Grounding in Electrical Equipments

Importance of Grounding in Electrical Power System- In Protection and System Operation

Introduction to System and Equipment Grounding:

Grounding is the connection to earth of an electrical system or its equipment. Both electrical distribution systems and electrical equipment are connected to ground (earth).  System grounding allows for proper and safe operation of the electrical distribution system. Equipment grounding is the connection of all exposed non-current-carrying metal parts to ground to protect personnel by removing any electrical voltage that may be present on non-current-carrying metal parts. Grounding in Electrical Power system for protection and need of grounding in power system is discussed in this article.

Grounding Electrode Introduction:

A grounding electrode is a metal component that contacts the earth and has a conductor connected to it. Per the NEC, a grounding electrode can be a 5/8′′ diameter or larger copper, stainless steel, or zinc-coated steel rod that is driven into the ground and contacts the earth for 8′ or more. In large commercial and industrial applications, grounding electrode systems are used.

Large commercial and industrial distribution systems require a large surface area to contact the earth. Grounding is accomplished by connecting components and devices to metal underground pipes, metal frames of buildings, concrete-encased electrodes, or ground rings.

A grounding electrode conductor (GEC) is a conductor connecting the grounding electrode to the equipment grounding conductor and/or the grounded conductor of the circuit at the service equipment. See Figure 1.

Electrical Equipment Grounding: Grounding provides a direct path for unwanted fault current to travel to its power source without causing harm to technicians or equipment
Figure 1. Grounding provides a direct path for unwanted fault current to travel to its power source without causing harm to technicians or equipment

A grounded conductor is an electrical cable, wire, or busbar that has been intentionally grounded. A grounded conductor is commonly referred to as a neutral conductor. (However, the NEC® specifically defines the neutral conductor and, in some cases, it may not be the same as the grounded conductor.)

Safety Precautions:

Safety precautions must be taken when working with grounded conductors. There is always the possibility of voltage potential on the grounded or neutral conductor. A grounded conductor is a current-carrying conductor and can be energized. There are also requirements for color-coding the insulation of grounded conductors. Typically, the insulation on grounded conductors must be white or gray or have three continuous white stripes along its entire length.

Grounding in Electrical Power system for protection:

Grounding in Electrical Power system for protection is of very importance. Some of main causes of using grounding in power system are given in details in following a few paragraphs.

Ground Fault and Protection:

Equipment grounding protects equipment and personnel from ground faults. A ground fault is a specific type of short circuit in which current inadvertently flows into non-current- carrying components of the equipment or system.

Examples of ground faults include current that travels into a motor frame or from a metal enclosure through metal conduit. Typical causes of ground faults include insulation breakdown or the inadvertent entrance of moisture into electrical equipment. Failure to ground and bond systems properly can result in serious damage to systems, equipment, and facilities, and electrical shock hazards.

Fuses and circuit breakers are also used to protect against ground faults through use of low- impedance grounding paths. When a ground fault occurs, enough current must travel back to the power source to allow the fuse or circuit breaker to operate and clear the fault. An effective ground-fault current path is a grounding path that contains very little opposition to the flow of fault current back to the source and allows the fuse or circuit breaker to operate.

The two reasons electrical distribution systems are connected to ground are to dissipate voltage surges in the distribution system into the ground and to stabilize voltage to ground during normal system operation.

Lightning and Grounding in Power System:

The most severe form of voltage surge to occur on a distribution system is a lightning strike to the outside distribution line. Although a utility has its own systems to help dissipate voltage surges, a large voltage surge can travel down the distribution line and into a facility. As a voltage surge travels through feeders and down to the branch-circuit level, damage to electrical equipment can occur.

When lightning strikes, it seeks the shortest path to ground. Since the distribution system is composed of conductors, a large amount of electrical energy from a lightning strike can travel through a distribution system. However, by connecting a distribution system to ground at or near the point where the electrical system enters the facility, a path for a voltage surge to travel into the earth is provided.

With a properly installed system with low impedance (total opposition to the flow of current in a circuit), a voltage surge can readily take an intentionally installed path into the earth rather than through the facility’s electrical distribution system. See Figure 2.

Grounding in Electrical Power system for protection: Proper grounding ensures that there is a low-impedance grounding path for fault current to earth ground.
Figure 2. Proper grounding ensures that there is a low-impedance grounding path for fault current to earth ground.

As a distribution system operates, the voltage can vary due to loads cycling on and off. Voltage, as measured from an ungrounded conductor in the system to ground, can also vary. By connecting an electrical distribution system to ground, the voltage from an un-grounded conductor to ground is stabilized during normal operation.

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