Cause: AFE Mains Undervoltage comes when the input AC voltage is lower than the drive's nominal rating, resulting in a reduced DC bus voltage.

"UrF" fault in Schneider ATV950 drive indicate " AFE Mains Undervoltage" condition. This means the DC bus voltage is insufficient because the mains (input) voltage is below the acceptable range. This condition can impair the drive 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 for a few minutes, and then power it back on to reset any transient faults.
• Review Fault Logs: Check the drive’s fault logs for additional details about the low DC-bus voltage event.

2. Inspect Power Supply:

• Measure Input Voltage: Use a voltmeter to measure the mains voltage at the drive’s input terminals. Ensure it is within the specified range for the drive.
• Power Quality: Check for issues such as voltage sags, dips, or fluctuations. These can cause the DC bus voltage to drop.

3. Verify Power Source:

• Stability: Ensure the power source is stable and capable of supplying the required voltage and current for the drive.
• Capacity: Check if the power source has enough capacity to handle the drive and other connected loads without significant voltage drops.

4. Inspect Connections:

• Wiring Integrity: Inspect the wiring and connections between the power source and the drive. Look for loose, damaged, or corroded connections.
• Cable Sizing: Ensure that 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 a suitable environment, free from excessive heat, dust, or humidity that might affect electrical connections.

6. Inspect the Drive’s Components:

• DC Bus Capacitors: Check the condition of the DC bus capacitors. Aging or failing capacitors can contribute to low DC bus voltage.
• Rectifier Stage: Ensure that the rectifier stage of the drive is functioning properly and not contributing to the low DC bus voltage.