Lightning Protection Risk Assessment and System Design

Lightning protection is one of the few hazards where the required level of protection is a calculated output rather than an engineering preference. IEC 62305 2 works from the local flash density, the collection area of the structure, the construction and contents, the occupancy and the consequence of loss, and returns a risk that is then compared against a tolerable value. That calculation decides the protection level, and the protection level decides the rolling sphere radius, the mesh size, the down conductor spacing and the surge protective device coordination. Our work runs that assessment properly rather than defaulting to a level and fitting a finial to it, and for a process plant it also covers the parts that are frequently missed: the risk to services entering the structure, the bonding of external conductive parts, and the surge protection of the instrumentation and control systems that carry the real loss potential on a modern site. On a tank farm we work to the additional requirements that apply to flammable atmospheres, where an unbonded floating roof or a poorly earthed vent is the ignition path that matters.