A VFL is used to detect faults, breaks, or bends in fiber optic cables by emitting a bright red light that is visible even through the fiber's jacket. It's a cost-effective and. The ST816B Visual Fault Locator is specially designed to allow quick and efficient maintenance of fibre optic networks and can be used for tracing and continuity checks allowing rapid identification of specific fibres. Note: Meant for use with polished, terminated fiber cables. For single mode, multimode & plastic fibers.