During normal operation of the current transformer (CT), the impedance of the circuit ZL is very low, equivalent to the state where the secondary coil is shorted.
The magnetomotive force produced by the secondary current opposes the magnetomotive force produced by the primary current, the excitation current is very small, the total magnetic flux through the iron core is very small, and the induced e.m.f. of the secondary winding does not exceed a few tens of volts.
When the secondary side is open, then the secondary current becomes zero, any demagnetization effects do not exist, while the E1 of the primary coil remains constant and the current becomes exclusively the excitation current, thereby causing the flux Φ in the core to rise sharply and the core is in a highly saturated state.
Further, the number of turns of the secondary winding is large and therefore a high voltage, possibly in the range of thousands of volts, will appear at both ends of the secondary winding; this poses hazards not only to the insulation of the winding but to human life as well.
Thus, it is absolutely prohibited to open the circuit for the secondary side of the current transformer.