This article addresses plugging a standard 5V USB connection into a 12V or 24V USB port on an Elo All-In-One Touchcomputer. 

The way the 12V and 24V USB ports are designed, the lower portion of the USB port delivers the standard 5V power and data communications. The upper portion of the USB port delivers power (either 12V or 24V). Therefore, connecting a standard USB device into the lower portion of the powered USB ports will not harm anything, it will behave as a standard USB port. It is physically impossible to connect a standard USB device into the top section of the powered USB ports.