VFD-rated motor cable
First part to price when insulation damage or output-cable leakage is found.
Siemens SINAMICS G120 Fault F30021, What It Means
Siemens SINAMICS G120 Fault F30021 means the power unit detected a ground fault. The drive sees leakage or a grounded condition in the motor circuit, power cable, brake circuit, or connected output components. The trip is there to protect the power module from catastrophic output-stage damage.
In the field, F30021 usually traces back to a motor winding fault, wet or damaged cable, contaminated terminal box, or brake wiring issue rather than a bad parameter.
Common Causes
- Motor winding short to ground. This is especially common on older motors running on long VFD leads.
- Damaged output cable. Crushed tray cable, liquid intrusion, or poor terminations can leak to ground.
- Moisture in the motor junction box. Outdoor fans and pumps often trip after washdown or weather changes.
- Brake resistor or brake wiring fault. Broken insulation or bad wiring around braking components can trigger F30021.
- Failed output reactor or filter. Downstream components can create a ground path the power unit detects immediately.
- Internal current transformer or power module problem. Less common, but possible if the fault remains with the load removed.
Step-by-Step Fix
- Lock out the equipment and disconnect the motor circuit from the drive. Separate the G120 from the motor leads before doing insulation testing.
- Megger the cable and motor separately. Test each phase conductor to ground on the cable, then test the motor windings independently. This is the fastest way to stop guessing.
- Inspect the motor terminal box. Look for moisture, conductive dust, carbon tracking, damaged insulation sleeves, or a loose ground lug touching a phase conductor.
- Check brake resistor and brake wiring. If the application uses dynamic braking, inspect those conductors and terminations carefully. Grounded brake wiring can look like a power unit fault.
- Inspect any output reactor, du/dt filter, or sine filter. Disconnect and test these components if present. A failed output accessory can trip the G120 even when the motor is healthy.
- Power the drive with the motor still isolated. If F30021 clears, the problem is downstream. If the fault stays active with nothing connected, suspect the power unit.
- Review cable routing and shielding. Poor VFD cable practice, damaged armor, or liquid-filled conduit can create repeat failures that come and go with temperature and humidity.
- Reconnect and perform a controlled test run. Bring the motor up slowly and watch whether the fault returns immediately or only under heat and load. That timing helps distinguish cable damage from motor insulation breakdown.
Parts Often Needed
| Part | Notes |
|---|---|
| VFD-rated motor cable | Best replacement when tray or conduit cable is leaking to ground |
| Insulation resistance tester | Essential for proving whether the motor or cable is the culprit |
| Brake resistor assembly | Relevant where the brake circuit wiring has failed |
| Output reactor / du-dt filter | Useful on long leads and can be the failed component itself |
| Replacement inverter-duty motor | Needed when megger results show winding breakdown |
Technician Tips
When diagnosing VFD faults, always check the drive’s fault history before resetting. Modern drives store the last 5-10 faults with the drive state at the time (running, accelerating, decelerating, stopped). An overcurrent fault that occurs only during acceleration points to incorrect motor data or mechanical binding. The same fault during deceleration suggests regeneration issues. A ground fault that clears after disconnecting the motor and returns when reconnected means the motor or cable insulation has failed.
Prevention: Schedule annual infrared scanning of drive cabinets. Loose power connections and failing capacitors produce heat long before they cause faults. A 5-minute scan can prevent hours of unplanned downtime.
Need Replacement Parts?
Before ordering, match the drive part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Professional
Call Siemens support or a qualified drive shop if F30021 remains with the motor and downstream components disconnected. That usually points to an internal power module or sensing issue, and continued field resets can turn a repairable problem into a full drive replacement.
See Also
- Siemens SINAMICS G120 Fault F00001, Overcurrent Guide
- Siemens G120C Fault Codes, Common Trip Guide
- Siemens VFD Fault Codes, What They Mean
- Siemens Micromaster Fault F001, Overcurrent Causes and Fix