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TheFullHitSpark

Alright guys, ime racking my brains and books trying to find out why my boss told me that you are not supposed to run PME outdoors to say out feeding a garage or a shed of some sort, i know about potential differences.... also i know some about special locations but i just cant understand why your not allowed PME, surely if the cable is protected by an RCD, the same as it would be if you was using an earth steak for TT what is the difference?? can someone elaberate on this for me please. if this is the case you would have to have an earth steak which would be a seperate means of earthing than provided from your source, but obvioulsly your (PME) earth would still have to provide protection for the cable feeding the (sub main).

Thinking outloud abit.. :p



If someone could show me the light i would be mostly appreciative.




Cheers guys, also as you all probably know this forum is the dogs go-nads...
 
hiya mate its something to do with pme not being a true earth, therefore if you run a pme earth to a metal framed external building a difference in potential may occur in the event of an earh fault,so you should not use the pme earth, you have to create your own earth with a rod. (i think), perhaps someone can explain it better, regards dave.
 
ok.

Houses usually (although not always) have insulating floors etc which mean that main bonding can establish a reasonably equipotential zone, so even if the electrical system's earth is at some higher voltage (e.g. PME with a broken CNE) it would be difficult to get a shock while inside the house. Go outside, or in an outbuilding with a damp concrete floor (no damp proof membrane) and voltage differences can rocket!
 
There appears only to be a problem if there are extraneous conductive parts in the outbuilding...

The impression I get is that the presence of extraneous conductive parts does not preclude the use of an exported TNC-S earth, it mearly means that they have to be bonded back to the submain MET with the same size cable as is appropiate for doing the same in the main building, and that the same sized or bigger earthing cable is used to provide the earth to the submain board, and as this is not always practical, a way of avoiding having to install bigger cables is to convert the remote installation to TT, and bond extraneous bits back to this

I think!!!!!!
 
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This is interesting, could some of you seasoned sparks provide a diagram?

It would be very useful and informative.

Thanks

Si.
 
In my view wire as a normal circuit provide the right type of protection for the circuit IE 30MA RCD/RCBO at correct rating for cable ETC and thats it 35 years exp and all the certs you can think of plus a degree in logic
 
Alright guys, ime racking my brains and books trying to find out why my boss told me that you are not supposed to run PME outdoors to say out feeding a garage or a shed of some sort, i know about potential differences.... also i know some about special locations but i just cant understand why your not allowed PME, surely if the cable is protected by an RCD, the same as it would be if you was using an earth steak for TT what is the difference?? can someone elaberate on this for me please. if this is the case you would have to have an earth steak which would be a seperate means of earthing than provided from your source, but obvioulsly your (PME) earth would still have to provide protection for the cable feeding the (sub main).

Thinking outloud abit.. :p



How do you have your Earth steak, Me? Medium rare!
 
In my view wire as a normal circuit provide the right type of protection for the circuit IE 30MA RCD/RCBO at correct rating for cable ETC and thats it 35 years exp and all the certs you can think of plus a degree in logic
yes but the reasone that the pme becomes a prob lem ifs if the neutral conductor fail/broke (yes 1000/1 chance)earth conductor could raise in voltage
as stated in a house all pipe work ect would raise so no shock risk
the use of a r,c,d will not save you in this senario as the earth which will raise in voltage even when r,c,d trips will stay at a dangerous voltage
 
are you even a spark beaver? you wouldnt be using the neutral as a means of earth feeding the sub, it would be a seperate conductor. You dont make any sense!
 
are you even a spark beaver? you wouldnt be using the neutral as a means of earth feeding the sub, it would be a seperate conductor. You dont make any sense!

I think you should read below before asking any one if they are an electrician it is you that is incorrect.

Defintions from Regs

TN-S A system having seperate neutral and protective conductors throughout the system.

TN-C-S A system in which the neutral and protective functions are combined in a single conductor in part of the system.

TN-C A system in which the neutral and protective functions are combined throughout the system.

You will normally find on a TN-C-S system that the neutral and earth are combined upto your cut-out where they will then be seperated to give you a seperate neutral and earth.
 
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I think you should read below before asking any one if they are an electrician it is you that is incorrect.

Defintions from Regs

TN-S A system having seperate neutral and protective conductors throughout the system.

TN-C-S A system in which the neutral and protective functions are combined in a single conductor in part of the system.

TN-C A system in which the neutral and protective functions are combined throughout the system.

You will normally find on a TN-C-S system that the neutral and earth are combined upto your cut-out where they will then be seperated to give you a seperate neutral and earth.
i thank you for you vote of confidance ian.settle1
 
This explains in clear terms:

The risks from a PME installation stem from the
possibility of a ruptured neutral in the supply to
the consumer’s premises. The break in the neutral
could occur anywhere in the supply, and could
affect just one house or several houses, depending
on the supply arrangements. This could cause the
neutral, and the mains earth which is bonded to it,
to rise above the ‘true’ earth potential. To avoid
the possibility of metalwork inside the house being
at a different potential from the mains earth
(which in this case is at neutral potential), the IEE
regulations state that all metalwork such as central
heating systems, water pipes, gas pipes etc should
be securely bonded to the neutral at the PME
bonding point near the consumer unit, as in Fig 1.
In the unlikely event of a ruptured supply neutral,
the current which would normally return via the
neutral will attempt to return by way of all the
other earthed conductors – including the RF earth​
to amateur radio equipment (Fig 2).

http://www.rsgb.org/emc/pdfs/leaflets/emc7protectivemultipleearthingmembers.pdf

Also, I understand that you need to consult with DNO anyway before exporting PME.
 
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just answered a similar question, perhaps this will help,

Subcategory - 1. BS7671 - Voltimum UK - Electrical Installation Products and Contracting



Subcategory - 1. BS7671

*
Local supply authorities*appear to have different requirements when taking supplies externally to*garages/outbuildings.
Here in West Sussex we are*told that we should take a Phase and Neutral supply only to a detached building and then install a local earth rod for the**external supply.
Some time ago an article*appeared in "The IEE Wiring Matters" by John Ware.*This stated that even with a PME supply to the house it could be achieved without an earth rod. The article appeared in Autumn 2005 issue 16.
I would appreciate your fact and regulatory*based comments.


*
BS 7671, and indeed, the article you refer to, give the requirements for electrical installations generally. Should local conditions, or in this case, the electricity supplier, require that further measures should be taken, their advice should always be followed.**
30-03-2006



*Back


For further information please contact:
Company:
IEE
Address:
The Institution of Engineering and Technology
Savoy Place
London
WC2R 0BL
UK
Telephone:
+44 (0)20 7240 1871
Fax:
 

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