Lightning, grounding and protection for control systems

Protection for installations that have been modernized or retrofitted presents a unique mix of problems that require a different perspective from a new installation. The paper addresses lightning, transients, and radiation that cause problems on programmable logic controllers, distributed control systems, and remote electronic transducers. While complex grounding grids and networks were not required for analog systems, newer digital equipment requires a very effective means of maintaining equal potential througbout the facility. Methods of protecting circuits include a variety of sophisticated and not-so-elegant techniques. Special consideration must be given to the impact that towers have on a protection system. The investigation ranges from no air terminals to lightning arrays. The protection problem is compounded when different soils such as clay and rock are encountered. Methods of calculating the grounding circuit resistance are identified. Altbough the paper is based on a retrofit, the techniques are appropriate for any industrial installation using distributed control