There've been a number of questions sent my way about this grounding business and perhaps it's worthwhile to touch on the opposite before we move forward with particulars about the grounded side of things.
Much of what we use in our homes is ungrounded. That means there's no third wire ground connected at all. Shavers, nearly all bathroom appliance that aren't a hair dryer, many consumer electronic items, clock radios, and even AV receivers. In fact,
most items within our homes aren't grounded, relying on the good old 2-prong plug that looks like this.
And if we've come to understand the third prong of a 3-prong plug is needed for safety and noise reduction, what's up with products that don't use them? Are they not safe?
For the most part they are safe but must be designed to a different set of rules than 3-prong electronics. First off, the 2-prong plugs are, for the most part, polarized. This means they can only plug in one way. You know, the plugs that are never turned the right way when you want to plug them in? The ones that always piss you off when you're in a hurry?
If you look closely at a 2-prong plug you'll notice one of the metal prongs is fatter than the other. Here's a picture.
The fatter one is
neutral, the smaller one is
hot.
Neutral is similar to ground in that you
should be able to stick your finger into it without getting shocked (disclaimer for idiots. Don't stick your finger in the AC socket). The idea here is that if your AC wall plug is wired correctly, you haven't any choice but to plug the thing in correctly and safely.
Secondly, use of a 2-prong plug has other design considerations as well. Special rules apply as to how you isolate the electrical circuitry from the outer chassis and those rules are pretty strict. You'll notice that many 2-prong plug units have plastic outer chassis, though not all of them. Most AV receivers are metal chassis and have only 2-prong plugs.
Some older homes have only 2-prong AC receptacles.
In short, properly designed 2-prong plug units are safe and more common than grounded 3-prong units, typical to high end audio.
