Voltage Rating. What exactly does mean?

This may be a dumb question, but I cant find an answer anywhere.

I have a couple a three prong twist-lock plugs and receptacles with a rating of 120v 30A. Can each prong handle 120v? Being that there will never be more than 120v in any wire, Can I do 120v-120v-G for a 240v element? Or does it need to be 120v-N-G? I don't want to melt anything by just trying it out...

They are standardized so that you can't accidentally plug an appliance into the wrong voltage or amperage.

They are UL listed for a specific purpose.any use outside of that listing would be a code violation and likely unsafe.

It means don't go there. ALWAYS use code approved (NEC or your local building code equivalent) connectors. If it's 240 volt, only use 240 volt connectors. If it's 120 volt, only use 120 volt connectors.

Yes, for a 240v element you will have two 120v lines powering it BUT the lines are 180 degrees out of phase meaning that while one is at +120v the other is at -120V. Therefore there will be 240v across the insulation of the cable even though the voltage from one line to the neutral or ground is only 120v.

Voltage ratings signify the strength of the insulation to protect against arcing while current ratings are influenced by the gauge of the cable and its ability to dissipate heat while carrying current.

So for example, car audio cable may be quite thick and be able to handle 30 amps but automotive systems are 12v and do not experience the same high voltages as in home wiring. This is a case where the voltage rating is a limiting factor.