Hi Guys, I was looking for the same solution. Moter is of 50hp and load is around plus minus 60-70 amperes (there is a fluctuation in current from 300 - 440 volts).
In My case I also have a magnetic starter and a relay is attached to the system to auto start the motor in case of power interruption (Load shedding).
Relay has three buttons two of which are labeled as "TIME" while the third one (the middle one) is labeled as "LOAD". We use Load button to choose the Load in amperes, if load exceeds the selected limit, relay turns off the motor (or in case of any fault).
A wire comes out the relay, which is normally called by electricians as earth or ground wire. We have a switch on this wire which is used to switch on and off the motor manually.
I want to use Arduino plus moisture sensor and other stuff to automate the motor. Do I need to incorporate another relay to that wire (which comes out of relay) or Arduino can handle it?