A pump motor controlled by a float switch occasionally fails to start, but the main circuit breaker is not tripped. When the float switch indicates 'high water', an electrician measures 120 V AC at the input of the control transformer, but 0 V AC at the output. What is the most probable cause?

Question: A pump motor controlled by a float switch occasionally fails to start, but the main circuit breaker is not tripped. When the float switch indicates 'high water', an electrician measures 120 V AC at the input of the control transformer, but 0 V AC at the output. What is the most probable cause?

Answer options:

• The float switch contacts are stuck open
• The motor has a winding short to ground ✅ The control transformer's primary or secondary winding is open
• The motor contactor coil is shorted

Correct answer: The control transformer's primary or secondary winding is open

Explanation: If there is voltage at the transformer primary but none at the secondary, it indicates an open circuit within the transformer windings (either primary or secondary) or a blown internal fuse, preventing voltage transformation. A stuck float switch would prevent current to the transformer primary. Motor winding shorts or contactor coil shorts would cause current draw, potentially tripping a breaker, but not necessarily zero output from a transformer with input voltage.