Abstract: An actuator such as an electrical switch which responds to an event such as a change in temperature by movement of an element between first and second positions, for example in which an electrical circuit is open and closed respectively, includes a dimensionally heat-recoverable component which, on recovery when the actuator is exposed to a high temperature, changes configuration so that a portion of the component is positioned between the element while in its first position and a support for the component, so as to restrain movement of the element towards its second position. This allows the actuator to be locked as a result of exposure to the high temperature, which can be desirable, for example, because of damage to equipment to which the actuator is connected.