Current transformer location
for variable-frequency drive ground-fault protection
Low-voltage adjustable-speed drives (up to 600 V) typically have a simple six-pulse rectifier input stage which creates a dc bus from which an inverter stage creates an output of controlled frequency and voltage. This output is used to control the speed of the attached motor and driven load for process control.
A core-balance zero-sequence current transformer (ZSCT), connected to a residual-current monitor, can be used to detect a ground fault in the drive, cabling, and motor. The zone of protection is downstream (load side) of the CT. If installed at the supply motor control center (location 1 in the figure), a ground fault in the motor and the cable between the motor and drive will also be detected.
This location provides the largest protection zone. If installed at the drive input (location 2), the drive is additionally included in the zone of protection. And, if installed on the drive load side (location 3), a ground fault in the motor and the cable between the motor and drive will be detected.