In my experience with grounding as lightning protection, a very important
aspect is that everything be kept at the same potential. All grounds need to
be common with the electric service ground. The tower, metalic plumbing, and
anything metalic of any length, must be bonded together such that the entire
grounding system is at a common potential. Then added to that ground system
should be several electrodes. What is often not expressed, is that a good
grounding system will greatly lessen the possibility of a lightning strike
by constantly discharging the rapidly building electrostatic potential
during a storm. An ungrounded wire antenna, in my experience, is an open
invitation for a lightning strike.
Ten years ago, when I put up my tower, I thought it would be a great idea to
separate the tower grounding system from the electric service ground. Two
years ago I learned that such a setup creates very large differences of
potential in a storm. I had brought a cable from the tower ground system, in
through the wall and lugged it to the back of the steel plate where all of
my coax bulkhead connectors were mounted. I had wire braids with banana
plugs hanging from this grounded plate, that I would plug into the center of
the female bulkhead connectors, to ground the dipoles.
Anyway, I took a lightning strike (melted the reflector on a beam) and where
the tower ground passed near the romex cable feeding my equipment, there was
signs of a fire ball and the romex jacket was blown open. Interestingly
though, the main conductors were just scorched while the romex ground
conductor had very obviously taken the strike. This helped equalize the
potential difference between my tower ground and my electric system ground.
Then there was a flashover across the duplex outlet under my desk. It
literally blew the plugs out of the receptacle, melted parts of the
receptacle and blew the cover apart. This helped equalize the difference of
potential between the grounds and the electric mains.
The radios were disconnected from the antennas. All that was plugged in in
the shack was a digital clock. The clock was on top of a keyer which was
connected to two radios. The clock exploded all over the room, melted the
metal cover where it arced to the keyer, took out several transistors in the
keyer and wiped out both radios. Many other electrical items in the house
ended up in the spare parts bin also.
Another interesting thing was that where the guys were anchored to the roof,
it blew through the shingles as well as through the plywood. But only on two
of the guys. One was untouched. The two damaged areas were where the guy
anchors were bolted near the front and back edges of the roof. I could see
where the aluminum drip edge under the shingles was melted where the
lightning literally blew through the roof. This drip edge is in contact with
the aluminum gutters and obviously there was a huge difference of potential.
There was no metal near the third guy and no sign of damage either.
I now have all grounds tied together, added surge protection to the main
panel, and keep everything disconnected during a storm. I also have a wife,
three kids, and a dog, who have an awsome respect (fear) for lightning.
Jeff - K1LE
East Lyme, CT
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