Handbook of Electrical Installation Practice - BeKnowledge

BS 6651 provides a simple mathematical overall risk factor analysis for assessing
whether a structure needs protection. It suggests that an acceptable lightning strike
risk factor is 10 -5 per year, i.e. 1 in 100000 per year. Therefore, having applied the
mathematical analysis to a particular set of parameters, the scheme designer will
achieve a numerical solution. If the risk factor is less than 10 -5 (1 in 100000), for
example 10 -6 (1 in 1000000), then in the absence of other overriding considerations,
protection is deemed unnecessary. If, however, the risk factor is greater than 10 -5 ,
for example 10 -4 , then protection would seem necessary.
A suitable analogy could be made with the odds in horse racing. The shorter the
odds (e.g. 5:1), the more likely it is that the horse will win the race. The longer the
odds (e.g. 100:1), the less likely it is that the horse will win.
The shorter the risk factor (e.g. 1 in 10000) the greater the risk that a structure
will be hit by lightning. The longer the risk factor (e.g. 1 in 1000000) the less likely
it is that the structure will be hit by lightning.
It is acknowledged that certain factors cannot be assessed in this way and these
may override all other considerations. For example, if there is a requirement that
there should be no avoidable risk to life or a requirement for occupants of a building
always to feel safe, then this will favour the installation of protection, even
though it would normally be accepted that protection is unnecessary.
The factors which should be considered for determining an overall risk factor can
be summarised as follows.
The geographical location of the structure
This pinpoints the average lightning flash density or the number of flashes to ground
per km 2 per year. For structures sited within the UK this figure can be taken from
the map in Fig. 9.4.
The effective collection area of the structure
This is the plan area, projected in all directions taking account of the structure’s
height. The significance is that the larger the structure, the more likely it is to be
struck (Fig. 9.5).
For a simple rectangular box shaped structure as illustrated in Fig. 9.5, the collection
area (symbol A c) is simply the product of:
2
Ac = LW+ 2LH+ 2WH+ pH
where: L = length
W = width
H = height.
The probable number of strikes to the structure per year (P) is the product of the
flash density and the collection area, thus
-
P = Ac ¥ Ng
¥ 10 6
Lightning Protection 201
where N g = the number of flashes to ground per km 2 per year. (The number 10 -6 is
included because Ac is in m 2 whereas Ng is per km 2 .)