Lightning arresters are installed on many different pieces of equipment such as power poles and towers, power transformers, circuit breakers, bus structures, and steel superstructures in substations. (United States Department of Labor)

The present invention relates to a lightning arrester comprising resistance elements made of zinc oxide as a main component, having a nonlinear voltage-current characteristic and being disposed in an insulating cylinder... (Free Patents Online)

This reduction in resistance continues until the lightning arrester acts just like a direct short to ground. Upon reaching this condition, the lightning energy diverts to ground away from the protected equipment, thus reducing the effect of the overvoltage. (EC&M)