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Why do we need grounding?

Grounding is defined as a conducting connection, whether intentional or accidental, by which an electric circuit or equipment is connected to the earth or to some conducting body of relatively large extent that serves in place of the earth. Simple as it is and defined a long time ago by IEEE.

Practices in the design and installation of a facility ground system

Grounding is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground current to and from the earth (or the conducting body).

Based on this definition, the reasons for grounding can be identified as:

  1. Personnel safety by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system
  2. Personnel safety and control of electrostatic discharge (ESD) by limiting potentials between all noncurrent-carrying metal parts of an electrical distribution system and the
  3. Earth fault isolation and equipment safety by providing a low-impedance fault return path to the power
    source to facilitate the operation of overcurrent devices during a ground fault
The IEEE definition makes an important distinction between ground and earth. Earth refers to mother earth, and ground refers to the equipment grounding system, which includes equipment grounding conductors, metallic raceways, cable armor, enclosures, cabinets, frames, building steel, and all other noncurrent-carrying metal parts of the electrical distribution system.

There are other reasons for grounding not implicit in the IEEE definition. Overvoltage control has long been a benefit of proper power-system grounding. With the increasing use of electronic computer systems, noise control has become associated with the subject of grounding.

Equipment Grounding

Personnel safety is achieved by interconnecting all noncurrent-carrying metal parts of an electrical distribution system and then connecting the interconnected metal parts to the earth. This process of interconnecting metal parts is called equipment grounding and is illustrated in Figure 1, where the equipment grounding conductor is used to interconnect the metal enclosures.

Equipment grounding and system grounding
Figure 1 – Equipment grounding and system grounding

Equipment grounding insures that there is no difference of potential, and thus no shock hazard, between noncurrent-carrying metal parts anywhere in the electrical distribution system. Connecting the equipment grounding system to earth insures that there is no difference of potential between the earth and the equipment grounding system.

It also prevents static charge buildup.

System Grounding

System grounding, which is also illustrated in Figure 1, is the process of intentionally connecting one of the current-carrying conductors of the electrical distribution system to ground. The figure shows the neutral conductor intentionally connected to ground and the earth. This conductor is called the grounded conductor because it is intentionally grounded.

The purpose of system grounding is overvoltage control and equipment safety through fault isolation. An ungrounded system is subject to serious overvoltages under conditions such as intermittent ground faults, resonant conditions, and contact with higher voltage systems.

Fault isolation is achieved by providing a low-impedance return path from the load back to the source, which will ensure operation of overcurrent devices in the event of a ground fault. The system ground connection makes this possible by connecting the equipment grounding system to the low side of the voltage source.

Methods of system grounding include solidly grounded, ungrounded, and impedance-grounded.

Solidly grounded wye power system
Figure 2 – Solidly grounded wye power system

Solidly grounded means that an intentional zero-impedance connection is made between a current-carrying conductor and ground. The single-phase system shown in Figure 1 is solidly grounded. A solidly grounded, three-phase, four-wire, wye system is illustrated in Figure 2. The
neutral is connected directly to ground with no impedance installed in the neutral circuit.

The NEC permits this connection to be made at the service entrance only. The advantages of a solidly grounded wye system include reduced magnitude of transient overvoltages, improved fault protection, and faster location of ground faults. There is one disadvantage of the solidly grounded wye system.

For low-level arcing ground faults, the application of sensitive, properly coordinated, ground-fault protection (GFP) devices is necessary to prevent equipment damage from arcing ground faults. The NEC requires arcing ground-fault protection at 480 Y/277 V services, and a maximum sensitivity limit of 1200 A is permitted.

Severe damage is less frequent at the lower voltage 208 V systems, where the arc may be self-extinguishing.

Title:Practices in the design and installation of a facility ground system – J. C. Whitaker
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Good practices in the design and installation of a facility ground system
Good practices in the design and installation of a facility ground system

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  1. Raoul Desir
    Dec 09, 2022

    I have a question , our team is working on a project that has an ungrounded system and we need to be compliant with NEC 250-21B , Basler has a relay that works with open delta system , we did get the schematic of the open delta that is supposed to energize the coil of the relay upon a ground fault , the system is 800V , how do we test it .

    The output of the open delta is on energized when there is a fault in the system.

    Thanks in advance

  2. Melvin
    Oct 29, 2022

    Thanks for sharing

  3. Javier Guadarrama
    Nov 04, 2020

    Excelent technical article, grounding is a topic very import for all companies to avoid accidents and damage in equipment.

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