On Sat, 20 Aug 2016 16:32:05 -0400, rickman wrote:
In studying for the Technician exam I am finding questions which seem to
be making a big deal of having sharp, right angle bends in the ground
wires for lightning protection...
You should not expect the answers to the exam questions to represent
an absolutely correct explanation. Some of the answers have
exceptions to overly simplistic answers. Some answers only apply for
specific frequency ranges. A few make no sense at all if you've had
any experience with the topic. Long ago, I did my best to fix some of
these oddities, and gave up for reasons I don't want to explain. You
should have little difficulty finding other problems in question pool.
Basically, if you want to pass the exam, you have to swallow your
pride and forget most of what you know about physics, and just supply
the conventional wisdom answer that is expected. Sharp bends are
considered bad because the narrowing at the bend causes the cross
sectional area of the wire to be reduced at the bend. Given a large
DC current, such as a lightning bolt, this can cause the wire to fuse
at the bend. You can test this yourself by just bending a piece of
wire at a right angle. Pass sufficient current through the wire and
it will fuse at the weakest point, the right angle bend.
A right angle bend makes a rather poor 1/4 turn coil, but that's
allegedly enough for the wire to induce substantial currents in any
nearby conductors. The damage caused depends on what is nearby,
usually the coax cables on a tower. There's plenty of induced
currents from just the grounding wire, but the right angle bend
allegedly makes is slightly worse. I find this somewhat difficult to
believe, because a wide radius right angle bend would produce the same
induced currents.
There's also the phenomenon where a lightning bolt "leaps" off the
right angle corner bend, to a nearby grounded object. I've never seen
this happen, but I've heard it mentioned often enough to suspect that
it might be real. Since I don't understand it, I won't try to explain
it.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558