I observed people doing experiments with 5V supply of the PC and after a while they came up with total fire? They could not believe that the PSU would deliver so much power (5V x 13A = 65W); because, it is just a 5V supply!
That's why in UNO Board, the manufacturer has installed "Polymeric PTC Resettable Fuse 15V 500 mA" type high quality fuse in the 5V line to protect the PC and the user's hardware setup in case of possible shot circuts/overload.