Dear Jeff: OK No tap on the LV side of the (3 phase) transformer. One
230 VAC wire and neutral wire go to house from the transformer.

The non-PME scheme: neutral is "earthed" only at the sub-station AND a local
earthing of a separate wire that seems to serve the same function as the
North American green wire is provided.

The PME scheme: no earthing takes place; at the house entrance, the neutral
is connected to what seems to serve as the green wire (the "local protective
earth wire").

Thinking now about installing an antenna and radio: with the PME scheme the
radio's case is connected to the "local protective earth wire" and yet the
outside of the coax is often going to be connected to an actual earthing. I
expect some current.

When I do the same thing here with the green wire connected to the radio's
case and the outside of the coax well attached to an extensive earthing
system, nothing happens because all of the earthings are connected together.
Most important of all, if I grab an exposed bit of the radio while holding
the unattached coax connector, no tickle is felt.

It sounds as if a standard part of each radio amateur faced with a PME
system is to use an isolation transformer.

Thanks too to you Jeff for significantly expanding my understanding of how
different people arrive at a solution to the same issue. By the way, are
you a radio amateur?

73, Mac N8TT



"Jeff" wrote in message ...

On 15/07/2012 15:50, J. C. Mc Laughlin wrote:
Dear Jeff: Many thanks.

It is especially interesting to learn that the expectation of
encountering a floating (no earthing) system is zero.

Expanding a bit on what you have communicated:
It appears that the transformer used between one phase of a HV 3 phase
line and distribution "wires" is 440VAC, center tapped. The
distribution "wires" to a house (or the like) are one side of the
aforementioned transformer's secondary and the transformer's center tap,
which is labeled as neutral. It is expected that another house (or the
like) is fed from the other side of the transformer's secondary and its
center tap.

No, there is no centre tapping, the supply is direct from one of the 3
phases at 230V+/- wrt neutral.


At the service entrance: one scheme connects the neutral to an
earthing. An alternative (called 'protective multiple earthing' (PME) )
does not connect the neutral to an earthing, but connects exposed metal
in the house to an earthing. Wow! The latter scheme is expected in new
construction.

No, one scheme leaves the neutral floating (at the consumer's end,
grounded at the sub-station) and provides an additional protective earth
wire locally via an earth stake (or in the past bonding to water pipes etc).

PME bonds the local protective earth wire to the neutral at the
consumer's end, and all exposed metal is bonded to that wire. NO
external earths are allowed (without modification of the scheme).

It is not immediately clear what the advantage of the PME scheme might
be. Your caution about RF grounds is well placed.

The claim is that because the earth is provided by the supply company is
it more reliable than a local earth stake. Personally I am not convinced
by the whole scheme.

Have a look at:
electrical.theiet.org/wiring-matters/16/earthing-questions.cfm?type=pdf

for more info
Jeff


J. C. Mc Laughlin
Michigan U.S.A.
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