A plug adapter, also called a ground plug adapter or sometimes a pigtail adapter, is a common little accessory that makes it possible to plug a three-prong cord into a two-slot outlet. You undoubtedly are familiar with these little adapters—one side has three slots to hold three-prong plugs, the other side has two prongs to fit into old-fashioned two-slot outlets. At the bottom, you'll see a round metal loop or green pigtail wire with a metal connector on the bottom.
It is very common to use these adapters in homes with older outlets that don't have three slots, but it is bad practice. In many cases, these older electrical systems do not have a dedicated ground pathway, which means that there is the potential for shock if you plug a three-prong plug into them via an adapter. While some two-slot outlets may be grounded by metal conduit running back to the service panel, this is by no means always the case. The grounding system is a safety feature that helps protect you against electrical shock, which can range from startling to painful to deadly. A true grounding connection is a good idea and is required by Code for all new construction; a plug adapter can make you think you have a ground connection when you don't.
Two-Slot vs. Three-Slot Outlets
The outlets we use today have a narrow "hot" slot, a wide "neutral" slot, and a roundish "ground" slot. Older receptacles had only two slots—the hot and the neutral. As mentioned, the ground pathway in an electrical system is for safety. If something goes wrong with the appliance, the cord, or the outlet, the ground system provides a secondary path for the electricity, leading it back to your home's breaker box and safely into the earth. While the physics are complicated, you can think of the grounding system as a means by which "leaking" electricity can find its way back to the ground without causing harm. Without a grounding system the electricity looks for the easiest path it can find, and sometimes that path is through your body. Pain or sometimes tragedy can result.
Appliance cords and extension cords with three-prong plugs provide a ground for the cord and often serve as the grounding path for the appliance itself. Some appliances and power tools have only two-prong cords. This is usually because the appliance is designed to be adequately insulated (often called "double insulated"). If there's a short circuit inside the appliance, the electricity won't travel to the housing, where you could get a shock by touching it. That said, if the appliance or tool is old, it may not be double insulated, even though it has a two-prong cord.
The Truth About Ground Plug Adapters
Plug adapters are advertised as safety devices. There's a theoretical kernel of truth to this, but in reality, adapters usually add no safety function whatsoever. The truth comes from the fact that some houses have electrical systems with metal boxes and metal conduit. If this is the case, a plug adapter that is meticulously installed and not tampered with can provide a genuine ground path that runs from the cover-plate screw to the metal strap on the outlet, to the metal box, to the conduit, and back to the service panel where the main grounding wire and grounding wire extends into the earth.
But for this system to work at all, you must screw the little metal tab or the green pigtail of the ground plug adapter into the cover-plate of the outlet.
However, ff you have a plastic wall box, the adapter will do nothing at all, since there is no continuous metal pathway back to the service panel. Even if there is a metal box, it doesn't necessarily mean there's a ground path. A lot of older houses have metal boxes and no conduit—just nonmetallic cable with no ground wire.
The best way to provide grounding at an older outlet is to install a new circuit cable with a ground wire and connect it to a new three-slot grounded outlet. If you're not ready to take that on, you can replace the old two-prong outlet with a GFCI (ground-fault circuit interrupter) outlet. This helps protect you from shock if there is a ground fault. For example, if you plug in an old appliance with wiring problems, the GFCI outlet likely will shut off the power before the appliance gives you a shock. Be aware that a GFCI outlet does not provide a true ground. This means that, among other limitations, a surge protector plugged into the outlet will not work as designed.
Testing for Ground
If you ever want to check an existing outlet for ground, plug in a $5 tool called an outlet tester or receptacle tester. It will tell you if the outlet is grounded and can indicate wiring problems, such as hot and neutral wires being reversed. A group of LED lights on the tester will light up in different patterns that can be "read" to tell you what problems, if any, exist with the outlet.