"Eric F. Richards" wrote:
"Michael A. Terrell" wrote:
wrote:
I don't want to start a pising contest, but you might want
to visit a local TV or radio station. They take lightning
hits all the time and very seldom have anything more then a
sec or 2 of off air to show for it.
Really? A friend of mine makes over half his income from repairing
lightning damaged radio stations here in Florida. While I was the
engineer at WACX TV lightning hit their studio building in Leesburg
Florida. It took ot the entire telephone system, the main computer, all
the terminals, all the LNAs on the C-band dish, most of the receivers
and the 11 GHZ CARS transmitter that fed the original transmitter site.
It took months to repair everything, including replacing the vaporized
grounding system. true, if you're in an area that rarely get nearby
lightning strikes, but there have been storms here with over 1100
strikes in 30 minutes. The continuous EMP weakens things, and there are
a lot of failures because of this. I lost all three video amps in a
computer monitor when lightning hit the water behind my workshop. It
wasn't plugged in, and the video cable was wrapped around the tilt
stand, yet enough voltage was induced into the cable to blow a crater in
the video amp chips. A battery powered digital thermometer hanging on
the wall exploded. The IC was vaporized and a hole burnt in the circuit
board. In truth, there is little that can be don to protect you from a
direct lightning strike. AM radio towers have a huge spark gap at the
base to protect the insulator, but damage to the antenna system are
common. Also, a lot of stations have a spare transmitter that's already
hot, so they can switch over and get back on the air, "In a couple
seconds"
Yes, some TV stations get damaged. But invariably they violate the
proper designs of a lightning mitigation system. All that it takes
is a single conductor that doesn't go through the ground window.
No, everything went through one ground, but the strike wasn't on the
tower, it hit the poured concrete building in multiple locations. It
was a single level, flat roof with a wall around the outside edge. The
concrete finish cap was missing in a lot of places after the storm.
Years ago I worked on a fairly expensive project, set up on a mesa in
Colorado, with an antenna higher than any object in about a 5 mile
radius. The other users of that site were habitual in plugging
something into the protected side of the ground window and then
putting on a metal rack on the unprotected side of the window.
Because of that habit, they will no doubt lose their equipment in a
direct strike.
...and there's no need.
As for damage to the antenna systems... some times it can't be
avoided. But many times it can. In that case, all it needs to have
is a low impedence path to ground in the case that lightning strikes.
Gas discharge tubes (or spark gaps, but far less controllable) provide
that during a strike. The tube has to be able to survive for the
lifetime of the strike, no longer. If it shorts, well... it did its
*job*.
--
Eric F. Richards
When the strike hits the building rather than the tower, its going to
do damage. There were chunks of concrete blown out of the building,
exposing rebar and 1" threaded rod that held the concrete roof to the
pillars in the parking lot. The ground conductors for the building
vaporized, but it was replaced with newer and heavier grounding,
including a 4" metal conduit run from the equipment racks to the
tower. I moved the microwave racks from the center of the building into
a tiny closet as close to the tower as I could, rebuilt all of the
electronics, and added another set of ground rods where the 4" conduit
entered the building. After that, there has been no damage, even though
the tower has had some strikes. It will fail again, some day because
the weather here in Central Florida corrodes everything over time.
--
Former professional electron wrangler.
Michael A. Terrell
Central Florida