Handbook of Electrical Installation Practice - BeKnowledge

7.3
Lightning Protection 223
10.3
10.3 6.2 5.4 6.2 10.3
6.2 5.4 6.2 10.3
3.1 2.3 3.1 7.5
Fig. 9.15 Distribution (%) of lightning current in a 15 stanchion structure.
For lightning protection purposes an all conducting building, with metal cladding
and roof, is an ideal structure. It effectively provides electronic equipment within
it with a ‘screened room’ environment. Many steel framed or reinforced concrete
buildings with metal cladding will approximate to this ideal. If lightning strikes the
building, a ‘sheet’ of current will flow all over the surface and down to earth,
provided that the cladding and roofing is correctly bonded together. Any small
differences in resistance will have little effect on current flow – flow paths are dictated
by inductance and not resistance, owing to the fast impulsive nature of the
lightning return stroke and restrikes.
Current flows in steel framed or reinforced concrete structures show a similar
preference towards external conductors. Figure 9.15 shows that even when lightning
strikes the centre of a building’s roof, the majority of the current will flow down
external conductors rather than the nearer internal conductors. The current flow
through the three internal stanchions is relatively small, creating small magnetic
fields within the building.
Thus, buildings with large numbers of down conductors around the edge of the
building will have greatly reduced magnetic fields inside the building, minimising
the risk of transient interference to electronic equipment from the building’s lightning
protection system, provided, of course, that power and data cables are routed
away from the down conductors.
Problems that transient overvoltages cause
The effects that transient overvoltages have on unprotected equipment depend
on the size of the transient and the sensitivity of equipment. Transient overvoltages