A contact thermal sensor comprising a sprung electrical conductor (5) serving at opposite ends thereof as the moving contacts of first and second sets of switch contacts which, in use, are held in closed condition against the bias of said sprung electrical conductor, the first set of switch contacts being arranged to be held in closed condition by a bimetallic switch actuator (1,2) which is adapted to allow the first contacts to open at a predetermined temperature, and the second set of switch contacts being arranged to be held in closed condition by a member (17) of fusible material which is adapted to allow the second contacts to open at a temperature above the operating temperature of the bimetallic switch actuator, the arrangement being such that, in use of the sensor, in the event of failure of the bimetallic switch actuator to open the first set of switch contacts the second set of switch contacts can be opened in response to melting of said member of fusible material.