Introduction The LWâ?¡-252/4000-50 self-extinction Ac circuit breaker which is outdoor type finds its universal range of applications for the electrical system with elevation not greater than 2000m, ambient temperature not lower than-30C, dirtiness grade not higher than IV, alternating current 50Hz and the maximum voltage 126kV. It is used for cutting the rated current, malfunction current or switching the circuitry, to control and protect the electrical system, and it can also be used for connecting with the circuit breaker The gas of the SF6 is employed as the arc-extinguishing medium and the dielectric material. The self-extinction technology and the new-type spring operating mechanism are introduced in the SF6 circuit breaker. Thus the circuit breaker has good qualities with long electrical life, low operation power, lower noise, high reliability, simple structure, small and long overhaul periods. It can fully replace the import same type one. Caracteristic of structure The breaker introduces the three-phase porcelain -pole with single interrupter structure. A spring operating mechanism is supplied and the mechanical action of the three-phase is inner connected. The gas of the SF6 is connected in the three-phase pole parts. A density controller with temperature compensation is used to monitor the gas of the SF6. Because of the self-extinction arc control principle and optimized design, it improves the efficiency of mechanism and reduces the operating power Excellent breaking performance and long electrical life High mechanism reliability and can be operated frequently Because of the high insulation intensity and excellent contact movement, the breaking capacity current will not re-burn The circuit breaker has high outer insulation level and ratio creepage distance is greater than. So it has excellent performance of resisting the filth and the earthquake, it is also can be operated safely under the environment with atrocious weather High carrying ability: it has high capability margin as the temperature of each contact rises when it is under temperature rise test in the circuit (1.2 times 0f 4000A) The appearance of the breaker is compact, and the installation and debugging is very easy. The maintenance is also very convenient