One of the most accepted expressions to determine the number
of direct strikes to an overhead line in an open ground with no nearby trees or
buildings, is that described by Eriksson (1987):

N = Ng
(58h^06 + n)/10 eq 10.3

where

h is the pole or tower height (m) — negligible for
distribution lines

b is the structure width (m)

Ng is the Ground Flash Density (flashes/km2/year)

N is the number of flashes striking the line/100 km/year.
For unshielded distribution lines, this is comparable to the fault index due to
direct lightning hits. For transmission lines, this is an indicator of the
exposure of the line to direct strikes. (The response of the line being a
function of overhead ground wire shielding angle on one hand and on
conductor-tower surge impedance and footing resistance on the other hand).

Note the dependence of the incidence of strikes to the line
with height of the structure. This is important since transmission lines are
several times taller than distribution lines, depending on their operating
voltage level.

Also important is that in the real world, power lines are to
different extents shielded by nearby trees or other objects along their
corridors. This will decrease the number of direct strikes estimated by Eq.
(10.3) to a degree determined by the distance and height of the objects.

In IEEE Std. 1410-1997, a shielding factor is proposed to
estimate the shielding effect of nearby objects to the line. An important
aspect of this reference work is that objects within 40 m from the line,
particularly if equal or higher that 20 m, can attract most of the lightning
strikes that would otherwise hit the line.

Likewise, the same objects would produce insignificant
shielding effects if located beyond 100 m from the line. On the other hand,
sectors of lines extending over hills or mountain ridges may increase the
number of strikes to the line.

The above-mentioned effects may, in some cases, cancel each
other so that the estimation obtained form Eq. (10.3) can still be valid.
However, it is recommended that any assessment of the incidence of lightning
strikes to a power line be performed by taking into account natural shielding
and orographic

conditions along the line route.

This also applies when identifying troubled sectors of the
line for installation of metal oxide surge arresters to improve its lightning
performance.

Finally, although meaningful only for distribution lines,
the inducing effects of lightning, also described in De la Rosa et al. (1998)
and Anderson et al. (1984), have to be considered to properly understand their
lightning performance or when dimensioning the outage rate improvement after
application of any

mitigation action.

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