The circuit breakers immediately interrupt the electrical current flowing in a high voltage transmission line when a fault occurs. To achieve this, they must have the ability of extinguishing arcs that occur during closing and opening the contacts within a very short time. RMU is also defined as the device that performs voltage and current measurements of the power grid in order to detect failures, and also provides information to isolate them and control by disconnecting the other part of the grid. In this paper, the design and manufacturing steps of external type 17.5 kV 630A 50/60 Hz SF6 gas isolated, arc splitted, self-powered RMU is studied. For this purpose, first of all the arc quenching and arc quenching methods are explained. Then, the design methodology is given in detail. Finally, the verification tests and modifications applied after these tests are specified. After the process, the new RMU combined with breaker/load break switch (LBS) is designed successfully for switching actions of smart grid systems.