| Lightning as a source of harm is a very high-energy phenomenon. Lightning flashes release many hundreds of mega-joules of energy. When compared with the milli-joules of energy that may be sufficient to cause damage to sensitive electronic equipment in electrical and electronic systems within a structure, it is clear that additional protection measures will be necessary to protect some of this equipment. The need has arisen due to the increasing cost of failures of electrical and electronic systems, caused by electromagnetic effects of lightning. Of particular importance are electronic systems used in data processing and storage as well as process control and safety for plants of considerable capital cost, size and complexity (for which plant outages are very undesirable for cost and safety reasons).
Permanent failure of electrical and electronic systems can be caused by the lightning electromagnetic impulse (LEMP) via: conducted and induced surges transmitted to apparatus via connecting wiring; the effects of radiated electromagnetic fields directly into apparatus itself.
Surges to the structure can be generated externally or internally: surges external to the structure are created by lightning flashes striking incoming lines or the nearby ground, and are transmitted to electrical and electronic systems via these lines; surges internal to the structure are created by lightning flashes striking the structure or the nearby ground.
The coupling can arise from different mechanisms: resistive coupling (e.g. the earth impedance of the earth termination system or the cable shield resistance); magnetic field coupling (e.g. caused by wiring loops in the electrical and electronic systems or by inductance of bonding conductors); electric field coupling (e.g. caused by rod antenna reception).
Radiated electromagnetic fields can be generated via the direct lightning current flowing in the lightning channel or the partial lightning current flowing in conductors (e.g. in the down conductors of an external LPS) |
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