On Tue, 01 Sep 2015 10:55:01 +0100, NY wrote:

As a matter of interest, what's the typical voltage of the three wires
connected to porcelain/glass insulators, mounted on wooden poles, that
feed the transformers (often pole-mounted) that step down to 240V for
overhead wiring to houses? I can't find anything on Google, using
phrases such as "overhead power line 240V" or "low voltage overhead
power line".

I presume the 240V overhead wiring is 4-wire (ie a star with a wire at
each tip and one at the centre) to allow one of the wires to be
designated as neutral and thus able to be earthed at each house just
before the "electricity board fuse". Does this neutral wire have to be
thicker than the other three because it is carrying the return current
for all three phases, or do the three return currents usually cancel
each other out (at least for the ideal case where all three phase
currents are the same magnitude and exactly 120 degrees apart)?

Without bothering to look at all the other replies, I can tell you that
a 3 phase with neutral LV distribution cabling system between the 240v
substation and domestic properties only subjects the neutral to a maximum
current equal to that of any of the phases in a worst case scenario
(either only one or two of the phases supplying their maximum rated
currents with either the other two or one disconnected). When all three
phases are supplying a balanced 3 phase loading, no current flows in the
common neutral back to the substation transformer.

However, for each connected property, the neutral will carry the return
current to the street joint onto the 3 phase supply distribution cabling
where it will be cancelled out to a greater or lesser degree by the
neutral return currents of adjacent properties fed from the other two
phases. There will be some current flowing between the tapping off points
along the street distribution 3 phase plus neutral cable but it will
never exceed, even in the worst case, the currents carried by any one
phase.

Compared to a simple single phase distribution using only two wires in
the street distribution cabling where each conductor has to be able to
safely carry 3 times the current of each phase wire in the 3 phase
system, there is a significant saving in copper investment (a 33%
reduction in copper costs) plus, as a bonus, elimination of volt drop in
the common neutral return under maximum balanced loading conditions.

As far as the rural distribution system with pole transformers that
you've described, I believe the voltage level on these lines is typically
6 or 11 KV (I'm not an expert on such rustic arrangements - 11KV is a
common voltage level and I have a vague recollection of 6KV being
mentioned).

The common neutral on an 11 or 6KV line cannot be used directly by the
connected properties. Each property's neutral simply terminates on the
neutral connection of the pole transformer's LV secondary (where it may
meet neutral connections from two other adjacent properties if within
reaching distance of the LV overhead cabling).

It is my understanding that the neutral connection on the LV secondary
will be independently connected to a local earthing spike or spikes
provided by the utility company responsible for the supply. It may not be
as good a low earth resistance connection as that provided in urban
substations but it'll be a damn sight safer than relying on an 11 or 6 KV
distribution neutral connection (assuming such a connection is provided
or even needed in the higher voltage levels of the distribution network).

I'm now going to hit send and take a look at what the 'experts' have
said and see how big a fool I may have made of myself. :-)

--
Johnny B Good