The fiber boxes (60) each contain fiber (62) and are removably mounted to a fiber injector (9) for injecting the fiber (62) into a material flow stream. The fiber boxes (60) each have an enclosure (61) for containing the fiber (62), preferably a single fiber (62). An aperture 70 provides a feed hole (70) for passing the fiber (62) from an interior of the enclosure (61) to an exterior of the enclosure (61). Contacts (64) having ends disposed on an exterior of the enclosure (61), spaced apart for registering with mating contacts (54) on a fiber distribution ring (40). A frangible element (68) extends between the contacts (64) and provides electrical continuity between the two contacts (64), such that when the fiber (62) is fully withdrawn from within the enclosure (61) the frangible element (68) separates and breaks electrical continuity between the two contacts (64).