Question

The precaution to be taken while using current transformer?

Answer 1

If you have a solid core current transformer, you have to break the high voltage circuit to thread a wire through the core. Turn it off first.
If you have a split core CT, you can just clamp it around the wire, which in principle can be done without turning it off, but one does need to be careful.
The most important thing about CTs is NEVER LEAVE THE SECONDARY OPEN CIRCUIT. Always have a load across the CT. It can develop very high voltages if left open circuit.