What happens to a current transformer (CT) when it is on an energized bus but not connected?

When a current transformer (CT) is on an energized bus but not connected to a load, it effectively has an open secondary circuit. In this scenario, the primary current that flows through the CT will induce a voltage in the secondary winding. However, because the secondary is open, the induced voltage can rise to levels far exceeding the transformer's design limits.

This excessive voltage can create a "burn-up" condition due to insulation breakdown or damage to the transformer itself. Current transformers are designed to operate with a connected load that provides a return path for the induced current. Without this circuit connected, the CT cannot dissipate the energy and this leads to overheating, potential arcing, and ultimately failure of the device.

The other options do not accurately describe what happens in this situation. For instance, the CT cannot function normally because it is not connected, nor does it produce a phase shift or generate excessive voltage in a controlled manner since it does not have a load connected to limit the effects of the induced voltage. Thus, the behavior of a CT under these circumstances aligns with the understanding that it could indeed be damaged or "burn up" due to excessive voltage.