Why Current Transformer (CT) Secondary Should Never be left Open?

The most important precautions must be taken to never open the secondary circuit of current transformer (C.T.) while current following in the primary circuit. Opening of the secondary under these conditions remove the opposing mmf produced in the secondary and allows the flux in the core to reach a value determine only by the primary mmf. The results is that the flux in the core is greatly increased. This situation leads the following harmful defects.

(1) The increase flux in the core may saturate it. The effect of saturation is to be leave the transformer with a large value of residual flux . This will seriously impair the accuracy of the Current transformer.

(2) The flux in the core change it very high rate  inducing a large voltage in the open secondary winding. The effect of excessive voltage may be to puncture the insulation of the transformer or to provide dangerous shock to the operator opening the circuit.