9.3.6 Fog characteristics The characteristics of the fog introduced to provide insulator wetting have an effect on the experimental results. The fog is characterized by l. Droplet size distribution. Fractional liquid volume (1.8-6.2 g/m 3. Fog temperature. (fog temperature rise) and 2 are interrelated and contribute to wetting and washing of the exposed insulator surface by droplet impingement. For contamination testing it is essen- tial that the natural washing conditions be simulated. Depending upon the source, both droplet size and the fractional liquid volume of natural fog vary over a wide range. At a fractional volume below 1.8 g/m it is difficult to control temperature and density due to heat loss. The fog temperature must be significantly higher than ambient in order to ensure effective wetting through condensation on the insulator surface. 9.4 Mitigation of contamination flashover There are a number of methods used to reduce or eliminate the possibility of contamination flashover of ceramic insulators. These include: use of insulators with optimized shapes periodic cleaning. grease coating. RTV coating, resistive glaze, replacement of ceramic insulators with composite. 94,1 Use of insulators with optimized shapes The shape and leakage distance of insulators can be varied to address environmental conditions. Generally the shapes are aerodynamically optimized to gather as little pollution as possible and to enhance self-cleaning through wind and rain. Standardized variation in shape parameters is available for service areas characterized by different environmental contamination processes. Special designs of varying shed profiles, diameters, spacings, leakage distance, et are available. 9.4.2 Periodic cleaning In many installations high-pressure water systems, or corn and CO2 pellet blasting, are utilized to periodically clean surface contaminants off insulators. of these, high-pressure water cleaning is predominant and by far the cheapest.