The term "ground" refers to a conductive body, usually the earth. "Grounding" a tool or electrical system means intentionally creating a low-resistance path to the earth. When properly done, current from a short or from lightning follows this path, thus preventing the buildup of voltages that would otherwise result in electrical shock, injury and even death.
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There are two kinds of grounds; both are required by the OSHA construction standard:
- System or Service Ground: In this type of ground, a wire called "the neutral conductor" is grounded at the transformer, and again at the service entrance to the building. This is primarily designed to protect machines, tools, and insulation against damage.
- Equipment Ground: This is intended to offer enhanced protection to the workers themselves. If a malfunction causes the metal frame of a tool to become energized, the equipment ground provides another path for the current to flow through the tool to the ground.
There is one disadvantage to grounding: a break in the grounding system may occur without the user's knowledge. Using a ground-fault circuit interrupter (GFCI) is one way of overcoming grounding deficiencies.
Summary of Grounding Requirements
- Ground all electrical systems. [for exceptions, see 29 CFR 1926.404(f)(1)(v)]
- The path to ground from circuits, equipment, and enclosures must be permanent and continuous.
- Ground all supports and enclosures for conductors. [for exceptions, see 29 CFR 1926.404(f)(7)(i)]
- Ground all metal enclosures for service equipment.
- Ground all exposed, non-current-carrying metal parts of fixed equipment. [for exceptions, see 29 CFR 1926.404(f)(7)(iii)]
- Ground exposed, non-current-carrying metal parts of tools and equipment connected by cord and plug. [for exceptions, see 29 CFR 1926.404(f)(7)(iv)]
- Ground the metal parts of the following non-electrical equipment:
- Frames and tracks of electrically operated cranes.
- Frames of non-electrically driven elevator cars to which electric conductors are attached.
- Hand-operated metal shifting ropes or cables of electric elevators.
- Metal partitions, grill work, and similar metal enclosures around equipment of over 1kV between conductors.
Methods of Grounding Equipment
- Ground all fixed equipment with an equipment grounding conductor that is in the same raceway, cable, or cord, or that runs with or encloses the circuit conductors (except for DC circuits only).
- Conductors used for grounding fixed or moveable equipment, including bonding conductors for assuring electrical continuity, must be able to safely carry any fault current that may be imposed on them.
- Electrodes must be free from nonconductive coatings, such as paint or enamel, and if practicable, must be embedded below permanent moisture level.
- Single electrodes which have a resistance to ground greater than 25 ohms must be augmented by one additional electrode installed no closer than 6 feet to the first electrode.
- For grounding of high voltage systems and circuits (1000 volts and over), refer to 29 CFR 1926.404(f)(11).
- 29 CFR 1926 Subpart K, Electrical. OSHA Standard.
- Electrical Safety: Safety and Health for Electrical Trades Student Manual. US Department of Health and Human Services (DHHS), National Institute for Occupational Safety and Health (NIOSH) Publication No. 2009-113, (March 2009).