Why CT Secondary Shall Never Be Kept Open?

Construction of Current transformer:

The current transformer basically consists of an iron core upon which primary and secondary winding are wound. The primary winding of the transformer is connected in series with the load and carries the actual current flowing to the load, while the secondary winding is connected to a measuring device or a protection relay.

The number of secondary turns is proportional to the current flowing through the primary; i.e., the larger the magnitude of current flowing through the primary, the more the number of secondary turns.

Working principle of current Transformer:

The current transformer works on the principle of electromagnetic induction. When alternating current flows in the primary winding of CT, the primary current generates a magnetic field (H= NI) in the core of CT.

The generated magnetic field sets up magnetic flux in the core. The magnetic flux links to the secondary winding of CT mounted around the core, and thus linked flux induces a voltage in the secondary winding of CT.

The secondary winding of CT is connected to the burden (measuring instrument or protective relay). If the secondary CT is connected to the burden, the alternating current starts flowing in the secondary winding of the transformer.

The current in the secondary creates opposing magnetic flux in the secondary winding that opposes the main flux created by the primary winding. The net flux in the core is equal to the difference of primary and secondary flux. The flux in the core remains within the rated flux rating of the core if the secondary is connected to the burden.