Sag Mill Tripping on Earth Fault

First, with this being only six months old you might be under warranty. Contact whoever installed this for you and claim that you never touched it.

If for whatever reason, contacting the installer is not an option, then I see two possible scenarios. First, your ground fault sensing circuitry itself could be faulty. Depending on the design one should replace or repair the ground fault detector. The numbers you've cited imply that no ground fault exists. Second, you have an unexpected switch in this motor or starter. In the motor, the switch can be either from an overheating of the windings causing an insulation breakdown or mechanical abrasion of the windings. Either of these motor switch scenarios should be handled by the motor fabricator or installer. Possibly your Liquid Resistance Starter for this slip ring motor itself has an intermittent ground fault.

Good Luck.

In addition to others, keep these in your investigation:

- Is the motor rated for inverter-duty?

- Is the VFD cable used?

- Is the common mode current causing this problem?

For better response from this forum, you also can provide some more information like

- Does the motor trip right after start or after running a while

- The grounding of your system: solidly, LRG or HRG

- CT location (one line diagram of the motor)

- Multilin relay model (is it 469?)

Good luck.

- MS

Hi MSamad,

Thanks for your replay.

Is the VFD cable used? No, we are not using VFD.

Is the common mode current causing this problem? Not sure about this.

Does the motor trip right after start or after running a while? No, tripped on start-up in less 3 seconds.

The grounding of your system: solidly, LRG or HRG? Yes, it solidly earthed/grounding system we had.

CT location (one line diagram of the motor)? CT's are installed and located at the load side of the Circuit Breaker.

Multilin relay model (is it 469?) We are using 369 Multilin Motor Management Relay and Vamp Relay.

I am getting a current injection test instrument to test our Multilin and the Vamp relay at this point of time, as soon as have the result then I will posted on.

Regards,

Look for something that has changed since the commissioning records' dates. Reviewing those documents will shed some light on the problem.