The current transformer is always used with secondary closed with an ammeter, relays, etc. The reason is secondary is never kept open when the primary side is energized.
The primary of the Current Transformer is connected in series with the line, which carries a very large value of current. When secondary is closed the opposing MMF of secondary is slightly less then MMF of primary so it reduces the overall MMF of primary. Therefore the value of emf induced in the secondary is very less
Now suppose if the secondary of the Current Transformer is made open by any mean then opposing MMF of secondary will reduce to zero while that of primary will remain the same, therefore the resultant MMF will be the same as MMF of primary.which will induce very large value of EMF in secondary.
So high voltage at secondary will occur, which will is dangerous to transformer insulation and the person who opens the secondary. Not only this but eddy current and hysteresis loss will increase very much and the core of the Current Transformer will get overheated and may get completely damage. Also it might be possible that core may become permanently magnetized which would always give ratio and phase angle error.
So to avoid this condition never open the secondary of the Current Transformer.
NOTE. Also checkout another post on How to differentiate between Current Transformer And Potential Transformer ?
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