A common question I get about older homes is whether two-prong outlets can safely be changed over to three-prong outlets. Most home buyers today don't want to be stuck with two-prong outlets throughout the house. Two-prong outlets can always be changed to three-prong, and this can be accomplished a few different ways. Today I'll give a very brief explanation of what the third prong is for, and I'll discuss a few ways to convert to a three prong outlet. I didn't consult an attorney before writing this article, so I feel like I should add a disclaimer before giving any electrical how-to advice: Don't do any of this work if you're not qualified. This is only an overview of the steps that a qualified person would take.
The third prong on an outlet is commonly referred to as 'the ground', and it provides an alternate path for electricity that may stray from an appliance or product. This is an important safety feature that has been required since 1962, which minimizes the risk of electric shock, and allows surge protectors to protect your electrical equipment, such as televisions, computers, stereos, and other devices.
The ideal way to upgrade a two prong outlet is to install a three prong outlet that has a continuous electrical path back to the panel. If the outlet is installed in a metal box, and that metal box has metal conduit all the way back to the panel, this will probably be pretty easy to do. To test this, you can use an inexpensive pig-tail electrical tester, which is available at any hardware store for about two dollars. With the circuit energized, touch one end of the tester to the hot wire, which should go to the smaller slot on the outlet, and one end of the tester to the electrical box (see photo below left). If the tester lights up, the box is grounded. Now all you need to do is attach a bare copper wire to the box, and use this as the ground wire for a three prong outlet (see photo below right).
If you perform the test with a pig-tail tester and the light doesn't light up when you touch the hot wire to the box, the box is not grounded (or you're not touching a hot wire). In this case, you could run a ground wire back to the panel, or you could replace your two-prong outlet with a GFCI outlet. A GFCI outlet provides the best possible protection against shocks, but without a ground wire, this outlet will not provide any protection for your electrical equipment. A surge protector plugged in to an ungrounded outlet will do nothing (you could fry your new plasma TV). You will need to add a sticker to the GFCI outlet that reads "No Equipment Ground" - this sticker comes with every GFCI outlet.
What about two-prong to three-prong adapters? These can only be safely used on a grounded two-prong outlet. A pig-tail tester will light up on a grounded two-prong outlet if you touch one lead to the smaller slot on the outlet, and the other lead to the screw in the middle of the outlet.
If this is the case, you can safely use a two-prong to three-prong adapter, as long as you secure it to the outlet with the cover plate screw. Any other use of a two-prong adapter is unsafe.
Reuben Saltzman, Structure Tech Home Inspections