Lightning Page 142

one day of thunderstorm activity in two years.

Protection requirements further depend on the amount of exposed wire and on the conductivity of the ground. It is much easier to protect equipment situated in a salt water marsh than in dry, sandy soil.

It should be realized that virtually nothing can protect an antenna or signal wire from a direct stroke. Considering the tremendous magnitude of current involved, it is easy to understand that even a very heavy ground conductor might be vaporized. However, proper protective equipment pays for itself, protecting equipment against a little-publicized phenomenon known as a ground surge.

In order to better understand a ground surge, picture for the moment the plates of a charged capacitor. When a conductor is connected between the plates to discharge the capacitor, current flows through the conductor from plate to plate, and from all points on the individual plates to the conductor. Comparing an electric charge in a cloud and the ground below it to the places of a capacitor, and imagining the path of a lightning discharge as the shorting conductor, it is easy to see that each time lightning strikes, current flows through the path of the stroke from the ground to the cloud; current also flows through the ground from all surrounding points to the point where lightning strikes.

If lightning struck a point several hundred yards from an antenna tower, current would flow down out of the tower and through the ground to the point where the stroke occurred. The magnitude of these surges can be as high as ten percent of the current in the lightning stoke itself — as much as several thousand amperes. If the equipment in your shack were grounded at more than one point, a ground surge might find an easier path through the interconnecting wires than through the ground. In that case, goodbye equipment. This one hideous thought clearly illustrates the reason for one and only one ground point in a well-protected shack.

Where remote control lines or other types of signal wires run long distances above ground, ground surges from lightning are a major problem. The railroads, using unshielded signal lines running for miles at a stretch, often go to great extremes to protect their delicate and costly signal equipment. Each line has a heavy-duty lightning arrester connected to ground at the point where it enters a wayside housing. Electronic devices such as carrier-current transmitters and receivers are further protected by high-speed gas discharge tubes on each terminal.