What is the voltage phase at the beginning of the voltage curve of the AC withstanding voltage test performed by TOS series?

All our withstanding voltage testers of TOS series are designed to start the testing at approx. 0-volt level; however, it is not specified by safety standards.
For TOS series with a variable-voltage transformer circuit, it is to prevent the output voltage surge.
For TOS series with a PWM switching amplifying (PWM-based amplifier) circuit, it is to improve the output voltage waveform.