Effective bonding, grounding: The backbone of electrical safety

Providing and maintaining an effective impedance path to ground is critical to maintain reliable, efficient, and safe operating facilities.

10/09/2012


Courtesy: Schneider ElectricA stable electrical generating source serves as the backbone of operations in any facility, building or manufacturing plant. Electrical failures or power 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 any power generation unit and associated power distribution system, a well-designed, professionally installed, and effectively bonded grounding system is of critical importance to any power generation facility or site location. Providing and maintaining an effective impedance path to earth that stabilizes the system voltage is a basic and critical component in maintaining reliable, efficient, and safe operating facilities.

The following points outline the five principal purposes for an effective bonding and grounding system via a low or intentionally selected impedance path, as well as a guide to the applicable differences between bonding, grounding, and earthing. By following these basic principles, operations and facilities managers will be able to ensure they are helping their power generation facilities to become as reliable, safe, and efficient as possible.

Three-phase, 4-wire solidly or resistively grounded “WYE” are preferred over the use of 3-phase, 3-wire ungrounded systems specifically because of the possibility for destructive transient overvoltages that can occur throughout the power system during any restriking phase-to-ground fault. Such destructive overvoltages resulting from a resonant condition being established between the inductive reactance of the system and the distributed capacitance to ground (earth) can be several times the normal phase-to-phase voltage in magnitude.

Experience has proven that these overvoltages may very rapidly cause failure of insulation at multiple locations throughout a power distribution system, particularly at motors and loads associated with sensitive electronic equipment. Transient overvoltages from restriking ground faults are the primary reason why ungrounded systems are no longer recommended and solid or impedance grounded systems of some form are predominantly installed.

Purposes of bonding and grounding

The ground fault current path must be capable of effectively carrying the maximum ground-fault current. Courtesy: Schneider ElectricThe principal purposes for an “effectively bonded grounding system via a low or intentionally selected 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 generation supply and power distribution system during normal operations.

The generating system voltage is determined by how the output windings are arranged and configured as well as how the windings are referenced to ground or earth. The distribution system voltage is determined by how the secondary windings of the respective power distribution transformer are actually configured, in addition to how the windings are referenced to ground or earth.

The primary function or purpose for grounding the XO terminal of generation source is to provide for an applicable reference to the planet Earth to stable generating system voltage at the origins of the generated power source.

Since the secondary windings of a power distribution transformer are a specific and separately derived electrical system, the primary function or purpose of the system bonding jumper is to stabilize the system voltage relative to planet Earth. The system bonding jumper is usually connected within the same enclosure as the power supply terminals, and the jumper is not normally sized to carry large magnitudes of phase-to-ground fault current.

  • 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. However, depending 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 generator or secondary windings of the power distribution transformer.

Consequently, it is desirable and preferred that the majority of the ground fault current flow 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.

  • 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 effectively low enough, then the overcurrent protective devices employed in the circuit as fuses and thermal-magnetic circuit breaker 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), in order 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.”

Figure 1: Solidly grounded power generator. Courtesy: Schneider ElectricThe 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.

Figure 1 (to the left) is an example of a solidly grounded power generator or the secondary windings of a power distribution transformer where no intentional impedance is supposed to exist between the XO terminal of the generator or secondary windings of the power distribution transformer and earth. In this type of installation there is no intentional impedance to any phase-to-ground fault current other than the impedance in the supplying cable, windings, and ground fault return paths. Therefore, the maximum magnitude of phase-to-ground fault current will flow during any unfortunate phase-to-ground fault condition. (XGO is the zero sequence reactance of the windings of the generator or transformer.)

Figure 2: Resistance grounded power generator. Courtesy: Schneider ElectricFigure 2 is an example of a resistance or resistively grounded power generator or the secondary windings of a power distribution transformer where there is selected impedance via a selected grounding resistor intentionally installed between the XO terminal of the generator windings and earth or the XO terminal of the secondary windings of the power distribution transformer and earth.

In addition to the inherit impedance associated with the supplying cable, windings, and ground fault return paths, in this type of installation there is an intentional resistance inserted to limit any phase-to-ground fault current flow. Therefore, the maximum magnitude of current resulting from an unintended phase-to-ground fault current will be limited to a designated magnitude by the selected value of RN (prescribed ohmic value of the grounding resistor) plus the XGO.

The reasons for limiting the current by resistance grounding include the following:

  1. To reduce burning and melting effects in faulted electric equipment, such as switchgear, transformers, cables, and rotating machines.

  2. To reduce mechanical and thermal stresses in circuits and apparatus carrying fault currents. (This also includes the windings of the supplying generator or transformer.)

  3. To reduce electric-shock hazards to personnel caused by stray ground-fault currents in the ground-return path.

  4. To reduce the arc blast or flash hazard to personnel who may have accidentally caused or happen to be in close proximity to the ground fault.

  5. To reduce the momentary line-voltage dip occasioned by the occurrence and clearing of a ground fault.

  6. To secure control of transient overvoltages while at the same time avoiding the shutdown of a faulted circuit on the occurrence of the first ground fault (high resistance grounding).

Resistance grounding may be either of two types, high resistance or low resistance, which are distinguished by the magnitude of ground-fault current permitted to flow.

High-resistance grounding employs a neutral resistor of high ohmic value. The value of the resistor is selected to limit the current to a magnitude equal to or slightly greater than the total capacitance charging current.

Typically, the ground-fault current is limited to 10 A or less, although some specialized systems at voltages in the 15 kV class or higher might require higher ground-fault levels to activate protective relays. In general, the use of high-resistance grounding on systems where the line-to-ground fault exceeds 10 A should be avoided because of the potential damage caused by an arcing current larger than 10 A in a confined space.

Low-resistance grounding is designed to limit ground-fault current to a range between 100 A and 1,000 A, with 400 A being typical. The neutral resistor is selected according to R = Vln/Ig, where Vln is the system line to neutral voltage and Ig is the desired ground-fault current. Since the combined effects of charging current and system source impedance will affect the ground-current value less than 0.5% in the typical range of utility supplied systems, it is permissible to ignore these effects in calculating the ground-fault resistance value. The general practice is to consider that the full system line-to-neutral voltage appears across the grounding resistor.

Figure 3: Reactance grounded power generator. Courtesy: Schneider ElectricFigure 3 is an example of a reactance grounded power generator or the secondary windings of a power distribution transformer where there is selected impedance via a selected inductive coil intentionally installed between the XO terminal of the generator windings and earth or the XO terminal of the secondary windings of the power distribution transformer and earth.

In addition to the inherit impedance associated with the supplying cable, windings, and ground fault return paths, in this type of installation there is intentional inductive impedance inserted to limit any phase-to-ground fault current flow. Therefore, the maximum magnitude of current resulting from an unintended phase-to-ground fault current will be limited to a designated magnitude by the selected value of XN (prescribed inductance value of the grounding reactor) plus the XGO.

The term “reactance grounding” describes the case in which a reactor is connected between the system neutral and ground, as shown in Figure 3. Since the ground fault that may flow in a reactance-grounded system is a function of the neutral reactance, the magnitude of the ground-fault current is often used as a criterion for describing the degree of grounding system.

In a reactance-grounded system, the available ground-fault current should be at least 25% (X0 = 10X1) and preferably 60% (X0 = 3X1) of the three-phase fault current to prevent serious transient overvoltages from occurring. The term X0, as used, is the sum of the source zero sequence reactance, X0, plus three times the grounding reactance, 3Xn, (X0 = X0 source +3Xn). This is considerably higher than the level of fault current desirable in a resistance grounded system, and therefore reactance grounding is usually not considered an alternative to low-resistance grounding.

Reactance grounding is typically reserved for applications where there is a desire to limit the ground-fault duty to a magnitude that is relatively close to the magnitude of a 3-phase fault. Use of neutral grounding reactors to provide this fault limitation will often be found to be a less expensive application than use of grounding resistors if the desired current magnitude is several thousand amperes.

  • 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 effectively bonded together via a low-impedance path. As partially explained above, without a very low impedance path for ground fault current to flow in a relatively controlled path, potential rises or step potential differences are likely to occur at other locations within the power distribution system.

However, during nonfaulted 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 will flow through the conductive surface, equipment enclosure, or the earth because a parallel path will have been created.

  • 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 systems.

When lightning strikes an asset, facility, or structure, the return stroke current will divide up among all parallel conductive paths between the 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, the lower the total impedance.

Differences between bonding and grounding

Understanding the difference between bonding and grounding is an important part of creating a stronger and safer electrical system. Courtesy: Schneider ElectricThe 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 the Earth. For example, airplanes do not have any connection to 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 are 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. However, all of the conductive surfaces of these orbiting laboratories and satellites must be effectively bonded together in order to avoid differences of potential from being induced across their surfaces from the countless charged particles and magnetic waves traveling through space.

The common method to effectively bond together different metallic surfaces of enclosures, electrical equipment, pipes, tubes, or structures is with a copper conductor, rated lugs, and the appropriate bolts, fasteners, or screws. Other bonding methods 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 current, induced currents, surge currents, lightning currents, or transient currents during such 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. However, 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 semiconductive neoprene rubber blankets. Gas pipes and aluminum rods cannot be employed as grounding electrodes.

The grounding electrode conductor is the designed conductor that is employed to connect the grounding electrode(s) to other equipment.

Frank Waterer is an electrical engineering of Schneider Electric Engineering Services.



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