What must be connected to the secondary of a current or potential transformer when it is energized?

When a current or potential transformer is energized, it is crucial to connect a load or ensure that it is grounded. This is primarily because these transformers are designed to produce a secondary voltage or current that is proportional to the primary values they are measuring. If the secondary is left open (not connected), it can lead to dangerously high voltages that may damage the transformer and pose safety risks.

Connecting a load ensures that there is a defined path for the current or voltage generated by the transformer. This load could be a measuring instrument or a protective relay, which allows the system to function safely and effectively. Grounding serves as an additional safety measure, providing a reference point to minimize potential shock hazards and ensure the reliability of the system.

In contrast, leaving the secondary open would cause excessive voltage buildup, which can damage the transformer. A switch could be useful in controlling the circuit, but it does not fulfill the immediate requirement of connecting a load to the secondary. An indicator light by itself does not represent an adequate load to protect the transformer or maintain proper functionality. Therefore, connecting a load or ensuring grounding is fundamental to the safe operation of current or potential transformers.