Yeah, it should be safe. Just make sure the PSU is disconnected from the MB and other sensitive components like GPU's.
And make sure when not to short anything when connecting the green wire to a black one on that main power connector. Green to black - not red to yellow (mistakes can happen
Most importantly do not go further and open the PSU, unless you know what you are doing, and even then its an unneccessary risk. Those caps can kill you if you dont know how to discharge them, and accidently touch them the wrong way