In the system shown in the illustration, the engine room station is unable to signal any other station, nor is any other station able to signal the engine room station. The engine room station can, however, ring itself by proper positioning of its selector switch. What is the most probable cause of this problem? Illustration EL-0093

Correct Answer: Option D — There is an open between terminal "C" of the problem station and the common wire of the multi-conductor cable to the other stations.

Sound-powered telephone and alarm systems utilize a shared multi-conductor cable containing a common return wire. An open circuit between a station's terminal and this common return isolates that specific station, preventing signal transmission or reception while allowing local loop testing.