This means that the surge arrestor has high resistance at the operating voltage and low resistance as the voltage increases. Thus when lightning strikes the overhead conductors in a substation, the arrestor acts like a conductor and discharges the surge to the ground.
Surge arrestors are usually constructed of MOV(Metal oxide varistor). Zinc oxide is a widely used non-linear resistor. The zinc oxide is the form of blocks which are stacked inside the arrestor.