In previous chapters, the insulation requirements for lightning and switching surges have been discussed. The remaining consideration of design is contamination, which is considered in this chapter. For transmission or distribution lines, contamination dictates the type of insulator and the length of the insulator string. In some contamination areas, the insulator length may exceed that required by lightning or switching surges, and therefore the final design of the line is based on contamination. Similarly, in stations, the required length of the insulation, i.e., bus supports, bushings, etc., may be dictated by contamination, so that the BIL of the insulation must be increased when considering contamination.