A fire activated device which closes off a wall opening through which a plastic pipe or other fusible member extends. The device includes a back plate which is mounted to the wall with an opening in the plate in alignment with the wall opening which receives the pipe. A spring loaded cutting blade is normally held away from the pipe by a fuse plug which melts in the event of a fire. The blade is then released and cuts through the pipe under the influence of the springs. A stop plate holds the blade in position to cover the opening to prevent fire from passing through it to the adjoining room. The springs are shielded from the heat by telescoping tubes which contain them.