[Elecraft] Antistatic mat without electrical ground

[David Woolley (E.L)]

Stephen Prior wrote:

> In older houses in the UK the neutral is bonded to a real ground at the fuse
> box, where the ground is usually a long copper rod just a few feet away.

This configuration (TT) is rather rare (mainly rural supplies by 
overhead lines).

> There is rarely in my experience more than a volt or two on the neutral.
> 
> More modern houses use PME (protective multiple earthing) where the neutral
> is bonded not at the house but locally at the final step-down transformer-

The key factors about PME is that there is no earth wire from the 
sub-station, the premises earth being regenerated from the neutral at 
each property (TN-C-S).  It also has neutral grounded to real earth at 
multiple points in the distribution network.

Whilst PME is increasingly used, "The Electrician's Gude to the 16th 
Edition of the IEEE Wiring Regulations BS 7671" (ISBN 0-9537885-0-4) 
suggests that the most common system is TN-S, where the the ground wire 
runs back to the substation and is physically grounded there.

> the argument being that this forms a higher quality ground I believe.  The
> real danger with PME is that the neutral may well then rise above 0 volts,

The danger is that mains earth, which is tied to neutral, can rise above 
true earth by a large amount (the example in the book is 96volts) if 
there is a break in the neutral between properties, as the return 
current then flows between ground rods.

> but in normal circumstances, because plumbing inside the house is bonded to
> 'ground', no potential (!) exists for electrocution.  Until, that is, a ham
> decides to ground his equipment 'properly' outside.  Then large amounts of
> current can potentially flow!


-- 
David Woolley
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