If your entrance is old, the three wires (115,115,0) are separated and
come in as roughly parallel wires. These may or may not be insulated.
The air gap is the primary insulation. Many times these also had rubber
insulation to begin with, but they didn't know how to make
sunlight-resistant insulation back then, and it cracked off.
If you lean an aluminum ladder against these, the ladder could cause a
short between any two or all three of them. If the ladder touches only
one of the 115-V wires, then there could be a short from the wire to the
ground through the ladder, or from the wire to the ladder to you then
ground. The latter case would be especially true if you were holding
the ladder itself off the ground (i.e. carrying the vertical ladder)
when you hit the bare wire.
More up-to-date entrances have sunlight-resistant insulation that is
good enough that they actually twist the three wires together, using the
ground wire (which is a steel cable) for strength. Obviously this
insulation must be good or it shorts out to itself. As long as this
insulation is good, there should be no danger hitting it with the
ladder. The question is whether you trust that there isn't a break in
the insulation where the ladder hits it, or that the ladder won't cause
a break when it hits it. This insulation is pretty tough, so you ought
to be able to brush it with the ladder and be okay. But why test it?
Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Bluffton College
Bluffton, OH 45817
(419)-358-3270
edmiston@bluffton.edu