The basic duties of circuit-breakers are discussed together with the various methods available to fulfil them. In the high power range it is shown that the arc plasma is superior to the semiconductor, but there are some compet- ing arc media such as air, oil, SF, and vacuum which meet the require- 6 ments in often contradictory ways. The art of circuit-breaker construction is to make the best use of the advantages, and to invent optimum compen- sation for the disadvantages of the chosen medium. Although all the re- quirements can be met with each medium, some are especially suited for specific applications. There are, however, broad areas where both techni- cally and economically there is little difference between them. Therefore, for the time being, none can be ruled out or generally preferred. The complexity of the plasma equations and their boundary conditions, especially when convection cooling becomes dominant, exclude exact calculations and even the application of similarity laws in circuit-breaker development. Thus it is necessary to make a great number of rather costly and time consuming full scale power tests on prototypes. On the other hand the increasing requirements due to both larger generators and stronger networks are accelerating the technical progress and requiring the develop- ment of a wide range of models. Both these reduce the number of breakers which can share the development costs which are no longer small compared to those of manufacturing.
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