What is a USF fault on the ATS48 (or ATS46) Soft Starters?



Why is my ATS48 tripping on USF faults?
Product line:  
Altistart ATS48 and ATS46
All models, all serial numbers
Run command on Run/stop without having mains voltage on L1 L2 L3 within 250 ms of the run command.    
The soft start checks for three-phase AC power at the L1, L2, and L3 terminals as soon as a RUN command is received. This fault appears if the soft start has a RUN command and does not have line power in all three phases within 250 ms. This fault is stored in the fault history. Probable cause on a soft start with separately supplied control power:
• The line-supply circuit breaker, line fuses, or isolation contactor is open.
• The power board is defective.
• Determine why the three-phase power is not present at the L1, L2, and L3 terminals of the soft start when it receives a RUN command. Rectify the power supply problem.
• Verify the soft start's line voltage detection circuit by supplying only control voltage and verifying that nLP displays. Then apply a three phase line voltage and verify that RDY displays. If not, verify all internal soft start connections. If no problem is found, replace the power board.