Plug adapters make it possible to plug a grounded plug into two-pronged polarized outlets. You see, some outlets have only two opening, a hot and a neutral. In some cases, the box is grounded and the center screw on the outlet is grounded through the case of the outlet connection to the box. In this case only, a plug adapter can be plugged into the outlet and the ground tab can be connected to the center screw of the outlet cover. This allows a path to ground for the connection.
In most cases, however, this is not the case. The two-pronged outlets have no ground and people try to cheat with their connection by adding a plug adapter. Oh yes, it will allow you to connect the appliance or device to the outlet, but at what cost?
The problem starts when their is a short to ground through the case of the appliance or device that you have plugged in. Without a path to ground through the supply connection, electricity is trying to find a path to ground. Then you show up at the device, touch the case and WHAM! You get electrocuted and wonder what hit you. And to think, all of this could have been avoided by replacing the outlet with a grounded type outlet and in some cases, replacing the non-grounded wire feeding it.
Grounded outlets come complete with a hot, neutral, and ground. Likewise, nonmetallic sheathed cable comes with a hot, neutral, and ground wire. In order to find out if the outlet is indeed grounded, use a voltage tester to test it. Place one of the leads into the shorter outlet slot (HOT) and the other on the center screw of the outlet. If the tester reads voltage, the outlet is grounded and I recommend you change the outlet to a grounded plug.