The H3DK-HCL 100-120AC is a device from Omron that is used for monitoring 3-phase AC systems. It has a wide range of features, such as an integrated current transformer and 100A per phase current measurement, high accuracy monitoring of voltage, current, power factor, frequency, and harmonic distortion, as well as a small size and low power consumption. It also includes several safety features, including over-current detection, over-voltage protection, and a built-in fuse. It is an extremely reliable and versatile device that is ideal for industrial automation and power supply monitoring applications.