The Importance and Selection of Optical Fiber Cable Outer Sheath Material and Fire Ratings

Why is the Outer Sheath Important? What Materials are Available?

Optical fiber cables typically consist of the fiber core, cladding, coating, strengthening element, and outer sheath. The outer sheath acts as a protective layer, providing fire and moisture resistance. In the event of a fire, the performance of the outer sheath is critical.

Different types of materials are used for outer sheaths, each with inherent properties suitable for specific scenarios. Common materials include PE, PVC, PVDF, LSZH, Plenum, and Riser.

While most outer sheath materials possess good flame-retardant properties, the material alone does not determine the fire resistance of the cable. The flame-retardant materials used must also pass testing to achieve a certain fire rating.

What Are the Fire Ratings of Optical Fiber Cables?

According to the NEC (National Electrical Code) in the United States, optical fiber cables are classified into several fire ratings:

• OFNP/OFCP: The highest level of flame resistance under NEC standards. If a flame is blown with a fan onto a bundle of OFNP-rated cables, the fire will extinguish within 5 meters without producing toxic or corrosive gases.
• OFNR/OFCR: The second-highest level. Similar to OFNP, but without smoke or toxicity specifications.
• OFNG/OFCG and OFN/OFC: Commercial and general grades used in environments with lower fire-resistance requirements.

How Do Different Outer Sheath Materials Affect Fire Ratings?

Although there is no direct correlation between fire ratings and sheath materials, generally:

• Plenum sheaths: Can theoretically achieve OFNP fire ratings.
• PVC or Riser sheaths: Can achieve OFNR fire ratings.
• LSZH sheaths: Can achieve OFN fire ratings.

How to Choose the Right Outer Sheath Material?

To reduce the risk of fire in data centers, it's essential to know the level and specific requirements of your data center. This allows you to choose the appropriate outer sheath material based on standards.

Choosing Based on Layout Area:

• Plenum cables: Suitable for horizontal cabling areas and air-handling spaces. The best choice for environments where air circulates and fire control is difficult.
• Riser/PVC cables: Suitable for vertical trunk cabling, connecting equipment rooms on different floors. OFNR-rated cables are adequate in these scenarios.
• LSZH cables: Common in general areas. These cables release non-toxic smoke, minimizing harm to rescue personnel during a fire.

Choosing Based on Data Center Size and Level:

• Plenum cables: Suitable for large/extra-large data centers and fault-tolerant/parallel-maintenance data centers.
• Riser/PVC cables: Common in medium-sized, redundant data centers.
• LSZH cables: Often used in small data centers, such as general enterprise server rooms.

Conclusion

While the outer sheath of optical fiber cables is just one aspect of fire prevention, comprehensive planning of cabling scenarios and fire warning setups are essential to minimize fire risks and damages, ensuring data center safety.

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