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Trees do not afford protection from lightning to nearby buildings, and in many instances they should be protected in view of their particular value to the owner. It is recommended that trees which have trunks located within 10 feet of a protected building, and have branches of which extend to a height above a building, be equipped with lightning protection not only to protect the tree, but also to avoid the possibility of lightning striking the tree and side flashing or grounding to the nearby structure. The following rules should be considered as applying to lightning protection systems installed on such trees.
One main cable should be coursed from the air terminal at the top of the main trunk or branch to the ground terminal. Acceptable secondary conductors should be coursed from miniature branch points, as far out on the main branches as possible, to the main conductor on the tree trunk.
To avoid possible injury to roots by locating depth groundings near trunks of trees, the conductor should be extended out and away from the base of a tree in a shallow trench to a distance of not less than 12 feet or to the extremity of the over hanging branches. This conductor should terminate in a ground terminal. (It should be kept in mind that usually the underground spread of the roots of the tree is equal in area to the spread of the tree branches above ground.) Depth groundings should be made outside the area of the root spread.
If the groundings of a protected building is withing 25 feet of a tree, the two systems should be interconnected. If the tree or grounding of the tree is within 25 feet of a water pipe or deep-well casing a connection should be made between them.
Trees with trunks which exceed 3 feet in diamter, and which have extra-long branches, should have two down conductors. They should be led down opposite sides of the tree and connected to the two ground terminals. These two ground terminals may well be joined by a circular or semi circular conductor or a counterpoise burried in a shallow trench.
If there are several trees in a row (all major trees), the ground terminals of two trees not more than 80 feet apart may be inter-connected by a trench conductor coursed to the base of each intermediate tree. THe down conductor of each intermediate tree may connect with the "trenched" interconnecting conductor. This practice avoids making independent groundings for each tree.
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