Danfoss VLT AutomationDrive FC 302 fault A4: Mains Phase Loss
Alarm 4 on a Danfoss VLT FC 302 is Mains Phase Loss: a phase is missing on the supply side, or the mains voltage imbalance is too high. Danfoss notes the same message appears for a fault in the input rectifier.
Diagnose this with AI
Try Free Drive Diagnostic Upload a keypad photo, the fault history or a parameter export from your Danfoss VLT AutomationDrive FC 302. InduQT compares the evidence and suggests the next useful checks.
When it happens
- While running under load
- After a fuse or breaker on one phase opens
- On a weak or unbalanced supply
Likely causes and checks
1Blown input fuse or open breaker pole
- Points to it
- Sudden onset; one phase reads near zero at the drive input.
- Check
- Measure all three phase-to-phase voltages at the drive input terminals and check fuses (with power locked out for the fuse check).
2Loose or burnt input connection or contactor contact
- Points to it
- Intermittent trips, heat discoloration on a terminal or contactor pole.
- Check
- Lock out, inspect and torque the input terminals; check contactor contacts.
3Supply voltage imbalance
- Points to it
- All phases present but unequal; other drives on the same supply may warn too.
- Check
- Measure supply voltages and supply currents to the drive under load, as Danfoss directs.
4Input rectifier fault
- Points to it
- Supply is balanced and complete but the alarm persists.
- Check
- Refer to qualified service; the rectifier is inside the drive.
Telling it apart
- Alarm 4 points to the input side; Alarms 30–32 Motor Phase U/V/W Missing point to the output side between drive and motor.
- Whether Alarm 4 warns or trips is set in 14-12 (Function at / Response to Mains Imbalance).
- A lost phase can also drive the DC link low and show as Alarm 8 DC Undervoltage.
Relevant parameters
| Parameter | Name | Why it matters |
|---|---|---|
| 14-12 | Function at Mains Imbalance (Response to Mains Imbalance in software 9.5x) | Selects how the drive responds to imbalance or phase loss. |
Evidence to collect
- Alarm log time
- Phase-to-phase voltages and phase currents at the drive input under load
- Fuse and contactor condition
- 14-12 setting
Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.