EET 323 – Electrical System Design Lecture 9: Grounding Radian Belu, PhD

Introduction System Grounding: the intentional connection of one the supply circuit conductors to earth at a particular location. Service Grounding: the connection of the supply conductors and service entrance equipment, such as meters, panels, and disconnects, to earth ground. Equipment Grounding: the intentional connection of equipment enclosures and raceways to earth ground. - The chapter will cover and discuss the reasons for grounding electrical systems, the types of systems that require grounding, the grounding electrode system, and methods of equipment and raceway grounding.

Lecture Objectives

Reasons for Grounding Minimize Over-voltages: The grounding is done primarily for safety reasons. In Figure 9-1 the lightning arrester is connected between the phase conductor and the ground, and fro normal power system voltage and current assumes an open-circuit. If the voltage across the arrester exceeds the voltage rating, it goes into the operation, resulting in a low-impedance path through the ground. In addition to providing a path to ground for the flow of over-currents, the connection of the service supply conductors to the to earth ground stabilizes service voltage, preventing the voltage floating under certain conditions.

Limiting Voltage Potential on Equipment Enclosures Grounding it is also a means to limit the voltage potential on equipment enclosures (Figure 9-2), assuring a path for the current flow, in the case of a equipment insulation failure, malfunction, etc. so will be no current increase in the feeder. Connecting the motor frame to ground by use of an equipment grounding conductor provides a low-impedance path between the motor frame and ground (safety reason). The connection of the equipment grounding conductor to the motor frame has ensure that the voltage on the motor frame is kept to a minimum in the event of a phase-to-ground short circuit in the motor windings.

Provide a Low Impedance Path for Fault Current Grounding it is also a means to provide a low-impedance path for the flow of fault current (Figure 9-3), in the event of a fault between the ungrounded conductor and the motor frame will be a path for the flow of current to return to the system. The result is that there is no appreciable increase in current through the motor feeder over-current voltage, allowing the operation of protective devices to operate and to clear the short circuit, and removing the dangerous condition and cosuing the motor to be shut down. In this case, the equipment ground serves as a means to allow a sufficient magnitude of short circuit current to flow, permitting the operation of protective devices.

Made Electrode A grounding electrode consisting solely of a metal underground water pipe must be supplemented with an additional grounding electrode, which may be the building structural steel, a ground ring, or a concrete-encased electrode. Section 250.52(B) of the NEC prohibits metal underground gas pipe or aluminum electrodes to be used as a grounding electrode. The most common practice is to use a rod and pipe electrode or plate electrodes as the supplemental made electrode. The grounding electrode conductor that serves as the connection to the made electrode is not required to be larger than the #6 AWG copper or #4 AWG aluminum.

Service Grounding Requirements Single Service Disconnect Multiple Service Disconnect Service Supplied from Another Building

Single Service Disconnect The requirements for grounding at the service entrance comprised a single panel-board containing the main service disconnect (Figure 9-6(A)) – the grounded service conductor is connected to the neutral bus in the service equipment, and a separate ground bus for connection of the equipment grounding conductors and grounding electrode conductors are also present. Most of panel-boards suitable to be used as service entrance equipment have a combined neutral bus and a ground bus, for connection of the grounded conductor and the grounding conductors of the system.

Single Service Disconnect In many installation, the main service disconnect is separated from the main service disconnect is separated from the main service panel (Figure 9-6(B)). An installation where the main service disconnect is located near the service drop and meter, with the panel inside the building, for example. To meet the NEC 250.6(A) Section requirements bonding of the grounded service conductor and the grounding electrode system may occur only at the service entrance equipment, and no other connection between them is permitted.

Grounding of Separately Derived Circuits A separately derived system is system in which there is no direct electrical connection between ungrounded conductors and grounded (neutral) conductor of the service and the ungrounded phase conductors and grounded (neutral) of the separately derived system. These systems are installed when there is a need to supply certain loads with a voltage different from that of the service voltage. For example: a service voltage of 480 Y/277 V as well as 120 V voltage; emergency generator is connected to the building electrical system by means of a transfer switch.

Equipment Grounding

Equipment Grounding The raceways in Figure 9-11 contain the service entrance conductors, they are considered electrical equipment on the line side of the service over-current device. The equipment bonding jumpers are sized based on Table 250.66 of the NEC code. Section 250.102(C) of the NEC requires sizing the equipment bonding jumpers at 12.5% of the cross-sectional area of the service entrance conductors for the service entrance conductors larger than 1100 kcmil copper and or 1750 kcmil aluminum. It also requires that if aluminum service entrance conductors are used, the copper-equipment bonding jumper to be sized based on the equivalent-size copper service entrance conductors. Section 250.102(C) of the NEC requires each equipment bonding jumper to be sized based on the cross-sectional area of the service conductors in each conduit.

Equipment Grounding The use of instantaneous trip circuit breakers or motor circuit protectors (they protect the motor feeder circuit against the short circuit fault current levels) to provide protection of the motor circuits are specified in the NEC, allowing to these circuits to exceed the ampacity of the motor feeder conductors to provide the necessary overload protection for the motor. These overload elements are generally sized at 115% to 125% of the full-load current of the motor. Section 250.12(D) –permits the required equipment grounding conductor to be determined using the rating of the overload current element, as specified in Table 250.122. The rating of the motor circuit branch circuit over-current device is used as basis for sizing of the equipment grounding conductor if inverse time breakers or time delay fuses are used to protect the motor branch circuit conductors.

Equipment Grounding Section 250.97 of the NEC specifies the requirements for bonding metal raceway and cables containing circuit operating above 250 V to ground. These requirements apply to branch circuit raceways operating at 277 V to ground supplied from a 480/277 V system. Section 250.92(B) of the NEC specifies the requirements for bonding service raceways, meter troughs, wire-ways, and other enclosures located on the supply side of the service over-current device, to guarantee the integrity of the ground path in the event of a line-to-ground fault involving the service conductors located on the supply side of the main service over-current device.

Ground Fault Circuit Interruption In addition to the operation and location of the ground fault protection requirements of the receptacles, the NEC (Sections 230.95 and 215.10) requires the use of ground fault protection on services and feeders. The ground fault protection on disconnecting devices must be rated at 1000 A or more on on solidly grounded wye systems of greater than 150 V to ground but not exceeding 600 V phase-to-phase.

Grounding of Instrument Transformer Current and Voltage (Instrument) Transformer; The most use instrument transformers is the current transformer used to monitor large ampacity services, or the voltage transformer used to step-down the voltage of services rated above 600 V

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