Incidence of lightning
Lightning depends on both atmospheric and geographical factors. It is usually associated with areas having convection rainfall. It requires presence of high moisture levels in air and high surface temperatures on ground. Since the protection to be given to buildings is a function of the probability of lightning strikes, the frequency of lightning occurrence has been extensively studied and the results are published in the form of annual isokeraunic maps for different world regions. These are contour maps, which show the mean annual thunderstorm days of the region involved.
A thunderstorm day for this purpose is defined as one when thunder is heard at the point where it is measured. This obviously cannot indicate whether it is a result of inter-cloud or cloud to ground discharge. It does also show the frequency/number of instances or severity of cloud to ground strikes. Further studies are under way to gather such data and may result in modifications to the present methods of lightning risk assessment.
Probability of lightning strike
The probability of lightning strike depends on two factors:
- the incidence of lightning strikes in the geographical area where the structure is situated and;
- the attractive area offered by the structure for lightning. The attractive area can be defined as the horizontal area within which a downward leader will be intercepted by an upward leader originating from the structure.
Method of lightning protection
Lightning protection to any structure consists of providing a low impedance conducting path for flow of lightning discharge currents to ground without allowing them to flow through the structures. Lighting protection consists of an air termination, down conductors and ground electrodes.
A lightning mast independent of the structure but near enough to divert any lightning occurring in the vicinity is one example of protection. The protection offered may not be complete if the attractive radius of the building to lightning extends well beyond that of the mast. In such a case, it is possible that some of the strikes may hit the building rather than the mast.
A sphere is rolled over the protecting structure and the shaded areas which the sphere cannot touch are within the protection zone. The radius of the sphere can vary between 20 and 60 m depending on the degree of protection required. The standard protection will consider a radius of 45 m and increased degree of protection can be obtained by reduction of the radius.
Not all buildings can be protected by a free standing mast, the approach in this case is to have the conductors placed on the building itself called as air terminals then connect them to the ground through down conductors. The air terminals can be vertical spikes placed on the top periphery of the building. These air terminations can be connected by a flat conductor run on the roof thereby offering multiple paths for the lightning current to flow to ground. The down conductors are connected to dedicated ground electrodes to offer a short conducting path to ground.
Lightning behaves like a current source. In other words, the flow of current is independent of the circuit impedance. If the current path to ground has high-impedance elements, the lightning current, which is of the order of several kiloamperes as we saw earlier, produces a very high voltage drop. This voltage appearing on the conducting elements can cause secondary flash to nearby earthed objects. It can also cause damage to building structures by forcing a path through non-conducting building elements. This explains why the lightning conductors should be of as low impedance or in other words as low a length as possible.