skit75 :
Ok, while it isn't a voltage... it is a Ground Sense return to the supply and you should not run the supply without a load. It will begin to overheat. Make sure you have something it can power, hooked up to it.
A PSU will not overheat with no load, they all have the minimum load resistors built in specifically to avoid this otherwise you'd be blowing out your PSU everytime you went into standby, doing the paper clip test to jumpstart the PSU is a legitimate test and doesn't require a load, but hooking a fan up gives you a better indication of if the PSU is outputting power especially with PSUs that may not kick the fan on until a certain temperature is reached.
As for the original question, the power switch on the back of a PSU will not turn the PSU on, many people think that it should do something when connected to the wall, it will not do anything unless you get something to send it the on signal, be it your motherboard or a paperclip