Lightning Basics

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From accurate measurements, it is now known that a typical strike contains less than 100 coulombs, but just how much charge is this? What is a practical application of a coulomb? If we have an ordinary light bulb rated at 100W and 100V, the charge traveling through it each second will be one coulomb. Based on this voltage and power, a current of one ampere will flow. This means one coulomb per second or 60 coulombs in one minute. This value falls within the range of a typical strike. If this is true, how can this relatively small charge do so much damage?

One analogy is a sledgehammer driving railroad track spikes. If a very strong man simply holds the sledgehammer atop a spike and pushes down as hard as he can, the spike will not be driven into a railroad tie. On the other hand, if the same man swings the sledgehammer rapidly, the spike will be partly driven into the tie. Several swings and the spike will be fully driven into the railroad tie. The length of time is the main factor. The inertial energy in the sledgehammer head is transferred into the spike in a very short time. This is analogous to the damage done by a lightning strike. Because the time frame is so short, the nearly instantaneous current within a strike is very high, but the charge (coulombs) lowered to earth is relatively small.

Previously, it was mentioned that the current emitted by a single, sharp elevated point is limited to a small value. This is why Franklin's rod seemed to be a failure and was later relegated to the task of directing a strike down to ground. However, when many sharp points are elevated, each point contributes a small current so that the total amount of current can be greatly increased. Because the charge in coulombs is a relatively small value, the energy can be dissipated faster than can be accumulated to initiate a strike. It is not necessary to completely discharge the electric field.

As the electric field associated with the onset of a thunderstorm builds from the ground up, the first ionization potential of the air (about 7 to 8kV) is reached and conduction of current from each sharp point begins. Because the field voltage is rated in volts per meter, the higher the points are elevated the sooner they begin to discharge the electric field. If the field voltage can be kept below the value required for a strike, the strike may be prevented from happening. It is not necessary to discharge the field voltage completely to zero.

Massive energy?

Technically, a lightning strike does contain a massive quantity of energy, but the energy it contains is released in an extremely short time. The damage is done because of the instantaneous release of a relatively small quantity of electric charge. Because of these facts, a lightning strike is not inevitable. It can be prevented by practical application of scientific principles. The charge can be discharged harmlessly into the air slowly. We can take advantage of this principle to protect towers, antennas, transmission equipment and any tall structure.

In the past, this method has been called dissipation. A better description would be "field discharge" because this is what happens when large numbers of sharp, conductive points are elevated in the proper geometric distribution. The electric field voltage is diminished by this conduction just as a capacitor is discharged by a "bleeder" resistor placed across its terminals. A storm cloud above the earth forms a giant capacitor with the surface of the earth. If the voltage gets high enough, the dielectric between the plates (cloud bottom and earth surface) breaks down and instantaneous discharge occurs.

If you are concerned about lightning damage, you should review your basic understanding of lightning strikes. Get rid of the misconceptions from thousands of years past and get a good understanding of the scientific principles behind the sources of a lightning strike. Lightning damage can be greatly reduced or eliminated by simply partially discharging the electric field that initiates a strike. Remember, it is not necessary to completely discharge the electric field. It is only necessary to decrease the field voltage below that required (about 10kV per meter of elevation) for a strike. You may not be able to part the clouds, but with the proper application of these simple scientific principles, you can prevent lighting strikes.

The author gives credit and thanks Mike Langner and Sumant Krishnaswamy, PhD, for their invaluable assistance.

Nott is director of research and development for Nott Ltd.

-- See page 3 for more on field discharge

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