A three-wire electric stove or dryer plug and cord would be like this:
Hot #1, Neutral, Hot #2
240 volts across hot #1 and hot #2. 120 volts across each hot and the neutral. You would also have to bond the chassis (the metal case) to the neutral.
A four wire plug and cord would be like this:
Hot #1, NEUTRAL , Hot #2
The added ground (always a green wire in the cord) is to bond the metal housing of the device to EARTH, so that way if any loose current-carrying conductor were to short-circuit with the metal cabinet the charge would get safely carried to the ground instead of through YOU.
The voltages across the two hots and across each hot and neutral (the white wire in the cord) are the same as the three wire configuration. There must be no voltage across neutral and ground (EARTH) conductors!!
I tried to use the simplest layman language I could to explain this. Please correct me if I have made ANY error.
[This message has been edited by SvenNYC (edited 04-09-2003).]