Cause : DC link voltage undervoltage comes when the power unit has detected an undervoltage condition in the DC link. This can be caused by line supply failure or line supply voltage below the permissible value.

F30003 fault in siemens 6RA80 DC Drive indicate DC link voltage undervoltage. This means the DC link voltage falls below its rated limit. This is a critical fault, as excessive DC link voltage can damage the drive. 

1. Check the line supply voltage.
2. Check the line supply infeed and observe the fault messages relating to it (if there are any).
3. Check the line supply phases.
4. Check the line supply voltage setting.