Grounding a tool or electrical system involves creating a low-resistance (conducting) path connected directly to the earth. This path prevents the buildup of voltage that could cause an electrical accident or shock hazard. Improper grounding is one of the most common electrical safety violations reported by the Occupational Safety and Health Administration (OSHA).
In an electrical circuit, current flows from the hot wire (black) to the powered device and back to the source through the neutral wire (white or gray). The neutral wire is the grounded wire in a typical single-phase system. A grounding conductor or wire (white or green) acts as an alternate path for the current to flow back to the source if an electrical short occurs. For example, if the hot wire in an electrical device becomes worn and makes contact with the device, the current might flow back to the user. If a grounding wire is present, however, the current will take the path of least resistance and flow through it to ground.
The metal parts that you touch in an electrical wiring system, such as switch plates or light fixtures, should be grounded with no voltage. If the system isn't properly grounded, these parts may become energized. Metal parts of motors, appliances or electronics plugged into improperly grounded circuits may also be energized. When a circuit isn't grounded properly, unwanted voltage can't be safely eliminated. If there is no safe path to return fault currents, exposed metal parts in damaged appliances can become energized.
Standards for proper grounding are covered in Article 250 of the National Electrical Code, published by the National Fire Protection Association. Your local building code may contain additional or separate grounding requirements. Consult an electrician or your local building inspector for more information. See OSHA for regulations and best practices about grounding and working safely with electricity.