Why is it dangerous to open the secondary winding of a current transformer while the switchgear is energized?

Opening the secondary winding of a current transformer while the switchgear is energized is particularly hazardous because it can lead to a high buildup of voltage. Current transformers are designed to produce a secondary current that is proportional to the primary current flowing through their primary winding. If the secondary is opened, there is no path for the current to flow.

This disruption creates a situation where the current transformer can generate a very high voltage across its terminals because the magnetic flux in the transformer’s core remains constant. The core reacts to the absence of load by increasing the induced voltage to potentially dangerous levels, leading to insulation breakdown and other failures. This phenomenon is often referred to as "open-circuit voltage," and in some cases, it could exceed the voltage rating of the insulation, causing arcing or equipment damage.

Understanding this principle is crucial in electrical safety and equipment operation, particularly in high-voltage applications where current transformers are commonly used to monitor and control power systems.