What is a probable cause for a relay failing to drop out?

• A. Power loss to the circuit
• B. Operating coil is de-energized
• C. Operating coil is energized
• D. Mechanical failure of the relay

Answer: (C)

A relay failing to drop out can often be attributed to the operating coil being energized. When the coil is energized, it creates a magnetic field that holds the relay contacts in place, preventing them from returning to their default (normally open or normally closed) position. This means that as long as the coil remains energized, the relay will not release, and therefore does not drop out as expected.

In contrast, if there is a power loss to the circuit, the relay would be expected to drop out because the coil would no longer be energized. Similarly, when the operating coil is properly de-energized, the relay should also drop out. Mechanical failure of the relay would indicate an inability of the relay to function correctly, but the most direct and typical reason for a relay not dropping out is that its coil is still receiving power, keeping it energized.