The internal humidity in arresters is usually the result of some failure in the sealing material. The main problem caused for the humidity is the occurrence of partial discharges, which can occur in different ways. The first one of them is the formation of a humid path along the surface of the varistors column, leading to insulations problems and electrical sparks. One another problem related to the humidity entrance is related to possible strange gases or particles of pollution which can follow the humidity in the interior of the equipment. Depending on the type of gas or particles, there can be a reduction in the internal isolation. Finally, the internal humidity can also be responsible for the premature degradation of the varistors.