Ground wires for individual branch circuits (or metal sheathing that acts as a ground) lead back to the neutral or ground bus bar of the service panel. The service panel itself must be connected to the earth so that the entire electrical system is safely grounded.
Usually a service panel is grounded by a thick wire -- bare copper or green insulated -- leading to a ground rod or to a cold-water pipe. Follow the ground wire from the service panel to find how it is attached to the earth.
A copper ground rod is driven at least 8 feet into the ground. Its top may be visible, or it may be sunk a few inches beneath the ground. The service panel ground wire must be firmly attached to the ground rod, either with a special toothed clamp or by welding. Recent building codes often call for two or more ground rods for added security.
The service panel ground wire may lead to a cold-water pipe, which is connected to supply pipes that lead deep underground. The connection must be firmly clamped. Hot-water pipes are not acceptable for grounds because they run only to the water heater, not into the earth.
Because municipal water pipes are buried, a firm connection to a cold-water pipe forms an effective ground.