Otis · DSD 412 DC Elevator Drive
The input AC line voltage sagged more than 10% below the nominal line voltage as set at parameter #9. This does not shut down the drive but is an indication that work is needed to avoid future F904 Low Line faults.
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Measure the input AC line voltage.
Check and correct the setting of parameter #9 (nominal line voltage).
Investigate the AC power supply for voltage sags or instability.