The three wire config is two lives plus a neutral.
There is 120 volts between each live conductor and the neutral and 240 volts between the two lives.
The four-wire config is essentially the same as far as the voltages across the lives and neutrals.
However there is a ground conductor (just like the round pin on the power cord on your computer or printer) that grounds the metal case of your dryer to protect you from a potential electric shock.
I imagine this green conductor is what's called the euquipment ground conductor, right?
PS I don't like that bottom picture. Where's the strain relief? A bit of rubbing from the sharp edge of that hole and you can have the conductors touch each other or the case itself. Hope it was only removed for demonstration purposes and this is not how the actual dryer is used.
(I have a 110-volt 15-amp electric dryer at home. It has a strain relief at the hole where the cord enters the cabinet )
The dryer frame is still grounded with the 3-wire cord, but the grounding is effected by bonding it to the neutral. In the top picture you can see the grounding strap onto the center terminal.
The neutral is normally at (or very near to) ground potential, but the drawback of the 3-wire system is that if the neutral suffers a break somewhere along its run, then the frame of the dryer will end up being energized at 120 volts. That's because although the heating elements run on 240V, the control circuitry is connected from one hot to the neutral to run on 120V. With a broken neutral, the frame is then connected to the hot wire by way of the controls. The 4-w system removes this potential danger by keeping the neutral and equipment ground separate.
Which brings up a query on the lower picture: There is a green wire still shown connected to the center (neutral) terminal. Where does the other end go? If this connects to chassis inside the dryer, then it should have been disconnected at the terminal block.
Some of the Appliances I've seen do fail to show or mention on their diagrams that something must be disconnected. The information is buried inside the installation manual somewhere though. I've got some pics around somewhere. I'll see if I can locate them.
ThinkGood In the previous thread you said the dryer was "now wired with 10/3" I was assuming you meant you repulled it with 10/3wg to upgrade it, or are you meaning 10/2 with ground. If it is 10/2wg you will gain no benifit from the 4-wire cord/rec. If you have the 10/3wg and 4-wire cord/rec, it will as the others have said avoid the possibility of line voltage on the frame by separating the grounded conductor and EGC.
The first pic shows the bonding strap I spoke of that would be removed with a 4-wire cord/rec. and the EGC from the cord will be attached to the frame.
Sorry for going on, hope this helps.
[This message has been edited by lwinter31 (edited 01-08-2003).]