Safety device for a vehicle and method for controlling the same

A safety device for a vehicle having a battery system with a plurality of battery cells, a method for controlling the safety device, and a system that includes the safety device and the battery system. The safety device includes an electronic unit; a first device configured to detect an insulation resistance between components of the battery system and a reference potential, which components are connected in an electrically conductive manner to at least one battery cell; a second device configured to supply and/or discharge heat to and/or from the battery cells via a heat transfer medium which circulates in at least one circular flow pattern; and at least a first cut-off valve configured to interrupt the circular flow path of the heat transfer medium and which is actuated by the electronic unit from an open position into a closed position when the detected insulation resistance is below a predetermined limit value.