Yorkie,
I could be wrong, but it sounds like Neutral-Screened cable.
N/S cable is used here in NZ for overhead services into residential installations.
The neutral conductor in the cable is used as the support for the cable at both ends (as in it is wrapped around an insulator on the pole crossarm and on the house), the only other wire in the cable is the centre core which is the PVC insulated phase wire to supply the house.
I'll see if I can get a pic of a typical install here to show you what I mean.

{Oh and btw, the whole cable is covered in thick black PVC sheathing}
{Edit:}
Here is the neutral screened cable that feeds the house next door, it's the cable that is on the top insulator on the pole:
[Linked Image]

Here is the other (feed) end:

[Linked Image]

Last edited by Trumpy; 06/02/07 07:58 PM. Reason: To add images