By Oleh J. Sniezko, TCI Communications, Inc
Fusing or protection against over-currents in the cable TV coaxial distribution network is an area in which many misconceptions found a way into coaxial plant powering design. The origins of these misconceptions are numerous, but most of them can be traced to the interpretation of the rules of fusing in the power plant and power installations. A direct implementation of these rules results in lower reliability of the network without meeting the objectives of equipment and personnel protection, and fault isolation. This paper presents the fusing analysis from the point of view of the two primary objectives of fusing: 1. to protect components and equipment from costly damage caused by overcurrents; and 2. to isolate a faulty part of the network from the remainder of the network once the fault has occurred. These two direct objectives are subordinate to the paramount objective of increasing system availability through: 1. improved or at least maintained current reliability of the network, and 2. reduced repair time. The first decision during the fusing design is whether a fose is required in a particular location and whether it can meet the objectives of fusing. If the answer to these questions is positive, the following criteria will decide about type, value, and other parameters ofthefuse selected: • Maximum operating load current, • Ambient temperature, • Aging, • Operating voltage, • Frequency of the powering voltage, • In-rush characteristics of the network, surge currents, etc., • Available maximum fault current, • J2t characteristics of the devices to be protected, • Allowed voltage drop across fuse, • Applicable standard agency, • Ergonomics, • Humidity, and • Standardization. The paper analyzes all these parameters and presents a detailed fusing design for HFC networks, supported by the up-to-date principles of fuseology. Finally, it presents an example of fusing design in modern HFC networks.