That is why we originally figured they were for a ground fault in the motor. We said the same thing about the phase CT's in the starter at the substation, but apparently they use the core balance CTs for that purpose at the motor as well. I am new to this industry so I can't really argue this...
I would share if I knew. The client with the motor has another contractor company do all of their power studies and Schweitzer relay programming and that is who I spoke to. They didn't provide me the details on the calculations they've ran. They just said the calculation show the 600:5A to work...
The graphic is correct as to how the motor is installed with the CT's in the terminal box and they are core balance CT's.
After speaking with someone that programs the relays for these motors, he told me the facility uses these CT's for phase to phase faults inside the motor. They use the HRG...
Edit: I am thinking the CT is not suppose to measure any current, so I don't think it would need to be rated for FLA, but I'm not sure. Also, the system is on an HRG, so if there was a ground fault in the motor it should see more than 5A of fault current.
Can anyone please explain to me how to select the size for the primary of a three phase motor's core balance (zero sequence) differential current transformers. The secondary will be 5A.
Thanks,