Medium voltage SF6 circuit breakers with a mechanical drive (spring mechanism) for primary distribution up to 40.5 kV, 3600 A, 50 kA. HD4 circuit breakers are used in power distribution systems to control and protect cables, overhead lines, transformer and distribution substations, motors, transformers, generators and capacitor banks. The use of self-blowing technology for shutdown eliminates the possibility of switching overvoltages in the HD4 circuit breaker and, therefore, such circuit breakers can be used to a large extent in the modernization and expansion of existing installations in which the insulating materials of motors, cables, etc. are especially sensitive to electrostatic overvoltage.