Tim,

The voltage rating on coax is a lot higher than what you would calculate
based on max power. I can't find the numbers right now, but seem to recall
that RG58 was rated at 750 V or more. The big stuff is good for over 2000V.
The reason you don't want the end of the wire exposed is that it could lead
to arcing or corona. Probably not at low power, though. Sorry, can't answer
your other question.

Tam/WB2TT