The working environment of ZnO protective devices, which may be gaseous (air, SF) or liquid (transformer oils) and solid (sidewall), plays an important role in the life span of the material. ZnO elements in SF
However, adding to SF66 are degraded more than those in air [69]. a small amount (3 per cent) of oxygen, the presence of which seems to play an important role in the chemical process, ensures less degradation [95]. Water vapour and moisture presence also cause an increased leakage current at low temperatures [95, 97]. The relatively high working temperature of ZnOvalve elements in transformer oils (∼95◦C) degrades these arresters at a faster rate. For an applied
voltage V = 0.7 V, the arrester life has been estimated to be as short as 0.36years [98].1mA6