What is the significance of 4th CT input in PowerLogic PM5500 series meter model.

The fourth current input (I4) can be used to measure current flow (In) in the neutral conductor, which can then be used to calculate residual current. The meter refers to residual current as ground current (Ig).

For 4-wire Wye systems, ground current is calculated as the difference between the measured neutral current and the vector sum of all measured phase currents.