I was installing my power supply on my cooler master case a while ago. The PSU is not plugged to any outlet. I also was wearing an anti-static wrist band just to be sure. Just to let you know I was working on my computer down stairs and not on area that's carpeted. I accidentally pressed the on button of my psu. Like I said the psu is not plugged to any outlet. Would this damage my psu?
The power switch on the PSU or the power button on the case? In either case you probably don't have a completed circuit for any static to pass through.
Anyway, if you didn't feel a static discharge on your finger when you pressed the button then there wasn't any static in the first place.
Have you actually tried running power through it? Has anything happened that's actually given you reason to think something's wrong? And if this happened a while ago, what's made you think something's wrong now?
Even if it were plugged in, you will not damage a PSU by simply turning it on. It's just doing what it's suppose to do. Now, turning it on while you are installing or removing other components is a different matter.