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TOWER OWNERS NEED TO BE HELD ACCOUNTABLE, BY LAW, FOR PRIVATE PROPERTY DAMAGE

There is a 50% probability that a lightning strike will be approximately 30kA. (Anderson & Eriksson 1980) If the self-inductance of the earth is estimated very conservatively at .5x10-6H, and lightning takes the form of a pulse which has a typical rise time of 2x10-6S, then from the equation, V=Ldi/dt; the estimated Ground Potential Rise (GPR) of a 30kA strike will be 7.5kV. Values of GPR could easily triple upwards of 25kV for higher current lightning strikes or strikes passing through higher inductance.

Towers attract lightning strikes. The taller the tower the higher the probability that lightning will strike the tower. Towers that are over 1000 feet in height can generally expect to be struck by lightning four or more times in a single year in much of the United States. The probability of a lightning strike to a tower can be specifically calculated using NFPA 780 and knowing the location and height of the tower.

When Lightning strikes a tower its grounding system elevates in voltage as the current passes into the earth through the tower system ground. This phenomenon is known as lightning induced Ground Potential Rise (GPR). This GPR represents waves of voltage that ripple out in circle type patterns away from the tower base. If the tower is not properly grounded to dissipate lightning strike energy, the higher frequency energy will travel on the surface of the earth for considerable distances as it spreads out. Towers or attached guy wire supports that are located within a few hundred feet of private property can represent a very serious personnel safety hazard and can cause much damage to equipment.

Equipment damage to private property from tower lightning strikes is widespread throughout the United States and the world for that matter. The scenario is always the same: "A tower is constructed and shortly thereafter damage begins to occur to equipment on nearby private property. The property owner suspects that it might have something to do with that recently erected tower. The property owner contacts the tower owner that unequivocally states that it cannot be their tower that is the problem, and the tower owners have the money and lawyers to wear down any private property owner to eventually try to find alternate solutions for themselves.

There are alternate solutions such as using fiber optic cable instead of copper cable for communications and possible very fast acting shunting devices to protect equipment that use SAD Technology. However, sometimes even shunting devices are not good enough and that the ultimate solution requires isolation devices that are quite expensive. Some damage just cannot be resolved by the private property owner and they live with repetitive damage during lightning season from a tower owner that denies that they are the instigator of the trouble.

So what can a responsible tower owner do to reduce the lightning induced GPR that his tower is causing to the nearby private property owners? The tower owner must demand that the tower grounding system be designed to properly dissipate lightning strike energy. To properly dissipate lightning strike energy requires division and control. This is an absolute must for success, because of the magnitude of the current and the resulting surge impedance of any single dissipation path.

Ten radials connected to a ground ring bonded to a tower, will divide lightning current up into ten smaller segments. This will help ensure that the lightning will more likely follow the designated paths for dissipation into the earth and lower the resulting GPR to the adjacent private property. This is an absolute must for success, because of the magnitude of the current and the resulting surge impedance of any single dissipation path. Ten radials connected to a ground ring bonded to a tower, will divide lightning current up into ten smaller segments. This will help ensure that the lightning will more likely follow the designated paths for dissipation into the earth and lower the resulting GPR to the adjacent private property owner.

The maximum length of these ten radials is approximately 25 m (80 feet). Longer length radials will offer little dissipation improvement, because the lightning strike energy will not remain on the radials for much over 25 m (80 feet). In very limited spaces, the recommended minimum grounding system is at least 60 m (200 feet) of buried bare ground conducting wire composed of five radials, each 12 m (40 feet) in length.

A greatly improved copper wire grounding system can be easily achieved by the use of conducting cement placed around the radials at the time of installation. The cement will harden into concrete, both protecting the grounding system (giving it many years of additional life), and making the system a much better (lower) ground resistance.


LPGI & Affiliates
962 Coronado Drive
Sedalia, CO 80135-8303
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Fax: 303-688-5551

 

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