Re: O/T Question - What are the purpose of lightni

Posted by KEB on April 05, 2007 at 08:42:18 from (192.91.171.42):

In Reply to: O/T Question - What are the purpose of lightning r posted by Turke Bros. Farms on April 05, 2007 at 07:16:00:

I spent a number of years doing electromagnetic compatiblity for military aircraft, including designing lightning protection systems.

The SOLE purpose of a lightning rod is to minimize damage to the protected structure in the event of a lightning strike by providing a low-impedance path to ground for the lightning current. They do "attract" lightning, but only within an area roughtly defined by a cone exending downward from the tip of the rod to the ground. Depending on who's work you believe, the width of this cone at the ground is between 1 and 3 times the height of the rod.

The idea that lightning rods (or any other sort of structure) dissipate charge and reduce the probability of a lightning strike has been repeatedly proven to be false. This idea continues to be espoused by people who sell such devices, but there's no scientific basis whatsoever for their claims.

All lightning protection is based on reducing probabilities of significant damage - there are so many variables involved in the physics of a lightning strike that its almost impossible to account for all of these variables all of the time. One significant variable is the peak current in the strike - a typical cloud to ground strike has a peak current of around 20,000 amperes. A severe strike may approach 200,000 peak amps, and there are anecdotal reports of strikes significantly exceeding the 200k amp range. For aircraft, we use 200,000 peak amps as a design point for flight safety. Damage from a lightning strike is generally driven by the amount of energy the strike deposits in whatever its flowing though. There are variations in the duration of the stroke, and hence the amount of energy deposited, as well as variations in the peak currents.

The idea that lightning is an overgrown version of a spark is also wrong. Lightning propagates through the air in steps on the order of a few 10's of meters long. These "stepped leaders" are essentially ionized channels in the air, a few inches in diameter. Charge propagates down the leader and accumulates at the end until the electrostatic field is stong enough to break down the air, creating another step. The sparks you see coming off of a Van deGraff generator are a small scale version of this phenomenon.

As the end of this leader approaches the ground, the electrostatic field surrounding it becomes strong enough to initiate corona discharges from objects attached to the ground. When the leader makes contact with one of these discharges, the circuit betwen cloud & ground is completed and the charged area discharges through the conductive channel. This is the part of the lightning strike that we see and that creates thunder. Note that these process occur in a area a few 10's of meters from the end of the stroke and there is a lot of randomness in the leader through the air, which means we cannot adequately predict the actual attachment by other than probabilistic means.

Keith

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