The difference between 3 pronged and 2 pronged power supplies

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!

• Redo your measurements: (1) Unplug the power supplies. (2) Switch your meter to resistance. (3) Measure from earth pin to barrel jack. Nov 4, 2017 at 0:49
• @Transistor The resistance is <1 Ohm. And the meter showed positive in continuity test. So can I deduce the sleeve is grounded?
– Den
Nov 4, 2017 at 0:58
• Do they both have a "square within square" symbol on them? Nov 4, 2017 at 3:59
• fyi, an example of a small appliance is a toaster or a blender. i think that you mean a small electronic device Nov 4, 2017 at 4:42
• @ThreePhaseEel Only the 2 pronged one has that symbol. That's a double insulation symbol right?
– Den
Nov 4, 2017 at 5:51