Oct 2008

Lightning Protection

For the period 1959-1994, the National Oceanic and Atmospheric Administration’s (NOAA) Publication “Storm Data” indicated over 3,000 deaths, 9,000 injuries and nearly 20,000 property damage reports for the United States. That is about 50 property damages per month. It is likely there were many more disruptions to industrial plants that were not reported.

In general, the southeast quadrant of the US is most prone to thunderstorms, and one would think to lightning strikes as well. However, some past studies have shown Pennsylvania, New Jersey and Wisconsin ranked among the leaders – not a rousing endorsement of predictability by geographic area, although the west coast seems less likely to experience lightning storms.

Lightning occurs when the atmospheric electrical charge in a cloud becomes sufficient to overcome the resistance of air – either to another cloud, to ground, or to an object such as a wire or building or equipment. The flash of light we see is pure energy, and measured discharges have been documented to be over 14,000 amps in more cases than not. While the rare cloud to cloud discharges and discharges to ground (earth) do not cause much concern, the others can have significant impact. They can cause direct major damages or surges that destroy equipment or equipment components. Surges can move faster than normal protective devices – such as fuses and circuit breakers – can react. We have come to rely more and more on sensitive electrical devices in computers or chips in equipment that can be damaged by relatively modest surges. The National Fire Protection Association’s “Standard for the Installation of Lightning Protection Systems” (NFPA 780) provides guidance for protective installations, based on present knowledge and realization that the behavior of lightning is not well understood.

A lightning protection system consists of strike terminations, conductors, grounding electrodes and surge protective devices. The strike termination devices can be air terminals, metal masts (commonly lightning rods), overhead ground wires or even building metal structural parts that meet specific requirements. These arrangements provide a non destructive path to earth for lightning.

The strike termination devices have to be connected with conductors to grounding electrodes or to each other. The conductors can be individually run to grounding electrodes or they can be bonded electrically and connected to the grounding electrodes. The grounding electrodes can be connected to form a loop conductor to which bonding or a main conductor can be connected. There are many variations to accommodate a multitude of building shapes and sizes but as long as there is total continuity, the selection is one of economics. The goal is to have everything within a “zone of protection” connected to earth. Mast is a longer version of a more typically used air terminal. The advantage would be in a larger “zone of protection,” so fewer would be required. As a general rule, the air terminal or mast casts a conical zone extending down form the tip at an angle of 45º. They are typically placed at the highest roof levels. When used on buildings with multiple roof levels, there are a number of rules that can be applied so the lower sections can be considered within the zone of protection.

A considerable more complicated way of proscribing a zone of protection is by the Rolling Sphere method. This is a bit difficult to describe in words but the methodology is presented in NFPA 780 complete with illustrations and figures.

Another method of establishing the zone is an overhead ground wire or wires, called a catenary system. This is useful for outdoor equipment and linear exposures such as overhead electrical wires. The overhead ground wire forms a tent-like zone with an angle of 20º from vertical. Sometimes, for lesser exposures, the angle is considered acceptable at 30 to 45º. When used, the overhead ground wire must have a connector to ground at specific intervals. When shielding an overhead electrical supply line in this way, the system should extend to one-half mile away, if possible. Where power lines are run underground to important equipment from above ground lines a surge arrestor should be installed at the tie-in and provided with a low resistance to ground connector.

Just a mention about surge arrestors: these and surge protective devices provide a shunt to redirect surge current before it reaches important equipment. These are usually installed at or near the equipment to be protected. These devices can be used for equipment-specific protection. They must be used at all power service entrances, at entrances of conductors to specifically selected equipment to be protected (such as TV and alarm systems) and where conductors leave one structure to supply another structure over 100 ft. away. They are not required to be installed for specific equipment where the surge threat is determined to be of little consequence.

There are numerous specifics involved in lightning protection systems such as component spacing, materials and sizes, methods of attachment and interconnection, etc. Considerable design considerations must be taken into account for each facility’s protective system. Once installed, each system should receive inspection and testing to assure continued effectiveness. An annual visual inspection should be conducted and an in depth inspection at 3-5 year periods. Appendix D of NFPA 780 outlines the elements of each inspection. Of key importance is continuity testing and testing of the resistance to ground.

If you are thinking of pursuing the development and installation of a lightning protection system, Risk Logic, Inc. stands ready to assist with your engineering needs and concurrent development of a full Preventative Maintenance and Test Program.