Safety issues in combining telephone and cable TV
By STANLEY KAUFMAN
Panel 16 of the National Electrical Code Committee is responsible for safety considerations relating to optical-fiber cables, communications (telephone and data) circuits, and cable-television circuits in buildings. The panel recently passed, and the National Fire Protection Association accepted, a new article for the 1999 National Electrical Code (NEC) that addresses the safety issues of hybrid fiber/coaxial-cable communications. Article 830, "Network-Powered Broadband Communications Systems," goes beyond the electrical safety issues of traditional telephony and cable TV to deal with issues related to the powering of the system.
Article 830 covers a variety of possible fiber and metallic cables that connect to the end customer`s premises. (Trunk and distribution cables are covered by the National Electrical Safety Code.) A typical system includes a cable that supplies power and the broadband signal to a network interface unit (NIU). The NIU can be inside or on the outside of the customer`s building and converts the broadband signal to the component signals--conventional telephone and cable-TV circuits. Typical configurations could include: 1) coaxial cable with both broadband signal and power on the center conductor, 2) composite metallic cable with a coaxial member for the broadband signal and a twisted pair for power, or 3) composite optical-fiber cable with a pair of conductors for power. Larger systems may also include network components such as amplifiers that require network power.
The most important issue is powering the NIU. Powering for the NIU must be independent of the building`s power system to provide reliable telephone service even when there is a power outage. Since the NIU will often require a higher level of power than is ordinarily encountered in either conventional cable-TV or telephone circuits, wiring methods have been specified in Article 830 for safely powering NIUs over the broadband communications cable. "Low" and "medium" powered systems have been defined and appropriate wiring methods have been specified for each powering level. The initial proposal included a "high" power system that raised many contentious issues and was consequently deleted from the final draft of the new article.
Article 830 addresses protection of network-powered broadband circuits in case of exposure to lightning or accidental contact with power wires. The NEC has a long-standing requirement in Article 800 for the provision of primary protectors on telephone circuits that have these exposures. Under the same exposure conditions, NEC Article 820 only requires the grounding of the outer shield of a cable-TV cable. The rationale for this requirement is that the customer is expected to be in frequent physical contact with a telephone handset, but not a TV set. Departing from this past practice, Article 830 requires protection of both the telephone circuits and cable-TV circuits derived from the NIU.
The 1999 National Electrical Code is now available from the National Fire Protection Association (website www.nfpa.org). This code is designed to be suitable for adoption as law by governmental bodies. November 5, 1999 is the deadline for submitting proposals for the next version, to be issued in 2002. q