In this paper, we propose collision detection and safe reaction for a non-backdrivable manipulator with a base force/torque sensor. Measurement of the base force/torque sensor is compensated for using quasi-static dynamic information of the manipulator. Collision point, magnitude of force, and direction of force are detected by assuming an external collision to be pure force. While the manipulator is not convex, a collision point may not be unique when only using this assumption, so we assumed that a collision is applied outside the convex hull of the manipulator. For a simple calculation of collision point candidates, a cylinder model is applied to each manipulator link. Three different safe collision reaction strategies are proposed for reactions to a variety of force positions and directions One other strategy is proposed to remove the remaining contact force and to create a margin to avoid additional collisions. We ran several experiments to verify the effectiveness and performance of the strategies.