How lightening occurs (basics like how cloud forms & how opp. charges form)? Why it strikes to earth?

what is the current carrying capacity of GI Strip (50X6) that is used for discharging of lightening? I want to know during lightening will this GI strip will not burn out when such a huge current flow through it.Also in our chimney (275M Height), we have used 70 Sq mm Cu conductor (cirferential ) at the top, why we have select 70 sq mm size, will it not be burn out or why not bigger size?