Lightning is a frequent cause of fires and deaths of people in many countries in the world. For example, each year some 240 US citizens die as a result of lightning. Lightning is a form of electrical discharge between charged clouds and the earth. The FM Data Sheet (1984) on lightning indicates that lightning strikes may range from 2,000 to 200,000 A as a result of a potential difference of 5 to 50 million V between clouds and the earth. The frequency of lightning varies between countries and areas depending on the number of thunderstorm-days per year for the locality. The damage that lightning can cause depends very much on the ground condition, with more damage occurring in areas of high earth resistivity.
Protective measures—buildings
The NFPA 780 Standard for the Installation of Lightning Protection Systems (1995b) sets out the design requirements for protection of buildings. While the exact theory of lightning discharges is still being investigated, the basic principle of protection is to provide a means by which a lightning discharge may enter or leave the earth without damaging the building being protected.
Lightning systems, therefore, have two functions:
· to intercept the lightning discharge before it strikes the building
· to provide a harmless discharge path to earth.
This requires buildings to be fitted with:
· lightning rods or masts
· down conductors
· good ground connections, typically 10 ohms or less.
More details for the design of lightning protection for buildings is provided by Davis (1991) in the NFPA Fire Protection Handbook (Cote 1991) and in the British Standards Institute’s Code of Practice (1992). Overhead transmission lines, transformers, outdoor substations and other electrical installations can be damaged by direct lightning strikes. Electrical transmission equipment can also pick up induced voltage and current surges that can enter buildings. Fires, damage to equipment and serious interruption to operations may result. Surge arresters are required to divert these voltage peaks to ground through effective earthing. The increased use of sensitive computer equipment in commerce and industry has made operations more sensitive to transient over-voltages induced in power and communication cables in many buildings. Appropriate transient protection is required and special guidance is provided in the British Standards Institute BS 6651:1992, The Protection of Structures Against Lightning.
Maintenance
Proper maintenance of lightning systems is essential for effective protection. Special attention has to be paid to ground connections. If they are not effective, lightning protection systems will be ineffective.