Dave wrote:
Bob Minchin wrote:
Dave wrote:
Has contacted me about the Ozzie practice of using mains sockets to
connect to wind generated 12 Volts as they are cheaper than the low
voltage ones. This could result in a 12 volt device being plugged
into a 12 volt mains socket and getting a bit of a shock. I have
tried to explain the problem, but I am not an electrician. Would
anyone care to correct the following before I post to him please.

start quote.

We call that PME over here. It is basically the same as you describe,
but there can be a problem with it. Our power supplies are earthed at
the transformers that supply the 230 Volts, from the centre of the 3
phase supply and again at the house input, by strapping the earth and
neutral together. (There is no true cable to actual earth) It can
happen that if an earth link fails at the incoming supply to a house
on the same phase the neutral can rise to the supply voltage, by
virtue of all the properties connected to that phase.

End quote

Regards

Dave
There would appear to be no commonality between the two section of
your post.

Yup - I was struggling a bit with that as well.

I have no idea what their low power socket looks like and since they
illegally use the same plugs for 12 Volts, as they do for 230 Volts, my
brother was getting more than a little concerned. What they do is remove
the live pin and consider it as a 12 volt plug, but I was trying to tell
him what the problems we have about the neutral rising to mains voltage
that we can have when fault conditions happen.

Ignoring the above, how close is my writing after 'We call that PME over
here to the truth?

Its not the best description I have read to be fair... sorry!

The particular fault scenario I think you are talking about is where you
get an open circuit PEN conductor (Protective earth and neutral) due to
a cable fault. The neutral will rise to something close to mains (since
it is now free to float, and is connected to mains via the loads in
yours and other's properties), however more worryingly, the the earth
will also rise toward mains voltage since it is connected to it.

Hence it is very important that the equipotential bonding is up to
scratch to ensure that if all the exposed conductive bits of the house
are going to float to mains potential - it really is *all* of them.

As a point of interest. the PEN conduction *is* however connected to
real earth - not only at the sub station, but quite possibly at multiple
other places along the route (hence the "multiple" bit of PME).

What any of this has got to do with 12V wiring and plugs though I can't
quite see at the moment.


--
Cheers,

John.

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