The incorporation of a circuit breaker within an RMU without additional cost and with a reduced volume has been made possible by use of a new interruption technique: autoexpansion. This approach uses the combination of a gas flow created by expansion due to the energy dissipated by the arc itself, and the arc rotation created by a magnetic field. The interruption energy is not therefore taken from the operating mechanism as in the classic puffer principle. The overall device is therefore lightweight, reliable and of reduced dimensions. Interruption is carried out under atmospheric pressure of SF6. Therefore, owing to the use of this interruption principle which allows considerable flexibility of design, the circuitbreaker fulfills three functions: circuit-breaker, isolator and earthing-switch. In addition this approach enables the incorporation of automatic electronic protection systems which give further benefits in terms of flexibility of operation and selectivity.