Protection of Subscriber Line Interface Circuit: Study of a Case

This paper describes the damage mechanism to the Subscriber Line Interface Circuit (SLIC) caused by 50 Hz overcurrents due to induction on the telephone loops from power lines. To protect the SLIC connected to these exposed lines, the need of an overcurrent protective device, with a suitable time to trip characteristic, is then pointed out. This is the result of both the survey on the overvoltages, which can arrive at the SLIC input, and the laboratory tests on 50 Hz current resistibility of the SLIC. The measuring system, the synthesis of the collected data and a few resistibility test results are also presented.