This sets the DC bus voltage threshold that, once the battery voltage sags to this level, disconnects load bank 3 from the bus to protect the batteries from over-discharge, per the description. Because the plant lets each of its three load banks disconnect at a different voltage, bank 3 is typically configured to disconnect last, at the lowest of the three thresholds, since bank 3 is typically wired to the site's most essential equipment. A setting of 400 (40.0 volts) means load bank 3 drops off the bus as soon as the battery sags to 40.0V during an extended AC outage.