Current limit module for power distribution in transient sensitive systems

One of the most important problems, when designing the power distribution in modern exchanges, is to minimize the effects of hazardous transients on the feeding voltage. Components which are used today - anything from transistors to intelligent integrated circuits - are more sensitive than their more simple precursors. Ericsson´s construction practice is to minimize transients to harmless levels that are within the static and dynamic characteristics of electronic exchanges. Transient suppression can be achieved in different ways. For large telecommunication plants, this is solved by Ericsson, with high ohmic (≫45 m¿) distribution. A short circuit current is, in this system, limited to maximum 1,000 A by the high resistance distribution cable. The voltage drop in the power distribution is kept at an acceptable level by means of voltage division between the battery circuit resistance and the resistance in the distribution cable. However, high ohmic distribution is not effective as transient suppression in small systems with short cables and small batteries, such as PABX-systems. In this paper an Current Limit Module for transient protection is described. It has been developed by Ericsson Power Systems, (a division of RIFA) for the Swedish PTT. The function of this module is to limit short circuit currents to values in magnitude of nominal currents of the equipment. The CLM prevents all types of surge currents, a.o. inrush currents etc. The CLM which is a very compact hybridized module can easily be built into any distribution system.