I see a lot of USB "charging" cables on the market with cutaways showing two small data conductors, one large power conductor, and shield.
The implication is that power is conducted in the shield.
Is that really appropriate for USB cables?
Specifically USB, with its wide variety of devices. I realise that there are plenty of application-specific phantom power configurations where the devices are designed appropriately for that situation.
Best Answer
Considering that shielding the data wires is desirable to keep interference away, and considering that shielding is virtually always grounded, it should come as no surprise to find the ground conductor and the overall shield combined. In fact, in most kinds of shielded cables, that is common practice.
There are rather complex "handshaking" protocols established for USB peripherals to negotiate for how much power they require from the host. And for "dumb" hosts like simple chargers, Apple has a scheme for setting the D+ and D- pins at fixed voltages to indicate how much current is available. For example...