I just plugged a 300V power cable into power supply that accepts 115-240V then turned on the power supply. The power supply was not connected to the motherboard. I don't have another cable so I can't really check if its still working nor do I want to try again with the 300V cord if it can potentially cause damage.
Where did you get a 300V power source? Why would you plug a 115-240V power supply into it? Didn't you need some sort of adapter to plug a pc power cord into 300 volts? Or was it just an ordinary power cord with 300V stamped on it? If that was the case, that would be the maximum safe voltage for the cord.
If you plugged a power cable running 300V then there definitely is a risk that you damaged the power supply. However I think you mean you plugged in a power cord with a rating of 300V. This simply means the power cord is designed to tolerate this much voltage. It would be difficult for a domestic user to have access to 300V - in the UK the domestic voltage is 240V, in the US 120V So no you haven't done any damage
It needs to be connected to the motherboard, before it will operate. 300V, just means the capacity of the cable.
Just because the cord is rated 300V does not mean that you would get 300V from it. That just needs to have at least as high of a voltage rating that you would be putting through it. What matters is what is the AC voltage of the wall outlet you would be plugging it into?
