OFNP fiber patch cords offer the highest fire resistance and are suitable for use in plenum spaces such as air ducts and ventilation shafts. The ratings OF, OFNR, and OFNP represent different levels of flame retardancy for optical fiber cables, particularly important for installation in various building environments. OF (Optical Fiber): This is a general designation indicating an optical fiber cable. Dual-Layer Protection: An additional layer of protection shields the internal fibers, enhancing the overall strength and. Optical Fiber Nonconductive Plenum (OFNP) and Optical Fiber Nonconductive Riser (OFNR) are two fire protection classes used for fiber optic patch cord.