Proper precautions for protection against lightning by grounding the aerial outside of the building should be taken.
The wisest plan is to install a heavy single pole double throw switch outside of the building where the rat-tail enters. The knife of the switch should be connected to the aerial, one contact to the house lead and the other to a heavy wire grounded on the outside of the building as in Fig. 24. When the apparatus is not in use the aerial should be grounded by throwing the switch on the grounded contact.
Fig. 24. Switch for Lightning Protection.
The rulings of the National Board of Fire Underwriters governing this class of work are appended below.
"1. Aerial conductors to be permanently and effectively grounded at all times when the station is not in operation by a conductor not smaller than No. 4 B. S. gauge copper wire, run in a direct line as possible to water pipe on street side of said water pipe within the premises or to some other equally satisfactory earth connection.
"2. Aerial conductors when grounded as above specified must be effectually cut off from all apparatus within the building.
"3. Or the aerial to be permanently connected at all times to earth in the manner specified above, through a short gap lightning arrester; said arrester to have a gap of not over .015 of an inch between brass or copper plates not less than 2 1/2 inches in length, parallel to the gap, and 1 1/2 inches the other way, with a thickness of not less than one-eighth of an inch, mounted on non-combustible, non-absorptive insulating material of such dimensions as to give ample strength. Other approved arresters of equally low resistance and equally substantial construction may be used.
"4. In cases where the aerial is grounded as specified in paragraph 1, the switch employed to join the aerial to the ground connection shall not be smaller than a standard 100-ampere jack-knife switch.
"Notice of wiring done for these installations should be sent to the Board, the same as for all other work."