A device for controlling at a fault condition. An apparatus is connected by a connection line to a grid point of a transmission net in an electric power network. A voltage raising apparatus includes a first branch connected to the grid point including a switching element and a second branch including a current resisting means. The voltage raising apparatus includes a computer for signal processing of a sensed fault condition on the network and for affecting the operation of the switching element such that on a fault condition at least part of the current is diverted through the voltage raising apparatus and for evaluation of further actions.