On Tuesday, September 1, 2015 at 10:55:02 AM UTC+1, 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)?

It's most likely to be 11kv. It could be 6.6kv 11kv or even very rarely 33v in the UK. It will be delta connected on the primary side. Some rural properties only get two phases on the primary side. Not in the UK, but in some countries abroad the primary side can be a single phase and the return is via the earth.

On the secondary there are multiple configurations. It will be star connected. Earth may or may not be combined with neutral on a single conductor or not supplied at all. There may be a single phase, two phases or three phases.

Some remote locations have a split phase arrangement on the secondary where the supply is a single phase centre grounded. This allows a supply a little bit more remote from a single phase supplied transformer.

The neutral will be earthed at the substation. A combined earth and neutral conductor should be earthed at multiple points along the route.

If the load were balanced completely there will be no current on the neutral conductor. In practice there will be current on the neutral conductor but it should not exceed the current on a single phase - it would take a very odd inductive load to do that.

Don't use this info for anything other than entertainment. I may be talking out of my arse.

Philip