wrote
I am trying to gain a better understanding of the issues associated
with lightning and grounding, particularly for an amateur radio
station. I would greatly appreciate any answers to my questions, given
below.
1.) It seems like lightning "finds" a path before actually discharging
through it. Is this correct? How fast is the process of finding the
path?
I have 5 minutes before supper to answer this, so it is an off-the-cuff
answer which you can look up in greater and possibly more accurate detail
later:
For several minutes before a thunderstorm arrives, it pushes a massive
wavefront of positive ions over the surface of the earth. This ion "wave"
saturates all objects and they begin to emit positive-charged "streamers"
skyward toward the thunderstorm cloud. Every object with these positive ions
(rocks, buildings, trees, human heads, etc) allows these streamers. Minutes
(or seconds) later, a stepped-leader from a cloud will start earthward and
the attraction to several of those streamers will grow. Hair could stand up
on your neck when this happens. Seconds or parts of a second later, a
step-leader will launch from the most favorable of those upward-flowing
positive-ion streamers, and the stepped leader will connect to it. A small
visible explosion is seen and heard about 150' above ground as this happens
(film shows this) and the cloud opens a channel of current to that stepped
leader. Milliseconds later a RETURN SROKE from the ground to the cloud opens
that stepped leader path wide open, and another pusle or two or three may
occur from cloud to ground following that. Lightning has "struck".
2.) Does lightning discharge over one path or many paths? In other
words, if lightning strikes a point, and there are two paths from that
point to ground, is the energy shared - like a current divider? One
would think that this is what happens, with the sharing based upon
relative impedances to ground.
Yes multiple paths are likely in most strikes. Current division occurs even
through air, we call it voltage division when more than one branch connects
with earth, as the current flowing (10-50ka) will be divided according to
the varying voltage potentials felt at various points. Voltage division is a
good thing when it is where we want it to be. Current then flows on
conductors of our choosing!
3.) This talk about "single point ground" can be confusing, especially
when the solution involves multiple ground rods. I believe we are
really saying that the station (equipment) should be connected to
ground at only one point - but you can have any number of ground
connections (i.e., like ground rods) otherwise.
That's correct. Ideally there would only be one single point ground for an
entire structure. But when radio room and AC service entrance grounds are
far apart, we bnd them and connect all forms of grounding systems along the
way.
4.) Does the size of an antenna affect the probability of its being
struck by lightning, all else being equal? What is the governing
relationship?
No, but the HEIGHT above ground does. Any object that a rolling ball 300' in
diameter would touch as it was "rolled" over your property, is an object
that is most likely to be struck if lightning occurs in that general area.
Lightning can sideflash horizontally for hundreds of feet, and even change
direction more drastically, but the rolling ball defines most likely
attachment points for a strike.
5.) Finally, consider an amateur antenna on a pole, located some
distance from the service entrance of the house. Let's say there is a
copper cold water pipe right at the service entrance. Is it wise to
place a lightning rod (or rods) right under the antenna, place an
arrestor on the coax there and then another arrestor where the coax
enters at the service entrance? Is a ground wire needed between the
remote ground rod and the cold water pipe?
A "Lighttning Rod" is a roof top or other high fastened "collector" of
lightning. The modern name for this is an "Air Terminal". From a lightning
rod, bonded "Down Conductors" carry the lightning energy to a "Grounding
Electrode" (Ground Rod).
A tower is already a lightning "air terminal" and all it needs is a good
ground system to divide the vltage and curent in as wide an area as
possible. Even the coax shield must be grounded many times along the height
of the tower, and an arrestor (or surge protection device) could be attached
at the base of the tower also. Yes it would also be used (as is shield
grounding) at the entrance to the structure.
Cold water pipes may be bonded as additional voltage division in a lightning
protection ground system, but they are only the source of primary single
point ground IF the structure's AC service entrance uses them as such.
Thanks in advance,
-JJ
More details are explained at:
http://members.cox.net/pc-usa/station/ground0.htm
Cheers,
Jack Painter
Virginia Beach, Virginia