Abstract: A warning device is responsive to the failure of an electric power supply for activation to a warning mode. The warning device includes an arm pivotally mounted to a frame member and latched by an electromagnet in a non-warning mode. In the non-warning mode, a collapsible warning sign carrier is enclosed within an enclosure formed by the frame and the arm. When the supply of power is interrupted, the electromagnet releases the arm so that the arm pivots to a warning position.