to me previously that TNCS was more an issue if it was supplied without PME in a 'lost' neutral scenario; think i've since read that elsewhere too but not sure where now.

thats the crack then in terms of those fault issues, in that its because TNCS its more likely used in short runs and single ownership (rather than general multi-installation supply) that its less a problem than PME which is more likely to affect many in a fault occurrence.

or in other words i would suspect that TNCS with PME is 'better' (if i can use the poor description) than TNCS no PME with all other things being equal ?

and thanks a lot for the input. its been appreciated.


Generally the other way round - in this case TN-C-S is safer than PME due principally to the reasons Andy clearly highlighted above.

Essentially, you fist want to decide if Section 717 is at all relevant - but in all cases you do need to understand the implications on the particular circumstances of using a PME earth - (lost PEN and a significant rise in MET potential for potentially many hours, diverted neutral currents elevating the MET potential in normal use, implications on sustained and often high current flow in the earthing and bonding system etc etc)

regards


OMS

-------------------------
Failure is always an option
 31 January 2013 05:09 PM
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psychicwarrior

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@OMS and AAJ

thanks fellas

@AAJ

no need to go to the trouble to do a diagram - i'll do a bit of work myself :-)
 01 February 2013 08:17 PM
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perspicacious

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From experience, find out what the DNO's own little rule book has to say as they can impose their own requirements over and above ESQCR and BS 7671...... And they wouldn't let me have sight of their little book citing that I have to seek their requirements having submitted the circumstances and they'll let me know each time.

In the end, I considered the wood clad shipping container as a caravan as the risk is exactly the same.......

Regards

BOD
 01 February 2013 08:27 PM
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perspicacious

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Just found p2 of the topic having only read p1

"porta-cabin and PME networks.
we dont offer a PME terminal for this type of installation.
BS7671?"


That is exactly what the xxxxx (deleted on legal advice) said to me until I challenged him to find it in BS 7671 and then realising it, the secret little book was allegedly quoted.

I wonder if anyone has a copy from any of the DNOs available?

Regards

BOD
 01 February 2013 09:13 PM
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Avatar for UKPN.
UKPN

Posts: 547
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there is a book, but DNO/Meter operator staff would not need to
reference it for such a basic query.

Regards.
 01 February 2013 09:32 PM
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perspicacious

Posts: 7267
Joined: 18 April 2006

"there is a book, but DNO/Meter operator staff would not need to
reference it for such a basic query."


Well the below poster seems to think that the answer lies in BS 7671 by his use of a ? mark mimicing the initial response I got......

"porta-cabin and PME networks.
we dont offer a PME terminal for this type of installation.
BS7671?"


Regards

BOD
IET » Wiring and the regulations » porta-cabin and PME earthing

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