Danfoss FC 302 Alarm 14: what to check first for an earth fault?
My Danfoss FC 302 is tripping on Alarm 14, Earth Fault. It happens at various speeds on a conveyor motor that flexes quite a bit. What should I check first?
Alarm 14 on a Danfoss VLT FC 302 indicates an Earth (Ground) Fault, meaning current is flowing from an output phase to ground. This can occur in the motor cable or the motor itself. The drive detects this by comparing the current going out to the motor with the current returning, tripping if the difference is too large. This fault can appear at startup, after idle periods or wash-downs, or during operation, especially under vibration or after drive service.
Likely causes and checks
1Motor cable insulation damage
- Points to it
- The motor cable insulation may be damaged, allowing current to flow to ground. This is common if the cable runs through wet conduits, flexes with the machine, or has been recently disturbed.
- Check
- Lock out power and wait for the discharge time. Disconnect the cable at both the drive and the motor. Use a megohmmeter to test each conductor to ground.
2Motor winding insulation breakdown
- Points to it
- The motor's winding insulation might have broken down, causing a path to ground. This is more likely in older motors, those that have overheated, or motors operating in wet or dusty environments.
- Check
- If the cable tests good, disconnect the motor from the drive and megger the motor windings to ground.
3Moisture or contamination in the motor terminal box
- Points to it
- Water, tracking, or debris in the motor terminal box can create a path to ground. This often causes trips on the first start after idle time or a wash-down, and may clear once the motor warms up.
- Check
- With power locked out, open the terminal box and inspect for moisture, tracking, or debris. Retest insulation after drying.
4Current sensor offset in the drive
- Points to it
- An offset in the drive's current sensors can falsely trigger an earth fault. This is particularly relevant if the alarm started after a power card change and the motor and cable insulation test good.
- Check
- Reset the individual offset in the three current sensors in the drive by performing a manual initialization or a complete Automatic Motor Adaptation (AMA).
Next check: Given the cable flexes and the fault occurs at various speeds, start by checking the motor cable insulation for damage using a megohmmeter.
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