Surge protectors are similar to zener diodes in that they are made of material that is essentially insulating until a predetermined voltage appears across the material. At this point, avalanche breakdown occurs and the surge protector acts as a current shunt. Metal oxide varistor (MOV) types of surge protector respond in nanoseconds and can bypass many joules of surge energy. However, a disadvantage of MOV surge protectors is that they draw a small current at all times and generally the failure mode of a MOV device is a short circuit. SiO (SOV) surge protectors are an improvement in terms of the MOV disadvantages.