The RDDRONE-FMUK66 includes three electrical fuses. They will blow when too much current goes through them, to protect the MCU and other components on the PCB.
Determining if a fuse is blown
It is easily determined if a fuse is blown by using a multimeter to measure if the fuse is still conducting. If you are certain that a fuse is blown, you will have to replace it by a similar fuse that can handle 500 mA.
The fuses have changed on recent boards to an automatically resetting type. You should never have to change them. If they do open, simply wait a very brief amount of time for them to reset. However you should determine *WHY* the fuse opened before attempting to use the board.
RDDRONE-FMUK66 has three fuses, two (F1 & F2) for the CAN transceivers, and one (F3) for the USB port.