I would like to know what protection class means here. As far as I know, there are 3 UPS protection classes, which are specified in the standards DIN-EN 5009-6, IEC 62040-3, IEC 61002-2 and VDE 0558. The highest protection class is protection class 1, which is resistant to all forms of mains interference. Systems with this protection class are independent of the voltage and frequency of the power grid, Voltage and Frequency Independent (VFI). The VFI classification states that the UPS output behaves independently of changes in the mains voltage and frequency. Protection class 2 protects against faults 1 to 5 and is independent of the input voltage, Voltage Independent (VI). And protection class 3 only protects against faults 1 to 3 and is dependent on the mains voltage and mains frequency, Voltage and Frequency Dependent (VFD). You describe the devices offered with protection classes from 1 to 9. I would like to have the explanation here.