Limited-Power Circuits for Telecommunications Systems

Power is often transmitted over communications wiring for signaling and for operating remote equipment. In the United States, the National Electrical Code allows the distribution of such power provided the power circuits are power-limited, classified as Class 2 and Class 3. To provide such circuits, telephone power supplies must incorporate suitable power- and current-limiting schemes and, if required, over-current protection. Power-limitation can be provided by: • Transformer impedance • Ferroresonant regulation • Positive temperature coefficient resistors (PTC). Examples are presented in this paper, along with tests for verification. Design requirements for telephone power supplies with power-limited outputs are relaxed by Underwriters Laboratories. Wirinig requirements are similarly relaxed in the National Electrical Code. Test data are presented that show wire damage caused by current pulses produced by a Class 2 circuit. The data also show that a fast-acting PTC can prevent damage to wires as fine as 27 gauge.