A Guide to BS EN 62305:2006 Protection Against Lightning Part 4 pps

Rolling sphere method

Given the lightning process already described in

Theory of lightning starting on page 4, it is logical

to assume that a lightning strike terminates on the

ground (or on structures) at the point where the

upward streamer was originally launched.

These streamers are launched at points of greatest

electric field intensity (see Figure 4.2a) and can move

in any direction towards the approaching downward

leader. It is for this reason that lightning can strike the

side of tall structures rather than at their highest

point.

BS EN 62305-3 | Rolling sphere method

38

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Figure 4.3: Striking distance (last step)

Figure 4.2b: Development of downward

leader/striking distance

Downward

leader

Vulnerable

points

Vulnerable

points Less vulnerable

(protected areas)

Striking

distance

(or last step)

Greatest field

intensity

++++

++++

For detail,

see Figure 4.2b

Striking

distance

(or last step)

This hypothesis can be expanded to explain why

corners of structures are vulnerable to lightning

strikes. Figure 4.3 illustrates a sphere rolling over the

surface of the building. The dotted line represents the

path of the centre of the sphere as it is rolled over the

building. The radius of the sphere is the striking

distance, or last step of the lightning discharge.

Thus it can be clearly seen that the corners are

exposed to a quarter of the circular path of the

sphere. This means that if the last step falls within this

part of the circular path it will terminate on the

corner of the building.

The position of the greatest field intensity on the

ground and on structures will be at those points

nearest to the end of the downward leader prior to

the last step. The distance of the last step is termed

the striking distance and is determined by the

amplitude of the lightning current. For example,

points on a structure equidistant from the last step of

the downward leader are equally likely to receive a

lightning strike, whereas points further away are less

likely to be struck (see Figure 4.2b). This striking

distance can be represented by a sphere with a radius

equal to the striking distance.

Figure 4.2a: Development of downward

leader/striking distance

BS EN 62305-3 Physical damage to

structures and life hazard