Lot of good info here, and some misinfo, so I’ll see if I can clarify a few things.
As many others have mentioned, the “third prong” is definitely important, as it is a safety ground. If you have an appliance with a 3-prong plug, the ground prong must be connected to the center tap of the transformer, and it’s usually done for safety’s sake. In other words, there must be a low impedance path between the appliance’s ground prong and the “common” bus bar at the breaker box.
Safety. Appliances that have a ground plug usually have an external metal chassis, or internal components with metal chassis (e.g. motor, compressor). If a fault occurs somewhere in the appliance, wherein there is a low impedance path between hot and chassis, the breaker or fuse will blow if the chassis is properly grounded. If a fault occurs and the chassis is not grounded, the entire chassis will float at 120 VAC. If you happen to be grounded to earth ground and you touch a “hot” chassis, you’ll get zapped.
Shielding. Some sensitive instruments utilize a shield to attenuate EMI from getting into the instrument and/or from being radiated from the instrument. This shield needs to be at ground potential, thus it is electrically connected to the infamous “third prong.”
Transient protection. As NutMagnet already mentioned, surge protectors need an earth ground. But a slight nit-pick: A surge protector needs an earth ground for normal mode protection. Even without an earth ground, you’ll still have common mode protection.
So don’t try to defeat it with those little orange adapters! Unless, of course, you ground the little tab. As NotMrKnowItAll pointed out, do not assume the plate cover screw is grounded! Perhaps it is, perhaps it isn’t. Check it by turning off the breaker and performing a continuity check between neutral and the screw head. (This is actually a “quick and dirty” method, but it’s better than nothing. A better method would be to measure continuity between the breaker box and the outlet.)
Someone mentioned that “the neutral is at earth ground.” This is a true statement if there is no current on the neutral line. If there is current, then there will be a potential. The voltage on the neutral line, which is usually very small even when a device is on, will be proportional to the current on the neutral line and the resistance of the neutral line. So why doesn’t the neutral do “double duty” as a safety ground? Well, if you were using an appliance where the neutral was connected to the chassis, and there was an open circuit anywhere on the neutral line, the chassis will become hot (even though current will no longer flow).
Now a word about polarized plugs. Many 2-prong (i.e. non-grounded) plugs have these, and for good reason – the hot conductor is, well, hot. As atypicalcarl pointed out, a lamp has a polarized plug to ensure the screw part of the bulb is connected to neutral. Also, if a non-grounded device has a power switch, the switch should be connected to the hot conductor, thus a polarized 2-prong is used. If a non-grounded device does not have a power switch, and/or the hot and neutral directly connect to a transformer, then a polarized plug is usually not needed.
A few tidbits about GFCI outlets. The NEC does allow you to replace a 2-prong outlet with either: a) a GFCI outlet, or b) a regular 3-prong outlet that is fed from a GFCI outlet. But you must do two things in order to make it “legal":
i) If you’re replacing a 2-prong outlet with a GFCI outlet, you must affix a sticker on the outlet cover that says “No earth ground.” Why? Primarily for reasons #2 and #3 above, i.e. some sensitive instruments need their shields connected to earth ground, and surge protectors need an earth ground for normal mode protection.
ii) If you’re replacing a 2-prong outlet with a regular 3-prong outlet that is fed from a GFCI outlet, then you must have two stickers: one that says “No earth ground” and another that says “GFCI protected.”
Keep in mind, however, that the “best” arrangement is a properly grounded GFCI outlet. Why? Let’s say you plug a device (with 3-prong plug) into an ungrounded GFCI outlet. The chassis will not be tied to earth ground. Now let’s say something breaks inside the device in the middle of the night, and a hot conductor makes contact with the chassis. What happens? I’ll tell you what: nothing. The GFCI will not blow, and the breaker will not blow. The next morning your barefoot 3 year old daughter goes up and touches it. Assuming she’s grounded, the GFCI outlet will (hopefully!) sense the current imbalance and blow. But this sucks, for the following reasons:
- It takes a few milliseconds for the GFCI to break the circuit.
- GFCI’s are notoriously unreliable, especially when compared to a properly grounded outlet. Always check them after a lightening storm!
I’m not saying GFCI outlets are bad, I’m saying that a properly grounded 3-prong outlet offers better protection for internal ground faults over an ungrounded GFCI outlet. In the latter situation, it is often the case that the GFCI has to “wait” for you to blow it via current running through your body. Kinda makes me wonder why the NEC allows it…