Specification, Section 7.1.2 & 7.1.5
USB 3.0 Specification, Section 220.127.116.11
In order to measure the power distribution of a USB device, the average current is measured during unconfigured, configured, active and suspend state with a digital multimeter and fixture to measure the Vbus current . The circumstances for measuring the average current are dependent on the speed of the device and the power mode the device is in (e.g. self powered, bus powered or battery charging capabilities). All High Speed measurements are performed by connecting the device under test after one self-powered High Speed hub. A Full Speed self-powered hub is connected to the first High Speed hub in order to force a High Speed device under test to enter its Full Speed mode.Beware that according to USB 2.0 Spec Vbus should stay between 4.75V and 5.25V - so this means that the worst case current will be measured at 5.25V.
For a High Speed device the average current is measured in High Speed and Full Speed mode. For a Full Speed device the average current is measured in Full Speed mode. For a Low Speed device the average current is measured in Low Speed mode. A Super Speed device is measured in Super Speed and High Speed mode.
A device should also be measured using the supported power mode in the following circumstances:
1) When a device is only capable of operating in self-powered mode, all measurements are per formed in self-powered mode. This means that the device is unable to enumerate without being connected to an external power.
2) When a device is capable in operating in bus-powered mode, all measurements are performed in bus powered mode even when the device claims to be self-powered (in its device descriptor).
3) When a device has battery-charging capabilities over USB, the power measurements are performed in worst case scenario, most probably this is when the product has a dead battery.
The USB 2.0 device under test is set in unconfigured state by using the tool USB20CV. The USB 2.0 unconfigured current must remain below 100mA at all times. For a USB 3.0 device the unconfigured state can be force by using USB30CV. A USB 3.0 device may consume 150mA in unconfigures state. The unconfigured current is measured in the above conditions.