It is used for earth or air feeders in distribution system power plants in different voltage levels. It is used for protection of low or medium voltage engine in industry like power station and oil refinery in order to warranty engine function and its safety such as assurance about overload status. Main protection of transformer means counting (unit) differential protection which is unit protection. With regard to transformer function importance toward continuous electrification , power transformer protection has a great importance. impedance protection is used for protection of transmission, so that it is possible to discover the kilometer area that the error was happened in it. This kind of protection is mentioned as main protection of airlines.