Hi!
Current transformers don't often go defective unless the primary is physically damaged or open circuit!
The difference between a voltage and a current transformer is that a voltage transformer is connected across a power supply and takes a primary current according to the load placed on its secondary (or secondaries), whereas a current transformer is connected in series between the supply and the load, and produces a given output current directly proportional to the primary current, the primary voltage being very small.
Because a current transformer has a magnetic flux in its core proportional to the primary current, it is IMPERATIVE that a load is always connected across the secondary of a current transformer, otherwise a very high voltage would be developed across the secondary enough to cause breakdown if the secondary is left open–circuit, but a short· circuit across the secondary of a c.t. normally does no damage as a rule, therefore it would not normally be necessary to replace it, unless o/c as per my first sentence!
Chris Williams