I'm just wondering what's the difference between a simple power supply with a ground prong and one without. I'm talking about those little DC power supplies with barrel plugs you can easy find on any small appliances. As far as I know a barrel plug has two contacts: one positive and one negative. What's the role of the third ground prong on the AC side for those have it? I hope I'm not mistaken but a small DC power supply like that use floating voltage right? With all plastic construction I don't think it can do much in terms of safety.
Actually I took two PS and toyed with it. One is relatively higher quality with a 3 pronged IEC C13/C14 plug. The other one has a cheapo 2 pronged design. I measured the DC voltage (with a multimeter) between the "wall-socket ground" and the sleeve of the barrel plug. The 3 pronged one resulted a 0 V, while the 2 pronged one showed a fluctuating ~300 mV potential difference. So does that mean the 3 pronged power supply has the negative terminal actually grounded? If it is what's the pros and cons of doing that?
I'll greatly appreciate any input! Thank you!