Lightening Arrester is connected in shunt with the lines so
that is protects from lightening.

when ever the lightening occurs, there is a voltage around
some thousands of voltage appear on line (3500V to 5000V)
for a fraction of seconds(25 to 30micro sec.s) which is more
than enough to damage all the equipment in the grid
connected to it. so when a lightening arrester is connected
then is provides low resistance path such that all that high
voltage is directed towards the earth resulting the
protection of all the equipments connected to the grid.