On power lines, how come down leads are often extended off the pole using fiberglass posts?

Answer 1

sometimes the down guys (guy wires) pass nearby energized conductors and in that case fiberglass guy strain insulators are installed to isolate the steel guy wire from completing a path to ground. The steel wire over time will build capacitance through induced voltage often enough to pose a danger to anyone that could come in contact with the down guy.

Answer 2

The device you're describing is known a strain insulator. they are generally made from fiberglass, but were originally made from apitong wood. The strain insulator is used to prevent the guy wire from being energized due to a damaged, cracked, or contaminated insulator.

The wires supported by those insulators are typically energized at voltages between 4,000 and 34,000 volts.

Answer 3

Fiberglass dosent conduct electricity untill it gets to about 1200 degrees.
They put it on the lines to keep people from getting shocked or struck by lightning.
There is a second reason as well, it's because when the downlines get old and a little rusty, or if something scrapes one, the wires can fray and they are really stiff, so they poke into people like needles. It really hurts! :)