The maximum voltage and time period that can be sustained between the motor case and the winding.
They are defined in the Electrical Appliance and Material Safety Act as follows.
Working voltage 30 V or less → AC 500 V for 1 minute
Working voltage 30 to 150 V → AC1000 V for 1 minute
Working voltage 150 V or higher → AC1500 V for 1 minute.