Re: Electrode
Do not know if this is the original subject or not but surges occur in either differential mode, or common mode. Which mode depends on where the surge occurs in relation to where you are.
For instance, if lightning were to strike the utility distribution line ahead of your service transformer you would recieve a differential mode surge. This is because of your transformer. Even though the surge was between ground and phase, once going through the transformer it changes the surge from phase to neutral, and phase to phase. This is why it is important to have a class "C" device installed at the service entrance with "all modes" (phase-to-phase, phase-to-neutral, and phase-to-ground)except neutral-to-ground. N-G isn't nessecary because of the N-G bond at the service entrance.
To add to what Wayne stated there are more than just MOV type of devices. The better designed units use hybrid devices. They come with MOV's, SAD's, and gas tubes. Each have advantages over the other. SAD's (silicon avalance diodes) are extremely fast, less than 1 pico-second, but cannot handle large amounts of energy. MOV's are fast, less than 1 micro-second, and can handle fairly large amounts of energy, but are not as fast as SAD's. Gas tubes are slow, but can handle extremely large amounts of energy. When all three are combined you get the best of all, untill you install them with leads longer than 6 inches, then they are useless.
The best units out today are built into the service equipment using a "Kelvin Clamp method with no lead lengths, and use some series techniques for added protection.