Why is the outer sheath of optical fiber cable important? What are the materials?
Optical fiber cable is generally composed of optical fiber core, cladding, coating, reinforcing element and outer sheath. As the protective layer of the cable, the outer sheath has the functions of fire prevention and moisture resistance, which has an important impact on the performance of the outer sheath in case of fire in the data center.
The outer sheath of optical fiber cable is divided into different material types. The outer sheath of each material has its own characteristics (different fire performance) and suitable for use scenarios. The common outer sheath materials are PE, PVC, PVDF, LSZH, plenum, riser, etc. the following table shows the performance and applicable environment of the above materials respectively.
Material of outer sheath | Advantage |
PE（Polyethylene） | It has good low temperature resistance, good chemical stability, resistance to most acid and alkali erosion, wear resistance, and high electrical insulation. Therefore, PE is the standard sheath material for outdoor cables. |
PVC（Polyvinyl chloride） | It has good mechanical properties, high electrical insulation, strong flexibility, strong and flame retardant, but poor stability to light and heat, so it is more suitable as the outer sheath material of indoor optical fiber cable. |
PVDF（Polyvinylidene fluoride） | It has many excellent properties, such as elasticity, low weight, low thermal conductivity, high chemical corrosion resistance and so on. At the same time, it also has better flame retardancy than PE, and hardly produces smoke even in case of fire. Therefore, PVDF material can be used in the outer sheath of more important trunk fiber jumper. |
LSZH（Low smoke zero halogen） | LSZH has the characteristics of low smoke, low toxicity, low corrosion and high flame retardant. As a safe and environmentally friendly material, LSZH is an ideal choice for indoor / outdoor installation. But also, the price of LSZH fiber optic cable is more expensive. |
Plenum | It is one of the main components of OFNP grade optical fiber cable, with high flame retardancy and no toxic gas or corrosive gas under extremely high temperature. Optical fiber cable with plenum material is the preferred wiring in air return pressurization system for ventilation duct or air handling equipment. |
Riser | It is one of the main components of OFNR grade optical fiber cables. Compared with plenum material, it has weak flame retardant performance and no toxic and corrosive gas specifications. Therefore, OFNR grade optical fiber cables with riser material are usually used in building trunk and horizontal cables. |
It can be seen that most of the outer sheath materials have good flame retardancy. Is the outer sheath material the only standard to measure the fire resistance of optical fiber cables? No, the flame retardant materials used in the cable must pass the test and reach a certain fire rating. For example, PVC sheathed cable, to achieve the best fire performance, also need to consider the overall cable to achieve what kind of fire rating.
What are the fire grades of optical fiber cables?
In NEC, the fire rating of optical fiber cable is often divided into OFNP / OFCP, OFNR / OFCR, OFNG / OFCG and OFN / OFC.
OFNP / OFCP, NEC standard flame retardant grade (pressurized grade), the highest grade. Suppose that the flame is forced to be blown to a bundle of flame retardant (pressurized) optical fiber cables by a fan, the flame on the optical fiber cables will be extinguished within 5m, and no toxic and corrosive gas will be generated.
OFNR / OFCR, the vertical level (trunk level) in NEC standard, ranks second. Similarly, a fan is used to force the flame to the bundled trunk level optical fiber cable. Although the flame will be extinguished within 5m, there is no smoke or toxicity specification.
OFNG / OFCG and OFN / OFC are commercial grade and general grade respectively. The optical fiber cables under these two standards are often used in the environment with low requirements for fire prevention and flame retardant.
How to choose optical fiber cables with different sheath materials?
In general, plenum fiber optic cable is suitable for horizontal wiring area and inflatable environment (transportation pipeline, air handling system). This kind of environment is ventilated and fire is not easy to control. Therefore, OFNP flame retardant grade fiber optic cable is the best choice; Riser / PVC fiber optic cable is suitable for vertical trunk wiring area, providing entrance equipment or computer room The connection with communication cabinets on different floors has little possibility of large-area fire, and the flame retardant grade can reach OFNR; LSZH optical fiber cables are often used in common areas, even if there is a fire, because the smoke released is non-toxic, it can avoid causing injury to rescue personnel.
Because the impact of fire in different size data centers is different, NEC standard recommends the optical fiber cables suitable for different environments. In general, plenum fiber optic cables are suitable for large / super large data centers and fault-tolerant / parallel maintenance data centers; Riser / PVC fiber optic cables are commonly used in medium-sized data centers and redundant data centers, while LSZH fiber optic cables are commonly used in small data centers, just like enterprise computer rooms.
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