The selection of a suitable line surge arrester depends on the following information:

The lightning ground flash density ,which characterises the lightning activity of the area under consideration.It is measured as the number of lightning flashes to ground per square kilometre per annum.

Effective shielding method which provides the shieding angle and the shadow width.Generally a positive shielding angle of 30° is adequate for towers not higher than 30m,and less for higher towers.

Probability of flashover from overhead shield wires to phase conductors.This depends on the coupling factor,Which is a function of the clearance between the conductors and shield wire.

Strike incidence and shielding failure.

Backflashover rates.

Theraftehr,The minimum energy density,discharge class and nominal discharge current of the arrester can be determined in accordance with the manufacturers’ application .