Introduction to Grounding System Design – Part Two

In Article " Introduction to Grounding System Design – Part One ", I explained the following points:

• Differences between Grounding and Earthing Terms,
• Differences between Grounded and Grounding systems,
• Differences between Bonding and Grounding,
• Grounding and bonding systems definitions.

Today I will explain the Second Part of the Introduction to Grounding System Design as follows.

1- How Earthing system works The grounding system is essential to complete an electrical path to ground if there is non-designed or unanticipated above-normal potential current or voltage surges during operating conditions. Personal injury, death or equipment damage can result if the grounding system is not designed and installed properly to guide these potentially dangerous charges safely to ground. The grounding systems under normal conditions carry NO current. The only time they carry current is under abnormal conditions when an electrical appliance or piece of electrical equipment is faulty and has become a potential shock or fire hazard. Under a fault condition the grounding conductor that is connected to the outer shell or sheet metal of the equipment or appliance must be able to provide a very low resistance path back to the source of the power (utility company's transformer) so that enough current will flow causing a breaker or fuse to open the circuit and automatically disconnect the hazard from the system. It is NOT the purpose of grounding system to send current through the ground. Sending equipment fault currents through the earth can be a fatal misunderstanding of how a grounding system works. For the most part, the only time you intentially send current into the earth is during a lightning strike or line surge due to a nearby lightning strike.

2- Construction of the Earthing System The following factors must be considered in the construction of the earthing system: Function, Type, Electrical properties, Construction material.

2-A Functions of Earthing Systems 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. A detailed explanation for each Function of Earthing System will be discussed in below paragraphs.

2-A-1 Functions of Earthing Systems: 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 or distribution 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). 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.  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.

2-A-2 Functions of Earthing Systems: 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 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.  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.  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.  Other unaffected equipment could be negatively affected or damaged by potential rises and unintended flow of current.

2-A-3 Functions of Earthing Systems: 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. 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 stand by generator equipment.  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 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.”  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.  Notes: Earth can not be considered as an effective ground-fault current path.  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, stand by 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 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 respect power source.

2-A-4 Functions of Earthing Systems: 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. Without a very low impedance path for ground fault current to flow in a relatively controlled path potential rises or step potential differences (touch and step voltages) are likely to occur at other locations within the power distribution system. 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.  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.

2-A-5 Functions of Earthing Systems: 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. 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.

In the next Article, I will continue explaining the Construction of the Earthing System . Please, keep following.

Back To Course EE-5: Grounding System Design Calculations