To avoid injury, what must the secondary winding of a current transformer be?

The correct answer is that the secondary winding of a current transformer must be short-circuited to avoid injury. In a current transformer, the primary winding is connected to the high-voltage circuit, and the secondary winding produces a lower current proportional to the primary current. If the secondary winding is left open-circuited while current flows through the primary, it can induce a dangerously high voltage across the open terminals of the secondary. This high voltage poses a significant risk of electric shock or damage to the transformer. By short-circuiting the secondary winding, the current flowing through it is limited to a safe amount, preventing hazardous voltage levels while allowing the current transformer to function correctly.

In contrast, having the secondary winding open, connected, or only insulated does not provide the required safety against the dangerous conditions that can arise from open-circuit operation. It's essential for safety and operational reliability that the secondary is correctly short-circuited during normal operation.