Polymer current limiters for low-voltage power distribution systems

Low-voltage power distribution systems are often protected from overcurrent conditions by fuses, low-voltage power circuit breakers and molded-case circuit breakers. As changes are made to the power distribution system, the short-circuit duty imposed on the switchgear often increases. This increased short circuit duty may necessitate the replacement of many of the devices in the system, including the circuit breaker if the rating of the breaker is exceeded. One approach to avoiding the replacement of over-dutied switchgear is to use a current limiter to decrease the short-circuit current. This paper discusses the use of polymer current limiters for the protection of over-dutied circuit breakers. Polymer current limiters (PCLs) are widely used in appliances, electronic power supplies and other low-power applications. Power distribution applications of polymer current limiters and simulation results showing potential problems and approaches to their solutions are given. The authors begin with an introduction to the functional properties of polymer current limiters, then describe how those properties drive the selection of associated components such as a shunt link and the molded case circuit breaker. Finally, they describe applications of the PCL in a hierarchic low-voltage power distribution structure. Particular emphasis is placed on methods for tailoring the time-current curve to achieve coordination with low-voltage circuit breaker characteristics.