In the case of large off shore wind farms, there is a need to connect the whole park to the grid on shore. The wind farms will be placed where the wind conditions are the best and since the distance to shore might be long, one would like to use only one cable. Since the wind power plants in most cases are equipped with rectifiers, a natural solution is to connect all wind power plants with a medium voltage DC grid, and connect this grid to shore, either by AC or DC. However, this creates a need for appropriate DC-breakers that today are not available on the market. The requirements of such a breaker will vary depending on the system, but the common needs will be fast operations due to the high short circuit currents, and long maintenance intervals due to the high costs related to the remote location. The connection of the wind park to shore is preferably done by high voltage DC, HVDC. A breaker for such a system will have totally different requirements where fast reaction and short opening times will be critical factors.