Response of the Western Electric Model 188A Test Set (Stop Lite) to ELF (Extremely Low Frequency) Voltages

摘要

The Western Electric Model 188A Test Set (Stop Lite) is a hand-held probe that enables a telephone craftsman to detect hazardous voltages on metallic facilities without being endangered by electric shock. The facilities under test--metal pedestals, cable shields, cable suspension wires, and power line ground wires and rods--are ordinarily at the potential of the local earth. However, 60 Hz coupling from power lines, in combination with bad connections and/or grounding systems, can cause unexpected voltages to exist in common situations. The 188A test set is designed to detect 60 Hz ac voltages between 50 V and 20,000 V(rms). Extremely low frequency (ELF) voltages can also be induced on telephone cable shields and metallic equipment in areas where the telephone plant is collocated with an ELF communications system. This report addresses the question of how the 188A test set responds to such ELF voltages alone and to such voltages together with 60 Hz voltages. Laboratory mock-ups of typical telephone plant field installations were assembled, and tests were conducted to simulate varios worst-case telephone fault conditions, such as a broken cable shield bond to ground. The report represents tabular and graphic data that document the response of the test set to a wide range of conditions.