Sometimes fiber optic cables are routed through and around machinery. A rule of thumb when specifying sheathing: if interlocked metal ( (SL)), plain or covered) sheathing is used, minimum bending radius is 4X the OD of the sheathing. If you were to take out a fiber strand and lay it flat, the strand would be longer than the. This Applications Engineering Note (AE Note) addresses estimating cable length or event distance using an optical time domain reflectometer (OTDR). This AE Note does not provide operating instructions for any particular OTDR. All lengths are calculated in a base unit, then converted. Reel count is ceil (Total ÷ ReelSize), and the rounded order length equals Reels × ReelSize. They have a central core surrounded by a concentric cladding with slightly lower (by ≈ 1%) refractive index.