Bonding And Grounding Issues In Power Distribution Systems
EP Editorial Staff | January 13, 2011
A safe, productive and viable operation calls for a stable power supply. How healthy is yours?
A stable power supply is the backbone of any building, plant or facility. Failures or instability resulting from a poorly designed power distribution system can negatively impact safety, production and the bottom line of a company’s operations.
As the foundation of an operation’s power supply and distribution capabilities, a safely and effectively bonded grounding system is critical in providing a low impedance path to earth that stabilizes voltage. Stable voltage, in turn, is a crucial component in maintaining the safety, reliability and efficiency of operations. The following points outline the five principles behind an effective bonding and grounding system via a low impedance path, and describe the applicable differences between bonding, grounding and earthing. By following these principles, operations and facility managers can help ensure that their power supplies are as safe, reliable and efficient as possible.
1.0. Principal purposes of a bonding and grounding system
The principal purposes for an “effectively bonded grounding system via a low impedance path to earth” are intended to provide for the following:
1.1. Provide for an applicable reference to earth to stabilize the system voltage of a power distribution system during normal operations.
The system voltage is determined by how the secondary winding of any power-class or distribution-class transformer is actually configured, as well as how the windings are referenced to ground or earth. The primary function or purpose of the system bonding jumper is to provide for an applicable reference to earth for the system voltage at the origins of the specific and separately derived system to stabilize the voltage (i.e., 600Y/347V, 480Y/277V, or 208Y/120V, 3 Phase, 4 Wire, Solidly Grounded, “WYE” Systems or a 240/120V, 3 Phase, 4 Wire Solidly Grounded “DELTA” System [see Fig. 1A and Fig. 1B]). The system bonding jumper is employed as a direct connection between the Xo terminal of a supplying transformer, generator or UPS output terminals and earth. This jumper is usually connected within the same enclosure as the power supply terminals and is not normally sized to carry large magnitudes of phase-to-ground fault current.
1.2. Create a very low impedance path for ground fault current to flow in a relatively controlled path.
The exact point and time where a phase-to-ground fault might occur cannot be determined. Depending, however, on the exact point of the phase-to-ground fault within a specific power distribution system, multiple return paths are likely to occur between the point where the fault conductor makes contact with a conductive surface and the Xo terminal of the supplying transformer or local standby generator. Consequently, it is desirable and preferred that the majority of the ground-fault current flows primarily in the specific equipment bonding jumpers and equipment ground conductors directly associated with the fault circuit. If the impedance in the equipment bonding jumpers and equipment ground conductors associated with the faulted circuit is too high, then significant magnitudes of phase-to ground fault current will likely take various other parallel paths in order to return to the source winding of the power supply. These various other uncontrolled and unexpected return paths can subject facility personnel to dangerous touch-potential differences—which can cause death, injury or permanent damage to internal organs. In addition, other unaffected equipment could be negatively affected or damaged by potential rises and unintended flow of current.
1.3. Create an effective and 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.
During the time of the phase-to-ground faulted condition, the subjected equipment bonding jumpers and the equipment grounding conductors are intended to function as a very low impedance path between the point of the fault and the ground bus within the service equipment or the standby generator equipment. Consequently, these affect equipment bonding jumpers, and the equipment grounding conductors constitute 50% of the total power circuit during the period in which phase-to-ground fault current is flowing. If the impedance in the ground-fault return path is not effective low enough, then the overcurrent protective devices employed in the circuit as fuses and thermal-magnetic circuit breakers will be ineffective to prevent substantial equipment damage. If the impedance in the ground-fault return path is too high, then the resulting flow of phase-to-ground fault current might actually be lower than the rating of the fuses and thermal-magnetic circuit breakers installed to protect the affected circuit.
Per NEC® 250-4(A)(5), to meet the requirements of an effective ground-fault current path “electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a permanent, low impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems.” The ground-fault current path must be capable of effectively and safely carrying the maximum ground-fault current likely to be imposed on it from any point in a specific power distribution system where a ground fault may occur to the return to power supply source. Earth cannot be considered as an effective ground-fault current path. Therefore, randomly inserting individual ground rods into the soil to connect to remote electrical equipment will not provide an effective return path for phase-to-ground fault current.
The primary function or purpose of the main bonding jumper (or MBJ) located within the service equipment is to provide a low impedance return path for the return of phase-to-ground fault current from the ground bus in the service equipment to the respective power supply source such as service transformers, standby generators or the output terminals of onsite UPS via the neutral conductors. The MBJ must be adequately sized to effectively carry all phase-to-ground fault current likely to be imposed on it. In addition, the MBJ is another bonding jumper that is often employed to stabilize the system voltage with respect to ground or earth. The MBJ, however, is only a small portion of the ground-fault return path for phase-to-ground fault current to return to the Xo terminal of the respective power source.
1.4. Limit differences of potential, potential rise, or step gradients between equipment and personnel, personnel and earth, equipment and earth, or equipment to equipment.
It is extremely important that all conductive surfaces and equipment enclosures associated with any power distribution system be effective bonded together via a low impedance path. As partially explained in paragraph 1.2 above, without a very low impedance path for ground-fault current to flow in a relatively controlled way, potential rises or step potential differences are likely to occur at other locations within the power distribution system. However, during non-faulted conditions, part of the normal load current will flow through the conductive surfaces, equipment enclosures and earth, if any current-carrying conductor is connected to earth at more than one location. For example, if any grounded conductor (neutral) were to become connected to any conductive surface or equipment enclosure downstream of the MBJ, then part of the load current would flow through the conductive surface, equipment enclosure or the earth because a parallel path will have been created.
1.5. Limit voltage rise or potential differences imposed on an asset, facility, or structure from lightning strikes, a surge event impinging on the service equipment, any phase-to-ground fault conditions, or the inadvertent commingling of or the unintentional contact with different voltage system.
When lightning strikes an asset, facility or structure, the return stroke current will divide up among all parallel conductive paths between attachment point and earth. The division of current will be inversely proportional to the path impedance Z (Z = R + XL, resistance plus inductive reactance). The resistance term should be very low, assuming effectively bonded metallic conductors. The inductance and corresponding related inductive reactance presented to the total return current will be determined by the combination of all the individual inductive paths in parallel. The more parallel paths that exist in a bonding and grounding system will equate to lower total impedance.
2.0. Differences between bonding and grounding
The terms “bonding” and “grounding” are often employed interchangeably as general terms in the electrical industry to imply or mean that a specific piece of electrical equipment, structure or enclosure is somehow referenced to earth. In fact, “bonding” and “grounding” have completely different meanings and employ different electrical installation methodologies.
“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 Earth. For example, airplanes do not have any connection to the planet Earth when they are airborne. However, it is extremely important for the safety and welfare of passengers, crew and aircraft that all metallic parts and structures of an airplane be effectively bonded together. The laboratories and satellites orbiting in space above the planet Earth obviously have no direct connection with the surface of our planet. All conductive surfaces of these orbiting laboratories and satellites, though, must be effectively bonded together to avoid differences of potential from being induced across their surfaces from the countless charged particles and magnetic waves traveling through space.
The common way to effectively bond different metallic surfaces of enclosures, electrical equipment, pipes, tubes or structures together is with a copper conductor, rated lugs and appropriate bolts, fasteners or screws. Other bonding means between different metallic parts and pieces might employ brackets, clamps, exothermic bonds or welds to make effective 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 currents, induced currents, surge currents, lightning currents or transient currents during abnormal conditions.
“Grounding” is a term used rather exclusively in North America to indicate a direct or indirect connection to the planet Earth or to some conducting body that serves in place of the Earth. The connection(s) to Earth can be intentional or unintentional by an assortment of metallic means.
A designated grounding electrode is the device that is intended to establish the direct electrical connection to the Earth. A common designated grounding electrode is often a copper-clad or copper-flashed steel rod. The designated grounding electrode, though, 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 aluminum rods cannot be employed as grounding electrodes.
The grounding electrode conductor is a designed conductor used to connect grounding electrode(s) to other equipment grounding conductor(s), grounded conductor(s) and structure (see Fig. 2).
“Earthing” is a term used in Europe or other countries that employ International Electric Commission (IEC) standards. The term “earthing” in European or IEC countries is synonymous with the term “grounding” in North America.
3.0. Common issues found with bonding and grounding systems in commercial and industrial power distribution systems
- All utilities (service-entrance ground bus, water lines, gas lines, telephone grounds, cable grounds, metallic sewer lines) are not effectively bonded together and connected to the building’s structural steel.
- All structures are not effectively bonded together.
- EMT conduits with set-screw couplings are employed as the ground-fault return path.
- No grounding bushings are employed.
- Bonding and grounding terminals are improperly installed.
- Bonding and grounding conductors are terminated in the wrong location.
- Bonding and grounding connections are loose.
- Grounding conductors are undersized.
- Mechanical grounding connections are oxidized and reduced.
- Lightning abatement system directs lightning currents into the building via connections to building steel.
- No direct access to external ground grid system is available for inspection or testing.
- The external ground grid system is deteriorated.
- The external grounding system is damaged (or was removed) during subsequent excavation work.
- No external grounding system is installed.
- Damage to an external grounding system could have been caused by previous lightning strike(s).
- Damage to an internal bonding and grounding system could have been caused by direct mechanical force, neglect and/or previous phase-to-ground fault.
- Records of initial ground grid testing do not exist.
- Records of regular inspections and maintenance of grounding systems do not exist.
- Drawings or records detailing the existing grounding system are unavailable.
The real bottom line
When designed and implemented correctly, bonding and grounding systems can optimize power distribution, thus allowing the efficient and reliable delivery of power to a facility’s multiple operating systems, buildings and other operational components. Additionally, a system can be strategically designed to minimize damages and power failure in the event of power outages and emergencies—thus making a well-designed power distribution system of utmost importance to your operations. MT
Frank Waterer is with Schneider Electric’s Power Systems Engineering Division. He has over 22 years experience with Square D in various engineering and R&D roles. Waterer’s activities in the area of engineering standards includes having served as chairman of the PES/IEEE Committee responsible for the design, development and installation of all IEEE Standards relating to all surge-protective devices.