You can manually focus most, but not all, Canon USM lenses without any power supplied from the camera. In fact, you can manually focus them when they're not even attached to a camera.
The EF-S 17-55mm f/2.8 is one such lense that allows the focus ring to move the focus elements via a mechanical connection that is not dependent upon power and other data signals supplied to the lens. This is the normal way that USM lenses are designed and made by Canon.
There are a very few USM lenses that are focus-by-wire and do require not only power but a specific instruction from the camera via the data connection to move the focus elements because there are no mechanical connections between the focus ring and the focusing elements.
Although not necessarily all-inclusive, here is a list of Canon USM lenses known to use focus-by-wire:
- EF 85mm f/1.2 L
- EF 85mm f/1.2 L II
- EF 50mm f/1 L
- EF 28-80mm f/2.8-4 L USM
- EF 200mm f/1.8 L USM
- EF 300mm f/2.8 L USM
- EF 400mm f/2.8 L USM
- EF 400mm f/2.8 L II USM
- EF 500mm f/4.5 L USM
- EF 600mm f/4 L USM
- EF 1200mm f/5.6 L USM
Only the EF 85mm f/1.2 L II, highlighted in bold, is a currently available, in production lens. All others have been discontinued.
Don't confuse the older non-IS Super Telephoto series lenses listed above with the first generation of IS Super Telephoto lenses. All of the first generation of IS (Image Stabilization) Super Telephoto lenses have mechanical manual focus capability. The same can be said about not confusing the listed older non-IS EF 400mm f/2.8 L II with the current EF 400mm f/2.8 L IS II. The current generation "II" versions of the Super Telephoto IS lenses all have both mechanical and focus-by-wire manual focus capability.
Do note that all STM lenses produced by Canon use a different type of focus motor and are all focus-by-wire.