[Gllug] OT: Mains electricity colour coding.

[Christopher Hunter]

On Mon, 2010-10-25 at 07:34 +0000, Chris Bell wrote:

>    The UK regulations require PME (Protective Multiple Earths) with the
> neutral connected to earth at the meter termination. 

This is only the case with new installations.  Where I work, we have
lots of outdoor electrical and electronic apparatus - we use PME
throughout, but Neutral is still considered to be a current-carrying
conductor.

Older installations (like most domestic installations) use (effectively)
two wires - Live and Neutral - from the sub-station.  The Neutral is
connected to a substantial Earth at the sub-station, but the resistance
of the cable ensures that the further you are from that sub-station, the
higher will be the voltage on the Neutral!  

The third terminal in your plug is for a protective Earth - this should
be local to the building (I have a large galvanised dustbin buried
beneath my patio!).  This is only there for safety, and has no part in
providing a current path, under normal circumstances!

You will notice, if a substation phase fuse blows, that one in three
houses loses power.  This is because there are actually three Lives
running along outside your house.  They are traditionally colour coded 
Red, Blue and Yellow (and Black is the Neutral return).  Each is at 230V
AC with respect to Neutral, but may be as much as 450V apart, Live to
Live.

I'll find a really good on-line resource for all this, and you'll be
able to read all about it....


Chris

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