DC current breaking tests were carried out on several vacuum circuit-breakers during the design of the energy transfer units for the RFX fusion experiment. The tests were performed with a synthetic circuit able to reproduce the expected stresses on the switching element during the breaking phase. The performance of a prototype for a medium-voltage AC switch with a self-produced axial magnetic field is discussed. The interrupter was found capable of breaking currents up to 55 kA with a recovery voltage greater than 35 kV and high values of di/dt in a large range of arcing time

The behaviour of four medium voltage circuit-breakers using different breaking techniques has been compared under actual service conditions with reference to industrial plants.Test conditions not fully covered by the International Standard were chosen.Description of the laboratory circuits and the test methodology have been reported.

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