What issue could result in 0 volts in the primary side of a transformer?

A faulty door switch could lead to 0 volts in the primary side of a transformer by interrupting the power supply necessary for the transformer to function. If the door switch is designed to prevent the system from operating when a door is open—such as in a safety mechanism in HVAC systems—when the switch fails or if it remains open, it will disrupt the circuit. Consequently, this results in no voltage being present on the primary side of the transformer, as the transformer relies on a complete circuit to induce voltage from the secondary circuit to the primary.

Understanding that the door switch is crucial for maintaining safe operation in certain HVAC systems emphasizes the importance of checking safety and interlock devices when troubleshooting electrical issues. In contrast, other issues such as blown fuses, short circuits, and open circuits would have different effects on voltage presence, focusing attention on those specific components rather than on a control device like the door switch.