Roller shutter installation comprising: - an apron formed of slats (2, 2a) which are articulated with one another, the rolling/unrolling of which about a shaft is effected by a drive means, the slats (2, 2a) of the apron being guided on each side by a slide (5a, 5b), the apron being capable of being unrolled as far as the bottom part of a window or door in a closed position, - at least one locking device for locking the apron in a partially open position in which the bottom slat is at a small distance from the bottom edge of the window or door, in particular a distance that is insufficient to allow a person to pass through, said locking device comprises - a frame extending in a direction substantially parallel to the axis of the slide, and - a slider extending along an axis substantially orthogonal to the plane of the apron, said slider being mounted in translation along an axis substantially perpendicular to the axis of the slide, in one of the slats (2, 2a), and connected to said slat by an elastic return device, the face of said frame facing the slats (2, 2a) of the apron comprises three portions over which said slider passes, namely - a top portion capable of compressing the elastic return device as the slider descends, - an intermediate portion provided with at least one recess which releases the compression of the elastic return device and prevents the slider from ascending, - a bottom guide portion for guiding the slider towards a rear track of the locking device in order to allow it to ascend, characterised in that said top portion, said intermediate portion and said bottom portion of the frame are each elastically deformable, and in that said slider, when it is applied to one of said portions of the frame, causes a variation in the position of the portion in question, the portion in question returning to its initial rest position when the application ceases. - an apron formed of slats (2, 2a) which are articulated with one another, the rolling/unrolling of which about a shaft is effected by a drive means, the slats (2, 2a) of the apron being guided on each side by a slide (5a, 5b), the apron being capable of being unrolled as far as the bottom part of a window or door in a closed position, - at least one locking device for locking the apron in a partially open position in which the bottom slat is at a small distance from the bottom edge of the window or door, in particular a distance that is insufficient to allow a person to pass through, - a frame extending in a direction substantially parallel to the axis of the slide, and - a slider extending along an axis substantially orthogonal to the plane of the apron, said slider being mounted in translation along an axis substantially perpendicular to the axis of the slide, in one of the slats (2, 2a), and connected to said slat by an elastic return device, - a top portion capable of compressing the elastic return device as the slider descends, - an intermediate portion provided with at least one recess which releases the compression of the elastic return device and prevents the slider from ascending, - a bottom guide portion for guiding the slider towards a rear track of the locking device in order to allow it to ascend,