>Path: infonode!uunet!europa.asd.contel.com!emory!kd4nc!ke4zv!gary >From: gary@ke4zv.uucp (Gary Coffman) >Newsgroups: rec.radio.amateur.misc >Subject: Re: Authoritative Book on Lightning Protection >Message-ID: <1991Dec19.202543.27000@ke4zv.uucp> >Date: 19 Dec 91 20:25:43 GMT >References: <10062@cactus.org> <14570231@hpnmdla.sr.hp.com> >Reply-To: gary@ke4zv.UUCP (Gary Coffman) >Organization: Gannett Technologies Group >Lines: 139 In article <14570231@hpnmdla.sr.hp.com> alanb@hpnmdla.sr.hp.com (Alan Bloom) writes: > >As I understand it, a lightning rod is not intended to withstand a direct >hit. The purpose of the rod is to PREVENT the lightning from striking >in the first place. The sharp point on the rod is important -- Its >purpose is to discharge the ionization of an incipient lightning strike >as it is forming. Before lightning can strike, it must form an ionized >column of air to conduct the current. The lightning rod bleeds off this >charge so that the lightning will hit somewhere else. This is *a* theory of lightning rod action. Lightning Protection Assoc makes a good living selling a system that resembles a barbed wire top hat as a lightning suppression device. The FAA ran a series of tests at their facilities in Florida on this and other devices. Plain sharpened lightning rods and LPA's dissapative arrays help during storm buildup and decline, but during the height of the storm you still get hit. Great chunks of the LPA arrays disappeared in the strikes that *did* get through. The plain lightning rods faired somewhat better. Though they prevented fewer strikes, when they were hit they did a better job of protecting the equipment from damage. Your average lightning strike has about 50,000 amperes of current flowing for about 100 milliseconds. That's a lot of current, but it doesn't last long. If your conductors are heavy enough, the current can be directed to ground without major damage. Number 8 can handle the current for the short time required, but poses too much inductance to properly protect equipment. Lightning is a step waveform nearly identical in character to a giant spark transmitter. The energy is mostly RF, not DC. Therefore, skin effect and inductance must be kept in mind when designing a lightning diversion system. 5 inch copper strap with no bends is the preferred conductor. Don't depend on a soft soldered connection anywhere in your lightning protection system. Join conductors mechanically with heavy hardware. The solder can melt and leave you with a potentially dangerous situation where your system has an open circuit. Then you have a lightning attractor worse than no system at all. Since we may be dealing with thousands of amps, even a tiny resistance will setup a large potential difference. It's this difference that kills our radio equipment. It's absolutely vital to setup a single point ground window so that our equipment never sees that potential difference. A single point ground window is a small area, no more than a foot square or so, through which *every* conductor enters or leaves our station. This window is usually a heavy copper plate that is connected to our single point ground field by 5 inch copper strap. *Each* conductor passing through this window is attached to the copper plate with suitable protective devices. This plate may rise to a very high potential during the strike, but *so will every conductor passing through it*. With equal potential on every cable entering and leaving our equipment, no damaging differences can develop. Note that power and telephone, if used, must also pass through the ground window. One big danger to our equipment is posed by the single turn transformer formed by our towers and coax cables. With 50,000 amps flowing down our tower structure, a large current can be induced in the parallel coax cables coming from our antennas. All cables should be routed *inside* the tower structure, and should be grounded top and bottom through suitable protective devices to short this potential single turn transformer. A proper ground field is essential to a good lightning protection scheme just like it is essential to good radio communications. An eight foot ground rod is *not* a sufficient ground except in the most exceptional soil. A good system of radials, no more than 6 inches below the surface and connected at a single point that becomes the station single point ground is required. This will couple the energy properly to the surface charge of the earth and the single point connection will prevent dangerous ground loops. Guard particularly against separate power company and phone company grounds. These must be rerouted to your single point ground if local codes permit. A heavy bonding wire direct from their ground point to your single point ground is usually sufficient. DON'T DAISEY CHAIN GROUNDS. For greater protection to your home, you may want to run a loop of heavy wire around the perimeter of your home and connect it with a single conductor to your single point ground. A bit more about how a lighning stroke forms and propagates. A lightning discharge occurs when the surface charge on the earth and the charge in the clouds develops a potential great enough to overcome the breakdown potential of the intervening air. Low energy ionized streamers first develop, some reaching up from the ground and some descending from the clouds. When they meet, the circuit is complete and the major current of the main bolt begins to flow. It will continue to flow until the charges are sufficiently equalized that the potential barrier becomes too large to bridge. The fault with dissapative arrays and pointed lightning rods is that, as the storm progresses, the amount of charge becomes too large for them to equalize and streamers develop from the points reaching towards the clouds. Then when these streamers meet the down coming streamers, BANG, a lightning bolt forms. Any lightning protective system must be prepared to deal with a main bolt strike. Another word on ground fields. Lighning is RF and is subject to the skin effect just like our radio waves. A single deep ground is practically useless. This is a surface sheet conductivity problem. A system of radials, lightly buried, couples the energy into the earth much more effectively than a deep rod. Soil may have conductivities ranging from hundreds to millions of ohms per foot. To form a low impedance connection to the surface charge, a large copper net shorts out most of this resistance and forms one plate of a huge capacitor that couples energy to the earth via the small capacitive reactance. To avoid differential currents from flowing, all connections must be at a single point. I can't emphasis this point too much. DON'T USE MULTIPLE GROUND CONNECTIONS. >The #10 or #8 wire used with most lightning rods would not withstand >the thousands of amps of a direct hit. Even if it did, the thousands of >volts developed would likely still jump to nearby rain gutters, window >screens, TV feedlines, etc. and cause damage. The best you can hope for >is to survive a near miss -- which can still induce destructive voltages >and currents. You can survive direct hits, broadcasters do it every time there is a storm. But you must take the right actions to prevent potential differences from entering your station. Since I did it right, I've survived two direct hits with damage only to a 2 meter Isopole and it's feedline. No equipment damage to the radio connected to the antenna and *operating* at the time. No damage to radios, computers, phones, or entertainment devices. But, you've got to do it right. There is no room for error. >If you live in a lightning-prone area, it is good practice to disconnect >the coax from all your radios when not in use. Each coax should enter >the house right at ground level and have the shield bonded with copper >strap to an 8 foot ground rod at the point where the coax enters the house. >A coaxial lightning arrestor installed at this point will provide some >protection for your radios in the event you forget to disconnect them. > >Any antenna tower or mast should be connected to a ground rod at its >base. This goes for TV antennas as well. Your local building code >specifies the required wire size. Disconnecting all equipment is the ultimate protection, but is often not practical for 24 hour packet stations, or when a storm blows up in the middle of the day while you are at work or at night while you sleep. Disconnecting only the coax is poor protection. Many strikes will enter via the power or phone lines. Only disconnecting *every* cable *every* time offers sure protection. Using separate ground points is a *serious* mistake with potentially deadly consequences. Gary KE4ZV