A "GFCI" is a ground fault circuit interrupter. These are inexpensive outlets when they are PROPERLY installed prevent over 200 electrocutions each year. The problem is we find them wired incorrectly several times a week.
The 'TEST" button is confusing if they are installed wrong. Today, I found one with the positive and negative connections backwards, (the test button still works) and more commonly we find them connected without a ground especially in older homes (again the test button still works). While it is technically legal to install without a ground, most electricians prefer not to install a grounded-type "three prong" outlet in an ungrounded situation. Once the outlet is installed, there is no way for anyone to know if the outlet is really grounded or not without testing it. There is a hidden shock hazard should an appliance or tool that needs grounding is plugged into this outlet.
We recommend that you buy a cheap tester like we have, without it, you have no way to assure it is properly installed. Read more about the importance of GFCIs here
You can find these testers at your local hardware store on online for less the $10
Though a GFI will activate if a grounded appliance develops an electrical short circuit to ground... such as when YOU touch a metal saw and become the path to ground... you will experience a momentary electrical shock. This could be a minor tingle or could be more catastrophic, especially if you are on a ladder or roof. This excerpt is from an OSHA (Occupational Safety and Health Administration) article on wiring in nursing homes and the dangers to employees working with ungrounded outlets...
"The ground-fault circuit interrupter, on the other hand, is a fast-acting device which senses small current leakage to ground and, in a fraction of a second, shuts off the electricity and interrupts its faulty flow to ground. The rapid response of the GFCI is fast enough to prevent electrocution and this protection is independent of the condition of the grounding conductor.
A GFCI can prevent an electrocution; however, it cannot by itself prevent an initial electric shock to an employee before it interrupts the circuit. This initial shock could lead to injuries of an indirect or secondary nature in which involuntary muscular reaction could cause bruises, bone fractures, and even death resulting from collisions or falls. Therefore, GFCIs are in addition to, and not in lieu of, equipment grounding conductor requirements."
(Here is a link to the complete article:
Are there any situations where a GFI will not protect me from a dangerous shock?
GFI's are not effective in certain circumstances. As mentioned earlier, they work by measuring the difference between the electrical current going INTO an appliance and the current going OUT of the appliance. This assumes that the person being shocked is grounded. If you were ungrounded and touched the hot and neutral wires at the same time, there would be no drop in current detected, so the GFI would not activate. Then, you would be at the mercy of the fuses or circuit breakers, which may or may not stop the current before its too late!
A second situation where a GFI will not protect you is when a second, unprotected circuit is involved in an accident. This can happen when a wire is accidentally drilled into or a metal screw penetrates a wire hidden in the wall. Unless this second circuit is also protected, you are at full risk of electrocution, even if the tool itself is on a protected circuit!