The Retractable Grounding Reel is a self-retracting conductor that provides a direct electrical connection between the tank shell and the floating roof. It is used to conduct the intermediate and long-duration components of lightning-stroke current.

According to API RP 545, the floating roof must be bonded to the tank shell with an appropriate number of bypass conductors. Each conductor, including its connections, shall have a maximum end-to-end electrical resistance of 0.03 Ω. The conductors must be of the minimum length required to allow full movement of the floating roof and should be evenly spaced at intervals not exceeding 30 m (100 ft) around the tank circumference, with a minimum of two conductors per tank.