Why CT Secondary Shall Never Be Kept Open?

Open-circuited CT can cause serious safety concerns like fire, electrocution, and protection failure. Therefore, you must ensure that CT secondary should always be close-circuited. Let us understand the CT construction and its working principle to know why the CT must not be open-circuited.

Construction of Current transformer:

The current transformer 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. The secondary winding of CT 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 the current flowing through the primary, the greater 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.

Why CT Secondary Shall Never Be Kept Open?

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 closed circuit CT diagram is shown in the below figure.

closed circuit CT

The current in the secondary winding creates an opposing magnetic flux that opposes the main flux created by the primary winding. The net flux in the core equals the difference between primary and secondary flux. The flux in the core remains within the rated flux rating of the core when CT is connected to the specified burden.

What happens when the secondary 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.

net flux in the CT when its secondary is connected to load

If CT is kept open-circuited, no current will flow in the secondary, and consequently, no secondary flux will be set up in the core. The diagram of open circuited CT is shown below.

open circuited CT

The net flux in the core of the CT is given in the following formula,

formula for net flux in the core of the CT

In the absence of a secondary current, the net flux will be higher than the flux generated in the core when CT is closed-circuited. The increased core flux induces more secondary voltage. The CT secondary voltage depends on the core flux, and the output voltage can be expressed with the following mathematical expression.

From the above expression, it is clear that the core carries higher flux during open-circuited CT, and as a result, a higher voltage is produced in the secondary. The CT secondary can not sustain the voltage more than its designed rated voltage, and it causes insulation failure of the winding. Also, this high voltage may cause an electric shock to the person working in the feeder. The magnitude of the secondary voltage with open secondary may be kilovolts.

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

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


The open-circuited CT causes-

  • High voltage at its secondary side causes flashover and insulation failure, which in turn creates a fire hazard.
  • High Flux in the core can damage the CT.
  • The protection of the equipment does not remain intact.
  • The person working on CT can get an electric shock.

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