about neutral ct

Are you asking about a current transformer on a neutral line?

The usual reason to put a CT on a neutral line is to find out what the neutral current is in the circuit. In a three phase circuit, if all of the three phases are perfectly in balance, there will be no current in the neutral line. There will be more neutral current as the imbalance between the phases increases. There will also be neutral current in a three phase circuit if there is a ground fault on one of the phase conductors.

In generator service the neutral current CT may feed into a protective relay to shut down the generator if there is too much neutral current.

I have seen a neutral CT used to find leakage current in a single phase circuit. If the current in the neutral line was not equal to the current from the CT on the hot conductor, there is a ground fault on that circuit and some current is returning to the feeder panel through the ground conductor.

I hope this made sense as you didn't give a lot of background.

Live a joyous life!
TT3

The CT on a neutral is used to measure the earth fault current, this is a standard protection system (51N).

Sometimes it is coupled with three current transformer on theother side of the generatoe or transformer this is named restricted earth fault, because it is protecting the equipment.

There is usually only 1 reason to measure the Amperage on a neutral; to monitor imbalance or ground fault. This is because the neutral serves as the unbalanced load current conductor. when there is a current on the neutral it is a result of either an imbalance between the 3 phase conductors or a ground fault.