In a dedicated circuit feeding a switch that controls a light, the light fails to illuminate even though voltage is present at the line side and load side when the switch is ON. What is the most likely fault?

• A. Defective switch
• B. Bad lamp
• C. Bad breaker
• D. Open neutral

Answer: (B)

When the switch is ON and you still don’t get a light, but you can measure voltage on both the line side and the switch’s load side, the switch is actually closing and delivering power to the fixture. The problem then is on the load side—the lamp or fixture isn’t allowing current to complete the circuit. The lamp could be burned out or otherwise defective, so even though hot power reaches the fixture, no current flows through the filament to produce light.

If the switch were bad, you wouldn’t see voltage on the load side when it’s ON. If the breaker were bad, the circuit wouldn’t have power at all. An open neutral would typically prevent a return path and could show odd readings or no light, rather than simply a non-illuminated lamp with a live switched leg. So the most likely fault is a defective lamp.