Hybrid cable power and protection

Hybrid cable power and protection

STANLEY KAUFMAN, AT&T

Communications carried by hybrid fiber/coaxial cable pose safety issues that are being addressed by the National Electrical Code Committee. Panel 16 of this committee is responsible for the safety of optical fiber cables, communications (telephone), data communications and cable-TV circuits in buildings.

A task group of Panel 16, chaired by Jim Brunssen, a member of the technical staff of Bell Communications Research, is writing a proposed article for the Code, tentatively titled "Network-Powered Broadband Communications Systems."

In a typical system, a communications company generally uses optical fiber cables, either up to, or close to, the customer. When an optical fiber cable is not directly connected to the customer, the company usually converts to coaxial cable. The task group is only dealing with the wiring that connects to the customer`s premises--not the trunk and distribution cables, which are covered by the National Electrical Safety Code.

The typical system envisioned by the task group includes a cable that carries power and broadband signals to a network interface unit. This unit can be located inside or outside the customer`s house and converts the broadband signals to conventional telephone and cable-TV signals. Different cable types include coaxial cable with broadband signals and power on the center conductor; composite metallic cable with a coaxial-cable member for the broadband signals and a twisted-pair cable for power; and composite optical fiber cable with a pair of conductors for power. Larger systems might contain network components such as amplifiers that also require power.

The task group is grappling with several issues, the most important of which is powering the network interface unit. Power for the unit must be independent of the household power system to maintain reliable telephone service even when a power outage occurs. Because the network interface unit often requires a higher level of power than is ordinarily encountered in either conventional cable-TV or telephone circuits, the task group is dealing with safely powering network interface units over a broadband communications cable.

Providing lightning protection and preventing accidental contact with power wires are other concerns of the task group. The National Electrical Code currently requires primary protectors on telephone circuits that are exposed to lightning or that might come in contact with nearby power wires. Under these exposure conditions, the Code requires only the grounding of the outer shield of a television coaxial cable. A rationale for this difference is that electrical protection of telephone lines involves safeguarding people who could be holding a telephone to their ear; electrical protection of cable-TV lines is primarily about safekeeping television sets from lightning damage.

In addition, the task group is considering protection requirements for the telephone and cable-TV signals derived from the network interface unit. q

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