Transformer

Dangerously high voltage at the terminals or leads; possible insulation breakdown in the windings.

if the secondary of Current Transformer becomes open during Load, then the secondary impedance now becomes infinite (you know it's open-circuited) and the CT core saturates. This in turn generates a very high voltage in the primary, which almost will be much more than the system volts. As a result, there is a flashover accordingly.

I hope this helps.

(CT) is a type of instrument transformer specially designed for protection and measuring purpose, it will operate by the current whenever it exceeds the set value. The C.T converts the 1000A/5A. In the secondary more tappings we will take depend upon the measurement. then Electrons will flow in the closed path only.when CT is shorted or connected to certain load, the impedance become negligible compared to circuit its connected, so voltage drop across CT will be negligible.

When CT gets open circuited, the impedance of CT become infinity, hence the voltage drop across CT primary will try to rise up to to the rated supply voltage, but in this process insulation fails.