A fall arresting device including a device housing, a shaft within the housing, a rotor assembly rotatably connected to the shaft that includes a drum and a disc having at least one region of a ferromagnetic material, an extendable lifeline connected to the drum, a magnetic sensor positioned stationary relative to the device housing and adjacent to the disc, and a that includes a hard-magnetic material. The magnet positioned stationary relative the device housing and the magnetic sensor, where the magnetic sensor is configured to detect a change in a magnetic field produced by the magnet when the disc rotates about the shaft, the change in the magnetic field induced by the at least one region of the ferromagnetic material being brought within close proximity to the magnet as the disc rotates.