A method and device are disclosed for supervising the operation of plural current transformers and for prevention of a malfunction, such as false tripping of power in an electrical system. Exemplary embodiments can include measuring current parameters (e.g., magnitude and phase angle) for each winding in current transformer (CT) sets. A negative sequence current can be calculated from measured phase currents and a fault condition determined. A phase angle difference parameter derived from phase measurements between two healthy CTs can be used as an additional condition parameter for reliable operation of equipment in the electrical system.