Science question, might bore most!

[Newboy1996]

In diagrams of electrical systems, earth and neutral appear to be the same thing? ending their path back at the sub station at the centre of "star".
Or in the case of TN-C-S (PME) earth is joined to the neutral conductor inside a property. Neutral is a Live conductor and earth is protective conductor, how can they be the same thing?
Is it just putting any residual current down the neutral as thats the return path, better than sending current to the mass of real earth?

I know there are some real pro's on this forum, so whats happening here?
(Better than talking about shower cables!)

 

[Spark123]

TN-CS uses the same PEN conductor for the earth and the neutral. The PEN is tied to earth at the distribution transformer and at various points along the length of its run, often referred to as PME (protective multiple earthing).
Earth and neutral even in a TN-S are normally around the same potential, ballancing the phases helps achieve this but also the impedance of the neutral conductor which can allow potential to be present is relatively low.
You probably won't be suprised though that if a PEN becomes damaged or comes adrift the voltage of the exposed metalwork connected to it can become live to mains potential, have a battle of the phases etc. It is tied down to earth at points along its length in order to reduce the risk associated with an open circuit PEN and assist keeping it at earth potential. Also, in some places it is illegal to use TN-CS such as with caravans and boats as a the risk associated with an open circuit PEN is much greater.

 

[Click-Sure]

Live radiators, I've not seen it but met a bloke who has. The very idea scares me.

I'm sure DNO's have contemplated providing customers with a stake to suppliment tns ad tncs supplies, with reason!

 

[Newboy1996]

TN-CS uses the same PEN conductor for the earth and the neutral. The PEN is tied to earth at the distribution transformer and at various points along the length of its run, often referred to as PME (protective multiple earthing).
Earth and neutral even in a TN-S are normally around the same potential, ballancing the phases helps achieve this but also the impedance of the neutral conductor which can allow potential to be present is relatively low.
You probably won't be suprised though that if a PEN becomes damaged or comes adrift the voltage of the exposed metalwork connected to it can become live to mains potential, have a battle of the phases etc. It is tied down to earth at points along its length in order to reduce the risk associated with an open circuit PEN and assist keeping it at earth potential. Also, in some places it is illegal to use TN-CS such as with caravans and boats as a the risk associated with an open circuit PEN is much greater.

If its "tied down" to earth along the return path to the transformer surely it would loose current to earth. Or am I missing something important here?? Or is real contact to earth only made during fault conditions (via a passive device, until there a surge of unbalanced supply).
Just curious.

 

[holmslaw]

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[dannyboi2003]

The neutral is not tied down at variuos points, it is earthed only at the transformer feeding your area.

Actually yes it is, at each joint or pole (in overhead areas) there is a cable from the neatral connected to a rod in the ground. Have a look at the overhead poles when you can and if PME is available each pole with have a cable running down from the neutral line to a rod at the bottom.

 

[holmslaw]

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[Spark123]

Yes the post is about LV systems, in PME the PEN conductor is tied to earth at various points along its lenght. Not sure where HV systems come into it?
It will also have connections to earth via consumers MEBs etc.

 

[Click-Sure]

HV systems don't have a Neutral or Earth, just 3 phases.

 

[RF Lighting]

I've never seen an overhead lv distribution system.

You obviously don't live in yorkshire then

 

[Click-Sure]

Or Gloucestershire, Worcestershire, West Mids, Warwickshire, Derbyshire, Leicestershire or Staffordshire (yes my own supply is overhead with an underground drop to the house, and its PME, baby)

 

[holmslaw]

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[RF Lighting]

This is an overhead PME supply to a house I wired a couple of years ago.

There are two cables coming down the pole. The one on the left is a straight concentric cable which feeds the cutout in the house, and the cable on the right is to an earth stake at the bottom of the pole.
You can see where it is tied to the CNE conductor to the right of the fuse on the pole.

 

[holmslaw]

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[Click-Sure]

How good is that ring of barbed wire on the pole, not seen one like that before.

RF - Sure thats a fuse? not seen one on a pole before, only ever seen phase isolators.