Use of Grounding In Electrical System – Electrical System Design
Minimize Overvoltage
Grounding is done primarily for reasons of safety. Figure 5.1 illustrate the need for grounding to prevent overvoltages from occurring on the power system. Note that a lightning arrester is located on the top of the pole mounted transformer, next to the transformer bushing. The utility phase conductor is connected to the arrester. The other side of the arrester is connected to ground. For normal operation, the lightning arrester assumes an open circuit condition. If the voltage across the arrester exceeds the voltage rating of the arrester, it goes into operation, resulting in a low impedance path to ground through the arrester.
On the low voltage side of the transformer, one of the secondary terminals is connected to earth ground. The grounded service conductor is routed, along with the ungrounded service conductors, to the meter socket, then to the service entrance panel of the building. Note that the grounded service conductor is also connected to earth ground at the service entrance through the grounding electrode system.
Assume a lightning strike occurs on the utility phase conductor. This lightning strike will induce a surge voltage in the phase conductor as the surge current travels along the phase conductor toward earth ground. A surge voltage will also be induced in the utility neutral conductor due to electromagnetic induction. When the lightning induced surge voltage reaches the pole mounted transformer, the large magnitude of the surge voltage causes the lightning arrester to operate.
Operation of the lightning arrester allows the surge to discharge to ground, travelling down the grounding conductor alongside the pole, and then to the ground rod located next to the pole. Essentially, a closed path to ground for the flow of surge current electrons has been established. Once the flow of electrons ceases, the lightning arrester once again assumes an open circuit condition to the normal system voltage.
In addition to providing a path to ground for the flow of electrons, the connection of one of the service supply conductors to earth ground stabilize service voltage. Without the earth ground, the service voltage may float and could become dangerously high under certain conditions. Likewise, the connection of the utility neutral to earth ground ensures that the voltage on the utility system neutral with respect to the surrounding earth remains at an acceptable level.
Limit Voltage Potential on Equipment Enclosure
On a 480V, three phase system, ungrounded motor or any other equipment, the result voltage during fault, is a voltage of 277V applied to the motor frame. If someone were to place one hand on the motor frame and the other hand on a grounded surface, such as the building structural steel, the path of current flow would be directly through the heart, resulting in electrocution.
Connecting the motor frame to ground by use of an equipment grounding conductor provides a low impedance path between the motor frame and ground. Since the equipment grounding conductor is connected to the grounding electrode system at the supply, the motor frame is essentially tied to ground potential.
Provide a Low-Impedance Path for Fault Current
For ungrounded motor, the motor will operate satisfactorily in the event of a fault between the ungrounded conductor and the motor frame with no obvious indication that the motor frame is not grounded, as there is no path to ground provided for the fault current. Also, the overcurrent device contacts remain closed, and the system remains energized which will result in equipment damage.
However, by providing a path for the fault current, the overcurrent device will operate to clear the short circuit, thereby removing the dangerous condition and causing the motor to shut down.
NEC Rule for Grounding
The NEC requires grounding of certain systems for safety purposes. Specifically, section 250-20(B)(1) of the NEC requires grounding of a system that can be grounded such that the maximum voltage between the ungrounded (phase) conductors and the grounded conductor does not exceed 150V.
The main grounding conductor sized according to table 250-66 of the NEC. However, this table covers service entrance conductors having a cross sectional area up to 110kcmil for
copper or 1750kcmil for aluminium. Above these maximum service entrance conductors, section 250-102(C) of the NEC requires the cross sectional area of the ground conductor to be at least equal to 12.5% of the cross sectional area of the service entrance conductors.
For cases where the service entrance conductors are installed in parallel in two or more raceways, the grounded conductor shall also be routed in parallel in each raceway. Section 250-24(B)(2) of the NEC requires the size of the parallel grounded conductor in each raceway to be based on the cross sectional area of the ungrounded phase conductor in each raceway. The grounded service conductor can not be smaller than #1/0 AWG for parallel installation.
Example: Determine the required copper main grounding conductor for the following service entrance conductors:
#4/0 AWG XHHW copper
Four 500kcmil XHHW copper per phase
Solution: The required copper main grounding conductor is read directly from table 250-66 as #2 AWG.
The total equivalent cross sectional area of all phase conductors is 4 x 500 = 2,000kcmil. The maximum required cross sectional area for the main grounding conductors is 12.5% x 2,000 = 250 kcmil.
The method of grounding a service supplied from another building shown in Figure 5.2. Note that the grounded service conductor is connected to the grounding electrode system at the main service panel. Separate equipment grounding conductors and grounded conductors are run from the main panel to sub-panel.
Ground Fault Circuit Interruption
The basic principle of operation and location requirements for ground fault protection of receptacles are as shown below.
In addition to the requirements for receptacles, the NEC requires the use of ground fault protection on service and feeders under certain conditions. Specifically, sections 230-95 and
215.10 of the NEC require ground fault protection on disconnecting devices rated 1,000A or more on solidly grounded wye systems of greater than 150V to ground but not exceeding 600V phase to phase.
Two configurations for the detection of ground faults are possible, as shown below in figures 5.4 and 5.5.
Section 230-95(A) of the NEC specifies that the trip setting of the ground fault sensor can not exceed 1,200A. In addition, the time delay can not exceed 1 second for ground fault currents of 3,000A or more. Operation of the ground fault detector must result in the opening of all ungrounded conductors in the system on the load side of the disconnecting device. It is also important to note that the ground fault detection schemes shown above will detect ground faults only on the load side of the GFCI sensing device. Also, the GFCI detector will not detect overloads or phase to phase faults not involving ground.
Grounding of Instrument Transformer
In many commercial and industrial applications, instrument transformers are used to transform the high load currents and voltages to smaller levels that can be applied to various metering and protective devices. The most common use of instrument transformers is as current transformers applied to large ampacity services. In addition, voltage transformers are commonly used to step down the voltage of service rated above 600V. All medium voltage installation use voltage and current transformers to provide the appropriate levels to the metering elements and protective relays.
Instrument transformer secondaries must be grounded to prevent dangerous overvoltages from occurring on the secondary system. Section 250-170 of the NEC requires that the secondary circuits of instrument transformers operating on systems where the primary voltage is 300V or more to ground must be grounded. Also, section 250-174(A) and 250- 174(B) require the cases of instrument transformers not mounted on switchboards or mounted on switchboards not having any live parts exposed, must be grounded in most locations. Section 250-174(C) specifies that cases of instrument transformers mounted on live front switchboards must not be grounded.
Figure 5.6 shows the proper location of the secondary ground conection for a current transformer. Grounding of a voltage or potential transformer secondary is shown in figure
5.7. In each figure, only one instrument transformer is shown for the purpose of clarity. Instrument transformers connected to other ungrounded conductors must be similarly grounded. Section 250-178 of the NEC also requires that the minimum size grounding conductor shall be #12 AWG copper or #10 AWG aluminium.
