A method for controlling an electrical system in a vehicle. The electrical system (11) comprises: at least a first switch (ES2) arranged such that operating the switches will isolate a part of the electrical system; and a control unit, the control unit being arranged to operate the switches. The method comprises: putting the control unit into a first operational mode, the first operational mode comprising at least a first arrangement of switches; receiving a request to enter a second operational mode, the second operational mode comprising at least a second arrangement of switches; determining the priority of the first and second operational modes at least in part by consulting a database of operational modes and their priorities; switching the control unit into the second operational mode if the second operational mode has a higher priority than the first operational mode; and continuing in the first operational mode otherwise.