The fault code "7802" on Siemens drives indicates a "Power Unit Not Ready" fault. This fault typically occurs when the power unit within the drive is not functioning properly or has failed to initialize correctly. Here's how to address it:

1. Check Power Supply: Verify that the power supply to the drive is stable and within the specified voltage and frequency range. Ensure that there are no issues with the mains power supply, such as fluctuations or interruptions.

2. Inspect Power Unit: Check the power unit components within the drive, including capacitors, rectifiers, and other electronic components, for any signs of damage or failure. If any components appear damaged or burnt, the power unit may need to be repaired or replaced.

3. Reset Drive: Attempt to reset the drive to clear the fault and allow it to reinitialize. Follow the procedure outlined in the drive's user manual or technical documentation for resetting the drive.

4. Verify Control Wiring: Check the control wiring connections between the drive and external control devices, such as PLCs (Programmable Logic Controllers) or control panels. Ensure that all connections are secure and properly terminated.

5. Review Drive Status Indicators: Check the status indicators on the drive's control panel or display interface for any additional information about the fault. This may provide more details about the specific issue with the power unit.

6. Inspect Cooling System: Ensure that the cooling system for the drive is functioning properly. Overheating can affect the performance of the drive components, including the power unit. Check for blocked ventilation openings, dirty cooling fins, or malfunctioning cooling fans.

7. Check for Ground Faults: Inspect the drive system for ground faults, which can affect the operation of the power unit. Use a multimeter or insulation tester to check for ground faults and address any issues found.

8. Consult Technical Documentation: Refer to the drive's user manual, technical documentation, or contact Siemens technical support for specific guidance on troubleshooting and resolving power unit not ready faults.

By following these steps and addressing the potential causes, you can troubleshoot and resolve the power unit not ready fault (7802) in your Siemens drive.