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

A current transformer (CT) operates by measuring the current flowing in a conductor. When a CT is placed on an energized bus but is not connected to any load or circuit, it can lead to a condition known as "open-circuiting." This scenario occurs because the CT is designed to produce a secondary current that is proportional to the primary current flowing through its core.

In an open-circuit condition, the primary winding of the transformer is not connected to a load, and hence the transformer does not have a closed loop for the magnetic circuit. As a result, the CT will generate a high voltage on its secondary side due to the lack of current flow, resulting in excessive electrical stress and heating. This overheating can subsequently lead to the insulation breaking down and potential damage to the CT, such as burning up or failure.

Therefore, in the context of a current transformer being energized but not connected, it is critical to ensure it is properly loaded to avoid such hazardous conditions.