Mike Dolt Enterprises
Mike Dolt Enterprises
User: tankfarms, Location: Texas by the Gulf, geat crab, great shrimp
Asked on: Monday, March 21, 2011 at 3:59 pm EDT
Answered on: Tuesday, May 14, 2024 at 3:04 am EDT
Question: For the past 2 days, I've been constantly getting a ground fault alarm (we have an ungrounded system here) on one of our 480V MCC bus. The alarm toggles simutaneously with the run status of a pump we have. (we have a DCS system here that we can monitor). So, it's simple, we isolate the pump, and check service/load.We got electricians check it, each phase-to-gnd megged good (2 Gohms), he disconnected the leads from the load side of the starter, and pulled the starter close, and no ground alarm; then he re-connected the leads back to the load side of the starter, and ran the motor, again, no ground alarm either, and from this point on, we no longer get this ground fault as we run the motor.Just some background infoThe pump is on a level control, so it's not continous runMotor FLA=86amp, 3-PH, 75HP, SF=1.15, 480Vit was measured that the motor was pulling approx. 100amps when it runcould it be possible that something (thermally) is going on inside the motor? as I know the motor is overloaded at the moment, which is a seperate issue we've been working on, but could it be this causing some intermittent insulation problem which then causes a ground fault alarm?At the time writing this, the motor kicks on/off with a level control, and no ground alarm is observed at the moment. kinda mysterious here.Any input is welcome. Thanks.
Possible Causes
Motor insulation breakdown
The overloaded motor could be causing excessive heat, leading to insulation damage and intermittent ground faults.
Loose connections
Thermal expansion and contraction could cause loose connections, leading to intermittent arcing and ground faults.
Moisture accumulation
Condensation or other moisture sources could create a conductive path to ground, causing intermittent ground faults.
Defective starter
A malfunctioning starter could be intermittently completing a circuit to ground, even when the motor is not running.
Faulty wiring
Damaged or loose wiring in the MCC bus or load circuit could create a ground fault path.
Transient overvoltages
Voltage spikes or surges can cause insulation breakdown, leading to intermittent ground faults.
Troubleshooting Steps
Monitor motor temperature
Check if the motor is overheating using thermal imaging or temperature sensors.
Inspect connections
Tighten any loose connections in the MCC bus, load circuit, and motor terminals.
Check for moisture
Look for signs of condensation or other moisture sources around the motor, wiring, and MCC bus.
Test starter
Isolate the starter and perform continuity checks or use a megger to ensure proper operation.
Inspect wiring
Check for any damage, loose connections, or insulation breaches in the MCC bus and load circuit.
Monitor voltage
Use a power quality analyzer or scope to check for transient overvoltages or voltage fluctuations.
Recommendation:
Given the intermittent nature of the ground fault alarm, it's crucial to continue monitoring the situation while troubleshooting the potential causes. If the overload issue is resolved and the ground fault alarm persists, consider further testing or replacement of the motor. Also, schedule a comprehensive inspection of the MCC bus and load circuit to identify any underlying issues that may contribute to the problem. Flag for review