In a locomotive control circuit, the warning horn will not sound if an open occurs in wire No. 5 while the throttles are retarded with only the right gear down.

• A. No. 13
• B. No. 6
• C. No. 5
• D. No. 4

Answer: (C)

The warning horn is energized only when a complete electrical path exists that is enabled by the throttle and gear conditions. In this scenario, when throttles are retarded and only the right gear is down, the horn’s energizing path runs through wire No. 5. If that wire opens, the circuit is broken and current cannot reach the horn, so it will not sound even though the other conditions would normally allow it. So an open in wire No. 5 stops the horn under those specific conditions, making No. 5 the critical wire.