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#32
June 6th 05, 08:03 AM
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"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 |
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