Siemens Power Transmission and Distribution has introduced the new high-current and generator circuit breaker 3AH37. The circuit breaker can handle operating currents up to 6300 A at 17.5 kV and short-circuit breaking currents up to 72 kA. The first vacuum generator circuit-breaker worldwide type-tested according to the criteria of the generator circuit-breaker standard IEE Std C37.013 was the 3AH38. With a rated voltage of 17.5 kV, it switches operating currents up to 4000 A and short-circuit breaking currents up to 72 kA.While the design of the post insulator improves the mechanical stability of the circuit-breakers, the vertical arrangement of the vacuum tubes ensures a compact design. The special construction ensures that no solid insulation is required, which considerably reduces the risk of fire. High current and generator circuit-breakers are used predominantly in power plants and in industry for switching generators and high powers. In these applications, the circuit-breakers are exposed to considerable thermal and mechanical stressing. The new circuit-breaker features a modular design that allows it to meet these requirements. With this design, the best materials can be used in each case for the current path, magnetic flux and cooling. The modular structure also allows the circuit-breaker to be installed flat. For this purpose the cooling elements are rotated by 90°, and even in this position optimum cooling through free convection is still possible. In this way circuit-breakers 3AH37 and 3AH38 can both operate without any additional forced-air cooling.