A PV system typically is mounted such that it is vulnerable to lightning. A flash of lightning is an electrical arc from the earth to a cloud that is the result of an electrical potential between the earth and the cloud. The resistance between the earth and the cloud is the ohmic resistance of air, hence the ohmic resistance will be less for a device that is physically above the earths surface (less distance to the cloud). By connecting a PV array to earth via a grounding conductor a path is provided for the discharge current.
For more information on Lightning Arrestors, visit our toolbox section here.