What is the function of neutral CT?

Answer 1

A CT is a current transformer, used to measure current flow in a conductor. Neutral is power return, usually grounded at the distribution panel. A neutral CT, then, is a device that measures the current flow in the neutral conductor. In a three phase star system, each phase returns current to neutral, but the three phases cancel each other out, resulting in effectively zero current in neutral. The neutral CT is used to detect an imbalance in the system, perhaps caused by a ground fault or by some failure in one of the phase loads. In a single phase, single ended system, there is current on neutral, so the value of a neutral CT is not so great. If you also had a hot CT, you could compare and detect imbalance between hot and neutral, which would be an indication of a ground fault. (Actually, a ground fault current interrupting device, also called a GFCI, usually compares current in hot and neutral simultaneously, because both conductors are wound together as the sensing transformer primary - any perceived current is a ground fault.) In a single phase, double ended system, such as the 120/240 split phase system used in the US, a current CT could indicate ground fault or a system imbalance but, usually, imbalance might be a normal situation as various loads are turned on and off.