What is the voltage drop in the AC withstanding voltage test (ACW) for TOS5300 series?

The voltage regulation in AC output is 10% or less of the maximum rated voltage, which is 5000 V.
This means that the voltage may fluctuate up to 500 V depending on the load (when changing from the maximum rated voltage to no load).
For more details, read the user’s manual – chapter of Specifications.