The Impacts of Lightning
Overvoltages of atmospheric origin arise from uncontrollable sources and their severity for the load depends on many parameters that are determined according to where the lightning strikes and the structure of the electrical network.
The impact of lightning on a structure produces spectacular results, but nevertheless is very localised. Protection against the effects of a direct lightning strike is provided by lightning conductors.
The consequences of lightning, i.e. the overvoltages created on the installations and equipment, can be appreciable over a radius of 10km.
Such overvoltages can be classified according to their point of impact:
- Near (indirect) or
- Distant lightning strikes.
For direct lightning strikes, the overvoltages are caused by the flow of lightning current in the structure concerned and its earth connections. For near lightning strikes, overvoltages are created in the loops and are in part linked to rises in earth potential due to the flow of lightning current.
For distant lightning strikes, the overvoltages are limited to those created in the loops. The occurrence of overvoltages due to lightning and their characteristics are statistical in nature and much data remains uncertain.
All regions are not equally exposed and for each country there generally exists a map that indicates the density of lightning strikes.
Ng = Annual number of lightning strikes on earth per km2
NK = Isokeraunic level
Ng = Nk/10
In France, the number of lightning strikes on earth is between 1 and 2 million. Half of these lightning strikes that reach earth have amplitude of under 30 kA, and less than 5% exceed 100 kA.
Protection against the effects of direct lightning strikes
The protective principle is to attempt control of the point of impact by attracting the lightning on to one or several specified points (the lightning conductors) that are placed away from the places to be protected and by letting the pulse current flow to earth.
Several lightning conductor technologies exist and can be of the following types: stem, meshed cage, taut wire or even priming device. The presence of lightning conductors on a facility increases the risk and amplitude of pulse currents in the earthing network.
Protection against indirect effects by SPD
SPD provides protection against transient overvoltages as well as protection against the effects of indirect lightning strikes.
Irrespective of statistical considerations for lightning and the corresponding recommendations set out in ever-changing installation standards, protection against overvoltages by SPD is today systematically demanded for any type of industrial or service activity.
For the latter, the electrical and electronic equipment is strategic and expensive, and not ponderable as certain domestic appliances might be.
Resource: Industrial switching & protection systems 2011 – SOCOMEC