KEB combivert R6-S Instruction Manual page 37

Troubleshooting and Diagnostics
Fault Cause
E.UP DC bus level is too low
E.PU Error on the power stage.
Solution
Likely indicates poor power quality.
Verify line input voltage. Sagging voltage or loss of
phase could cause E.UP error.
Check all grounding including X0 of transformer.
General error on the power stage. Check grounding
of the transformer. If the error is always active, then
the unit must be sent in for evaluation. If intermittent,
it may indicate excessive line sag conditions.
US - 37