Cause: Supply Mains Under Voltage comes when the electrical power supplied to a system or device is lower than the required or nominal voltage level. 

"USF" fault in Schneider ATV950 drive indicate "Mains Supply Under Voltage" condition. This means the input voltage from the mains supply is below the drive's acceptable range. This can impact the drive’s performance and potentially cause operational issues.

Here’s a structured approach to troubleshoot and resolve this issue:

1. Initial Checks:

• Restart the Drive: Power off the drive, wait a few minutes, and power it back on to reset any transient faults.
• Review Fault Logs: Check the drive’s fault logs for additional details about the mains undervoltage event.

2. Measure Input Voltage:

• Voltage Measurement: Use a voltmeter or multimeter to measure the input voltage at the drive’s input terminals. Ensure it is within the specified range for the drive.
• Three-Phase Balance: If using a three-phase supply, check that all three phases have balanced voltages.

3. Verify Power Source:

• Stability: Ensure the power source is stable and capable of supplying the required voltage and current for the drive.
• Capacity: Confirm that the power source has enough capacity to handle the drive and other connected loads without significant voltage drops.
• Voltage Fluctuations: Identify any patterns of voltage fluctuations and determine if they coincide with other equipment starting up or heavy loads operating.

4. Inspect Connections:

• Wiring Integrity: Inspect the wiring and connections between the power source and the drive for any signs of damage, loose connections, or corrosion.
• Cable Sizing: Ensure the cables used are of the correct size and rating to handle the load without excessive voltage drop.

5. Check for External Factors:

• Load Distribution: Ensure that other high-power equipment connected to the same power source is not causing significant voltage drops.
• Environmental Conditions: Verify that the drive is operating in an environment free from excessive heat, dust, or humidity that might affect electrical connections.

6. Inspect the Drive’s Components:

• Rectifier Stage: Ensure the rectifier stage of the drive is functioning properly and not contributing to the low input voltage.
• DC Bus Capacitors: Check the condition of the DC bus capacitors as aging or failing capacitors can affect voltage levels.