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, more the number of secondary turns.

Working principle of current Transformer:

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

What happens when the secondary of CT is Open- Circuited?

It is very clear that the net flux in the core is equal to flux due to primary current minus flux due to secondary current. If CT is kept open circuited, no current will flow in the secondary, and consequently no secondary flux will set up in the core. In absence of secondary current, the net flux will be higher than the flux generated in the core when CT is connected to load. The higher flux generates higher voltage in the secondary, and as a result of higher secondary voltage, the insulation of the secondary winding can fail. Also, this high voltage may cause electric shock to person working in the feeder. The magnitude of the secondary voltage with open secondary may be of kilo volts.

When CT of secondary is open, the VA is totally consumed in core heating as an core loss. As per the law of energy  conservation the energy balancing is always there, the VA consumed by CT is equal to the core loss of the CT when secondary is open circuited.

For this reason, CT should be never kept open- circuited. While working on measuring equipment like energy meter, ampere meter or protection relay, the CT must be short-circuited on test terminal block (TTB).