[Elecraft] Antistatic mat without electrical ground

[Stephen Prior]

Thanks for the extra details David,

I've never lived in a house with PME, but maybe that's because I've never
lived in a town!

73 Stephen G4SJP


On 14/05/2008 23:02, "David Woolley (E.L)" <forums at david-woolley.me.uk>
wrote:

> 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!
>