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
• Common return (C terminal) in multi-station ringing circuits • How self-ringing proves the local ringer coil and selector switch contacts are OK • Difference between a local fault inside the station and an open in the shared cable to other stations
• Trace the current path when the engine room calls itself: which components and wires MUST be good for the bell to ring? • Now trace the current path when another station tries to call the engine room: what additional wire or connection is required that is not used when it rings itself? • For each answer choice, ask: would this fault still allow the engine room to ring itself, while completely preventing all traffic between it and the other stations?
• Verify whether an open ringer coil could still allow the station to ring itself. • Check if a stuck-open local switch would also block incoming calls from other stations, or only outgoing calls. • Identify the role of the C terminal/common conductor in linking this station to the rest of the system and whether an open in that common matches all the symptoms.
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