CT or PT functions on the principle of electromagnetic induction. The electrical energy is first converted into magnetic energy and again the magnetic energy is converted in the electrical energy. The electrical current first converted in magnetic energy. The magnetic energy is nothing but converting electrical current into flux. The flux then flow through the core. The core of the CT or PT has its maximum flux carrying capacity. if the more flux rated capacity of the core flow through the core, the core may get saturate and eventually the core will get damaged due to core insulation failure.
The flux in the core depends on the v/f or volts/Hz ratio. If 50 Hz CT is operated at 60 Hz system, the flux(v/f) flowing in the core will be less than the rated flux carrying capacity of the core.
If 60 Hz CT is operated at 50 Hz system, the flux flowing in the core will be more than the rated flux carrying capacity of the core.
The designer of the CT design the CT, taking into account at what frequency the CT is to be operated. The turns in the primary and secondary is designed according to the core in the flux, and the flux in the core depends on the V/f ratio.