Re: Lightning Damage

Pierre Vandevennne wrote:
"w_tom" <w_tom1@xxxxxxx> wrote in
news:1146612531.233885.105750@xxxxxxxxxxxxxxxxxxxxxxxxxxxx:


It's not complex. Earthing is also why Ben Franklin's lightning rods
work. Note repeated references to a simple concept - 'less than 10
feet'.


Does that mean that Franklin's lightning rods are unable to protect buildings higher than ten feets?

Just a question...


From what I gathered, Lightning is not DC where you worry about the resistance but a complex waveform that requires a low inductance and is treated as a R.F. problem. Some of the early work on lightning protection came from measuring the rise time on strikes to the Empire State Building with low bandwidth cathode ray tube oscilloscopes. This lead to an underestimation of the current rise times and standards that
did not go far enough in specifying very low inductance paths.

In a metal building lightning rods may not offer additional protection as the building itself is the lowest impedance structure. For a wooden building you want to control the attachment point and a rod is used to do this. One talk I went to on the statistics of lightning strikes in Tucson presented a strike density plot that had a correlation length of 50 meters. The speaker concluded that even with a structure high enough to exceed the 70 degree "protection cone" you still had a chance for a direct strike 50 meters away from the structure. A solar telescope on Kitt Peak uses a wire antenna that creates a protection perimeter as is used to protect fuel storage tanks. The grounds I have used are two types, wet grid... a mesh that is spread out with a watering system
or a set of radial grounds as high frequency current are subject to the skin effect and a lower impedance is obtained with multiple grounds
designed a a counterpoise like used in high power low frequency transmitting antennas.

A class of expensive fancy rods known a dissipative system some times used radioactive sources or many fine points to "drain the field" preventing or diminishing the strike intensity. A series of tests debunked this class of devices and demonstrated no advantage over a blunt rod. (New Mexico Tech ?) The E field of a thunder storm is centered many thousands if not 10s of thousand feet in altitude and it seems not to care about the details of the attachment. The path inductance and current density in the grounding system does however effect the peak voltages induced into the structure and can cause secondary breakdowns if the impedance is two high. In addition large strikes e.g. 250,000 amps will generate secondary discharges off of high field regions caused by corners which cause large field gradients and rods at the corners of a structure help to control the current path even though they are "only" 10s of amps.

All of this is covered in the Lightning book by Uman (sp?)


d.




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