IEEE defines earth as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the Earth, or to some conducting body of relatively large extent that serves in place of the earth. The connection(s) to Earth are done by an assortment of metallic means intended to be employed as a designated grounding electrode. A designated grounding electrode is the device that is intended to establish the direct electrical connection to the earth rod. The designated grounding electrode might be a water pipe, steel columns of a building or structure, concrete encased steel reinforcement rods, buried copper bus, copper tubing, galvanized steel rods, or semi conductive neoprene rubber blankets. Gas pipes and aluminium rods cannot be employed as grounding electrode The grounding electrode conductor is the designed conductor that is employed to connect A common designated grounding electrode is often a copper clad or copper flashed steel the grounding electrode(s) to other equipment grounding conductors, grounded conductor, and structure.
Purpose of Earth Grounding
- Limit potential difference of neutral for system stability.
- Allow for operation of relays and system protections devices.
- Personnel safety.
Practical Ground Resistance rods have much less than 160 Ω. Typical rod resistances will be less than 25 Ω in most soils because of the existence of proximate utility ground references
Low ground resistance (< 10 Ω ) can be achieved with:
- Multiple Rods bonded together
- Counterpoise system
- Coupled rods
Grounding is thought by many to be synonymous with electrical safety. “If equipment is grounded then it is electrically safe” gives a false sense of security. The process that many consider to be “grounding” can and may minimize the severity of electrical faults when performed with an understanding of what happens during an electrical fault. “Grounding” is very specific; it means a connection to the earth (ground). With respect to systems, such as transformers, generators, or batteries, “grounding” generally means providing a connection from one conductor of the system to an electrode that is buried in the earth. However, not all systems are grounded nor is the electrode always in the earth. When referring to equipment, the term “grounding” can have various meanings. It may mean bonding or it may mean a direct connection to the earth. The term “bonding” sometimes may mean “grounding,” and sometimes, it may mean a short or a long connection.
Earth Bonding
“Bonding” is a method by which all electrically conductive materials and metallic surfaces of equipment and structures, not normally intended to be energized, are effectively interconnected together via a low impedance conductive means and path in order to avoid any appreciable potential difference between any separate points.
The bonded interconnections of any specific electrically conductive materials, metallic surfaces of enclosures, electrical equipment, pipes, tubes, or structures via a low impedance path are completely independent and unrelated to any intended contact or connection to the Earth.
The common mean to effectively bond different metallic surfaces of enclosures, electrical equipment, pipes, tubes or structures together is with a copper conductor, rated lugs, and the appropriate bolts, fasteners, or screws. Other effectively bonding means between different metallic parts and pieces might employ brackets, clamps, exothermic bonds, or welds to make an effectively connections.
In addition to preventing potential differences that may result in hazards, effectively bonded equipment can also be employed to adequately and safely conduct phase-to-ground fault current, induced currents, surge currents, lightning currents, or transient currents during such abnormal conditions.
The principle purposes for an “effectively bonded grounding system via a low impedance path to earth” are intended to provide for the following.
- Provide for an applicable reference to earth to stabilize the system voltage of a power distribution system during normal operations.
- Create a very low impedance path for ground fault current to flow in a relatively controlled path.
- Create a very low impedance path for ground fault current to flow in order for overcurrent protective devices and any ground fault protection systems to operate effectively as designed and intended.
- Limit differences of potential, potential rise, or step gradients between equipment and personnel, personnel and earth, equipment and earth, or equipment to equipment.
- Limit voltage rise or potential differences imposed on a power distribution system from lightning, a surge event, any phase-to-ground fault conditions, or the inadvertent commingling of or the unintentional contact with different voltage system.