In August 1979, late in the morning, lightning struck the antenna and communications center of a police station in central Florida––a high lightning-incidence state. Pieces of communications equipment and telephones were damaged, and several workers in the communications room were injured.
As occasionally happens immediately after this type of event, repairs and modifications are performed so rapidly that certain aspects of the pre-existing installation can only be inferred from the nature of the occurrence. The damage and injuries occurred primarily because lightning protection provisions for the combination of antenna and communications center structure were inadequate or completely lacking. If appropriate lightning protection had been provided, it is likely that damage and injuries would have been greatly lessened or nonexistent. Specific departures from accepted lightning protection practices were noted, with reference to applicable codes.
An electrical connection was required between the antenna tower and the main lightning grounding conductor within the building. Its purpose was to prevent a side-flash, which apparently occurred after the tower had been struck. There was no metallic bond between these two structures. It did not appear that the communications center even had an independent lightning protection system.
All services entering an occupancy must have common bonding of all grounding conductors, per the 1971 Edition of the National Electrical Code (NEC), Rule 800-31 (7). These services would include communications (antenna lead-in surge protector ground), telephone, CATV, electrical power, water and gas. Lack of a common grounding system results in substantial differences of potential between different metallic structures while high levels of lightning current are flowing, thus encouraging flashovers. In this case, the grounds were not bonded, which encouraged the occurrence of damage traceable to flashovers.
Telephone company standard practices call for bonding of their surge protector grounding conductors to electrical power system grounds. There was confusion in this case as to whether the city, which owned the communications center, was supposed to complete the telephone system ground if it did not already exist.
The radio antenna for the public safety communication system was installed on a tall tower adjacent to the communications building. The tower was supposed to be grounded in accordance with certain minimum requirements, which are at least partly determined by the character of the soil. Grounding electrodes were required to be a minimum of 8 feet long, per NEC Rule 250-83 (e), or 10 feet long, as required by Rule 63 of UL 96A, June 1963, Installation Requirements for Master Labeled Lightning Protection Systems. It could not be determined from available construction data whether this requirement had been met.
Furthermore, the tower was required to have a minimum of two earthing terminals or electrodes, one being connected to the grounding conductor of the main building’s system ground. In soil that is characterized as dry sand/gravel, three electrodes are required, with a minimum spacing of 6 feet between them, per Rule 65 of UL 96A. Based upon extensive radial trenching that was performed after the lightning strike, it was concluded that only a single electrode had been present at that time.
Overall system grounding resistance varies inversely with the number of grounding electrodes. Residual lightning energy that is not dissipated into the ground, but instead finds its way by induction and metallic or ionized conduction into neighboring objects, is roughly proportional to the grounding resistance. Thus two to three times an amount of residual lightning energy was transferred to the communications center from the tower than if the required practice in terms of numbers of grounding electrodes had been observed.
A surge arrester was required on the antenna lead-in conductor. This conductor provided a direct metallic path from the overhead antenna to the radio equipment in the communications room. The presence of a properly installed surge arrester would have minimized equipment damage or left none at all. Such a device, required by NEC Rule 810-20, and by Rule 2181 of the 1968 Edition of the Lightning Protection Code was supposed to be installed “as near as practicable to the entrance of the conductors to the building.” No post-event trace or mention of such a device was obtained.
Secondary power service surge arresters were required at their entrance to the communications center, per NEC Rules 230-107 and 280-11. These arresters would have minimized dissipation into the communications center of lightning energy coupled through the ground, the occupancy’s power wiring and the power service lines. Again, it could not be established that service entrance surge arresters had been present at the time the lightning damage occurred.
Thus, the communications center installation did not meet basic requirements for lightning protection appearing in authoritative industry standards, despite repeated lightning strike damage and injury incidents dating back several years. EC
MAZER is a consulting electrical engineer who specializes in electrical safety issues. He may be reached at 202.338.0669 or firstname.lastname@example.org.