Building codes have changed over the years and differ from region to region. As a result, grounding methods can vary widely. You may find any of several configurations in your home.
No matter the method, it's important that the ground circuit provides an unbroken path to the earth. Ground wires must be firmly connected at all points. If conduit or sheathing is used as a ground path, connections must be tight. A receptacle analyzer tells you whether your receptacles are grounded.
Metal boxes: In this arrangement both the receptacle and metal box are grounded. Ground wires are spliced together and attached with a pigtail to the box and receptacle. The grounding wire nut shown has a hole in its top that makes installing a pigtail easier.
In a system with metal boxes, the pigtail method is considered the most secure. Other methods also work well if installed correctly; one such method is a grounding clip that clamps the ground wire to the box.
If a house is wired with armored cable or conduit, there often is no ground wire. The cable connector joins the metal sheathing or conduit to the box to provide the path for ground.