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Put Lightning in the Ground
Jon Titus, Contributing Editor -- Design News, January 6, 2008
One afternoon I saw lightning strike a telephone pole about 50 ft away. The lightning split the top of the wooden pole and destroyed a neighbor's electric range and our TV set. But even if lightning strikes some distance away, it can still damage sensitive electronic equipment. Contrary to popular belief, you don't need a direct strike to turn expensive test-and-measurement instruments or sensors into junk. Lightning within a few kilometers of equipment can induce destructive voltages and currents on conductors. In one case, the magnetic field from a lightning discharge three miles away produced a voltage as high as 70V/m in a conductor (Ref. 1).
To some extent, geography determines the need to protect equipment against transients caused by lightning. You can find isokeraunic maps on several websites that show thunderstorm and lightning activity in the U.S. If you live in Florida, for example, prepare for much lightning activity. Areas along the U.S. Pacific Coast, though, experience few lightning storms.
Although test equipment with I/O and communication lines includes devices that protect against small transient voltages, these devices cannot handle the voltage and current induced by a nearby lightning strike. So, if you plan to make outdoor measurements or control exterior devices, your control and test equipment may require additional protection. Spark gaps, gas-discharge tubes, Zener diodes, thyristors and metal-oxide varistors (MOVs) will shunt high voltages to ground and you can use these devices individually or in combinations. These and other protective devices have capabilities and limits you should understand before you apply them. Several companies provide ready-to-use lightning-protection devices specifically for Ethernet, telephone and serial I/O lines.
Lightning protection always begins with a single low-resistance, low-impedance earth ground of 25 or less. All protective ground paths should terminate directly to this single earth-ground point. If induced current cannot quickly flow to your ground point, it will seek other paths that may not seem obvious. So, use heavy gauge conductors to connect protective devices to your ground.
The lightning strike I mentioned earlier occurred about 150 ft away from wires I ran from my basement to a grounded outdoor motor and feedback potentiometer. I had protected the five control wires with MOVs connected to a deep ground rod nearby. I could not determine whether these devices protected my indoor electronic equipment, but it suffered no damage.
You can do a quick cost-benefit analysis to determine what to protect and how to protect it. In some cases, the cost of protective devices may amount to more than the cost to replace a device. Most of the time, you can reduce the possibility of damage caused by lightning at reasonable cost. But, the cost to eliminate all effects of lightning can amount to quite a large sum.
Ref. 1: McCurdy, Patrick, "Transient Lightning Protection," PipeLine and Gas Technology, January/February 2004.
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 A gas-discharge tube and a transient-suppression diode protect a circuit. The diode turns on quickly and grounds some of the transient current until the discharge tube can conduct current and short the remaining transient current to the ground. |
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