Unmesh: even using an optical couple with 1500 volt isolation is not enough as induced spikes and back emf can exceed 3 times the supply voltage. I did a similal system for a seagoing dredge that uses four 3000 volt motors and four 3000 volt generators in a series loop for load sharing the power plants were railroad locomotive engines. I suspect that your dragline is also self powered. due to the power levels involved and the safty hazard to both the equiptment and to people there may be no ground connection , but there should be a fault detection circuit that may cause you serious problems , both to your circuit and to your health. You can not upset this circuits operation with your sensing. Use a contactless sensor powered by a properly isolated supply then further protect your electronics by opto isolation. never work on this whin the hv supply is live and never work on it without a knowledgeable safty partner. If you need more info , i will see if i can find the schematic and help you with the isolation details. work safly and enjoy Terry