Charles C. Roberts, Jr.

A lightning stroke is a discharge of electrical energy that usually occurs between clouds and the earth. As the electrical potential (which may be as high as tens of millions of volts) increases between the clouds and earth or between two clouds, a lightning stroke often forms. The currents in a lightning stroke may be on the order of tens of thousands of amperes with a duration on the order of 100 microseconds. The lightning follows a path of least resistance which may include people or property. Because of the high electrical currents involved, property damage and personal injury (or death) are known to occur. The basic behavior of lightning is not well understood by the scientific community because of the randomness of the lightning strike and difficulties in making measurements. This adds to the difficulty in analyzing a property loss allegedly caused by a lightning stroke.

tree roof

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Figure 1 is a view of a tree that was struck by lightning. The arrow points to damaged bark that shows evidence of a path taken by the electrical current from the upper part of the tree to the ground. The home owner witnessed the strike and indicated that several electronic appliances failed to function after the storm. Appliance failure can be a result of a direct strike from lightning or be induced by the strike in the form of a power surge or induced currents in electronic equipment. When several appliances fail simultaneously, a surge related failure is a likely cause. However, if only one appliance failed, then other failure modes such as end of life, vibration, impact, etc., may be a cause.

Figure 2 shows a home where a fire started in the attic just prior to a heavy rain storm. Figure 3 shows severe charring of wood sheathing in the vicinity of a metal plumbing stack at the roof . Burn pattern analysis (see Claims, Oct. 1987) suggests a fire origin at the interface between the plumbing stack feed through the roof and the wood sheathing. Analysis of ignition sources ruled out any other source with the exception of lightning. Apparently, the nature of the strike on the home was such that sufficient resistive heating occurred at the roof/plumbing vent interface to ignite wood sheathing.

heater pipe

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In some situations, a lightning strike can cause current to flow through a building piping system, causing damage to the interior of the building as opposed to the exterior of the building. Figure 4 is a view of a domestic water heater showing severe arc damage at insulator couplings that occurred just before a severe thunderstorm. Figure 5 is a close-up of the pipe coupling located above the insulated union. An insulated union is often placed between pipes of dissimilar metals to prevent galvanic corrosion (see Claims, Sept. 1993). Apparently, current traveling down the copper water pipe encountered the high resistance of the insulating coupling, which resulted in side flash to ground through the metallic structure of the water heater. Several other appliances were also rendered inoperative as a result of the lightning strike.
siding clock tree

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Figure 6 is a view of another home showing evidence of high voltage related power dissipation. Figure 7 shows another view of arc damage to the aluminum siding. Several appliances were rendered inoperative. Figure 8 is a view of a clock in the home, that was damaged from the high voltage surge, showing heating of the plastic housing. A definite lightning related loss? Not so quickly. Figure 9 shows the real culprit, a pine tree growing in the vicinity of the electrical transformer serving the home. The tree (which is shown substantially trimmed) was growing close to the utility wiring, such that it contacted both the high voltage wire (about 4000 Volts) and the 120V wiring entering the home. This caused a conductive path between the high voltage wire and the home wiring which delivered high voltages to appliances, the home wiring system and home grounding system. The over-voltage caused the flash burns to the siding as well as damage to electrical appliances.

Lightning related losses usually manifest themselves in high voltage related damage and heating. So do other causes, such as equipment malfunction and utility related malfunctions. Differentiating between the two often requires obtaining storm related data, analysis of damage to the home and failure analysis of appliances.