The outer sheath of a fiber optic cable protects the fibers from environmental, mechanical, and chemical damage, with material selection depending on installation conditions and safety requirements.

Purpose of the Outer Sheath

The outer sheath is the protective layer surrounding the optical fibers, designed to shield them from physical damage, moisture, UV radiation, chemicals, and fire hazards. It also contributes to the cable's mechanical strength, flexibility, and long-term durability .

Common Outer Sheath Materials

1. Polyethylene (PE) / High-Density Polyethylene (HDPE): Ideal for outdoor installations due to excellent UV resistance, weather durability, and low water absorption. PE is lightweight, non-toxic, and provides good electrical insulation .
2. Low Smoke Zero Halogen (LSZH): Preferred for indoor use where fire safety is critical. LSZH emits minimal smoke and non-toxic gases during combustion, making it suitable for buildings and confined spaces .
3. Polyvinyl Chloride (PVC): Widely used for general-purpose applications. PVC offers good chemical resistance, weathering resistance, and low cost, but produces dense smoke when burned .
4. Polyurethane (PU): Used for underwater or submarine cables due to excellent water, abrasion, and impact resistance .
5. Other specialized sheaths: Thermoplastic elastomers (TPE) for industrial chemical resistance, rodent-proof sheaths, and armored sheaths for high mechanical protection .

Mechanical Protection Considerations

• Armored vs. Non-Armored: In areas prone to physical damage, armored cables with metallic or non-metallic layers beneath the sheath are recommended. The outer sheath must be compatible with the armor to maintain protection .
• Installation Guidelines: Avoid sharp bends, excessive pulling tension, and contact with sharp objects. Use figure-8 loops to prevent twisting, and ensure clamping intervals support the cable weight without exceeding long-term load limits .

Fire and Safety Ratings

For indoor applications, fire performance is critical. Sheath materials must meet standards such as OFNP/OFCP (highest flame retardant grade) or OFNR/OFCR (vertical trunk level) according to NEC codes. The overall cable fire rating depends on both the sheath material and the cable construction .

Summary

Selecting the right fiber optic cable outer sheath involves balancing environmental exposure, mechanical protection, chemical resistance, and fire safety. Outdoor cables typically use HDPE or PE, indoor cables prioritize LSZH, and specialized environments may require PU, TPE, or armored sheaths. Proper installation practices are essential to maintain the integrity and performance of the cable over its service life .