No, we cannot open the Secondary Circuit of a Live CT, because it will make the CT to Explode. Here one question my arise that both CT and Power Transformer are Transformers but Open Circuiting CT Secondary, results in Exploding of the CT while Open Circuiting Power Transformer Secondary, does not results in Exploding of the Power Transformer. We will find out why in this Article, by comparing the CT with a Power Transformer.

Primary Current:

In Power Transformer, Primary Current depends on the Connected Load.

 I1 = Io + I2′ , where Io is the No Load Component and I2′ is the Load Component.

When Power Transformer Secondary is Open Circuited, Secondary Current I2=0. And so, Primary Current (I1) is equal to Magnetisng Current (Io), which is very small.

 In Current Transformer, Primary being placed in Series with the current carrying the conductor, the Primary Current (I1) is indepedant of the Secondary Loading. That is even if the Secondary is Open Circuited, the Primary Current is going to remain the same. 

      Net Flux in the Core (N1xI1 – N2xI2):

      When the Power Transformer Secondary is Open Circuited, the Secondary Current (I2) being zero, the Opposing MMF ( -N2xI2) is zero. And now the Net Flux in the Core is just N1xI1 . We have seen in the above section, that the Primary Current (I1) is equal to the Transformer Magnetising Current (Io) which is very small. So now the Net Flux in the core is equal to (N1xIo), which is very small (approximately equal to zero).

       

       When the Current Transformer Secondary is Open Circuited, Secondary Current (I2) being zero, Opossing MMF (-N2xI2) is zero. And so the Net Flux is (N1xI1). Now since the N1 is large and I1 is large, the Resultant MMF is very Large , as there is no Opposing MMF.

          So thus when the CT Secondary is Open Circuited, the Resultant MMF induced in the Core is very very high. Due to this Large Flux in the core, the Flux Linkage of the Secondary Winding will be Large which in turn will produce High Voltage accross, the Open CT Secondary Terminals.

          1. High Voltage leads to Insulation Failure of the CT and lead to a Short Circuit or a Fault inside the CT.

          2. Excessive Core Flux Saturates the CT Core Permanantly, therby giving error in the Transformation Ratio.

          3. Because of excessive core flux, the hysteresis and eddy current loss will be very high and the CT will get overheated. As CT is oil filled, because of overheating, the oil of CT will get boil and start to vaporize. Because of vaporization of CT oil, the CT housing will get pressurized and blast. 

          Exploding of the CT results in Power Interruption and consequently Loss of revenue. Also, the exploding CT may damage the nerby equipments, leading more Loss.

          Also, if someone accidently touches the Open Circuited CT Secondary, due to such High Voltage, he will get electrocuted and it may be a Fatal accident.

          Also, if someone accidently touches the Open Circuited CT Secondary, due to such High Voltage, he will get electrocuted and it may be a Fatal accident.

          Thank You so Much. See you in some other Post.