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Lightning has caused man to marvel since the beginning of time. One can almost visualize the fear and awe that lightning elicited in primitive man. Today, even with all of our knowledge about lightning, we are still fascinated with the raw power and the incredible pyrotechnic light shows of a lightning storm. This raw power can cause terrible damage to buildings and structures, especially their electrical systems.
Before protection could be provided, however, an understanding of the lightning was necessary. Benjamin Franklin, often considered the "Father of American electricity," studied the phenomenon of lightning and provided a basis for our understanding of this incredible force.
Franklin wrote to Peter Collinson, a colleague in France, providing documentation of the process involved in his famous experiment with lightning. The letter, titled "Electrical Kite," was written on Oct. 19, 1752 in Philadelphia and used the style of that time:
As frequent mention is made in public papers from Europe of the success of the Philadelphia experiment for drawing the electric fire from clouds by means of pointed rods of iron erected on high buildings, it may be agreeable to the curious to be informed, that the same experiment has succeeded in Philadelphia, though made in a different and more easy manner, which is as follows:
Make a small cross of two light strips of cedar, the arms so long as to reach to the four corners of a large thin silk handkerchief when extended; tie the corners of the handkerchief to the extremities of the cross, so you have the body of a kite; which being properly accommodated with a tail, loop, and string, will rise in the air, like those made of paper; but this being of silk, is fitter to bear the wet and wind of a thunder-gust without tearing.
To the top of the upright stick of the cross is to be fixed a very sharp-pointed wire rising a foot or more above the wood. To the end of the twine, next to the hand, is to be tied a silk ribbon, and where the silk and twine join, a key may be fastened. This kite is to be raised when a thunder-gust appears to be coming on, and the person who holds the string must stand within a door or window, or under some cover, so that the silk ribbon may not be wet; and care must be taken that the twine does not touch the frame of the door or window.
As soon as any of the thunder-clouds come over the kite, the pointed wire will draw the electric fire from them, and the kite, with all the twine, will be electrified, and the loose filaments of the twine will stand out every way, and be attracted by an approaching finger. And when the rain has wet the kite and twine, so that it can conduct the electric fire freely, you will find it stream out plentifully from the key on the approach of your knuckle.
At this key the phial may be charged; and from electric fire thus obtained, spirits may be kindled, and all the other electric experiments be performed, which are usually done by the help of a rubbed glass globe or tube, and thereby the sameness of the electric manner with that of lightning completely demonstrated."
National Electrical Code (NEC) 250.60 covers air terminals and driven ground rods, pipes or plate electrodes connecting the air terminals to ground. These electrodes must not be used as general electrical system grounding electrodes. Even though the electrodes for the air terminals cannot be used for the electrical system, Section 250.106 does require the lightning-protection system to be "bonded to the building or structure power grounding electrode system."
In previous NEC editions, any metal parts of the electrical system within six feet of the lightning-protection system down leads required bonding from the metal part to the lightning down lead. These requirements were removed and replaced with the reference to NFPA 780, The Standard for Installation of Lightning Protection Systems, which contains detailed information on grounding, bonding and spacing from lightning protection systems.
Protection of people, buildings, structures and electrical equipment from the effects of lightning began in the mid-1700s and continues to be a primary goal of designers and installers 250 years later. EC
ODE is staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or via e-mail at [email protected].