Which of the following is the most likely cause for a streetlight burning during daylight hours?

• A. The lamp bulb has arced over between electrodes
• B. The photocell has failed
• C. The ballast has failed
• D. Timing clock malfunction

Answer: (B)

The key idea is that streetlights are usually controlled by a daylight-sensing device—a photocell—that turns lamps on at dusk and off at dawn. If the photocell fails in a way that keeps the circuit closed (or it shorts), the control never senses daylight and the lamp stays energized regardless of ambient light. That’s why burning during daylight hours is most likely due to the photocell failing.

Other faults don’t fit the symptom as cleanly. An arc inside the lamp would create electrical issues with the lamp itself but doesn’t inherently cause lights to stay on only in daylight. A ballast problem might prevent the lamp from starting or cause flicker, not reliably keep it on all day. A timing clock malfunction could alter on/off times, but daylight operation is most directly explained by a photocell failure, since the daylight-sensing control is what determines daytime shutdown.