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9 Recommended Practices for Grounding
9 Recommended Practices for Grounding (photo credit: ag0n.net)

Basis for safety and power quality

Grounding and bonding are the basis upon which safety and power quality are built. The grounding system provides a low-impedance path for fault current and limits the voltage rise on the normally non-current-carrying metallic components of the electrical distribution system.

During fault conditions, low impedance results in high fault current flow, causing overcurrent protective devices to operate, clearing the fault quickly and safely. The grounding system also allows transients such as lightning to be safely diverted to earth.

Bonding is the intentional joining of normally non-current-carrying metallic components to form an electrically conductive path. This helps ensure that these metallic components are at the same potential, limiting potentially dangerous voltage differences.

Careful consideration should be given to installing a grounding system that exceeds the minimum NEC requirements for improved safety and power quality.

1. Equipment Grounding Conductors

The IEEE Emerald Book recommends the use of equipment-grounding conductors in all circuits, not relying on a raceway system alone for equipment grounding. Use equipment grounding conductors sized equal to the phase conductors to decrease circuit impedance and improve the clearing time of overcurrent protective devices.

Equipment grounding conductor
Equipment grounding conductor

Bond all metal enclosures, raceways, boxes, and equipment grounding conductors into one electrically continuous system. Consider the installation of an equipment grounding conductor of the wire type as a supplement to a conduit-only equipment grounding conductor for especially sensitive equipment.

The minimum size the equipment grounding conductor for safety is provided in NEC 250.122, but a full-size grounding conductor is recommended for power quality considerations.

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2. Isolated Grounding System

As permitted by NEC 250.146(D) and NEC 408.40 Exception, consider installing an isolated grounding system to provide a clean signal reference for the proper operation of sensitive electronic equipment.

Isolated grounding system for branch circuits
Isolated grounding system for branch circuits (photo credit: iaeimagazine.org)

Isolated grounding is a technique that attempts to reduce the chances of “noise” entering the sensitive equipment through the equipment grounding conductor. The grounding pin is not electrically connected to the device yoke, and, so, not connected to the metallic outlet box. It is therefore “isolated” from the green wire ground.

A separate conductor, green with a yellow stripe, is run to the panelboard with the rest of the circuit conductors, but it is usually not connected to the metallic enclosure. Instead it is insulated from the enclosure, and run all the way through to the ground bus of the service equipment or the ground connection of a separately derived system. Isolated grounding systems sometimes eliminate ground loop circulating currents.

Note that the NEC prefers the term isolated ground, while the IEEE prefers the term insulated ground.

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3. Branch-Circuit Grounding

Replace branch circuits that do not contain an equipment ground with branch circuits with an equipment ground. Sensitive electronic equipment, such as computers and computer-controlled equipment, require the reference to ground provided by an equipment grounding conductor for proper operation and for protection from static electricity and power surges.

Failure to utilize an equipment grounding conductor may cause current flow through low-voltage control or communication circuits, which are susceptible to malfunction and damage, or the earth.

Surge Protection Devices (SPDs) must have connection to an equipment grounding conductor.

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4. Ground Resistance

Measure the resistance of the grounding electrode system to ground.

Take reasonable measures to ensure that the resistance to ground is 25 ohms or less for typical loads. In many industrial cases, particularly where electronic loads are present, there are requirements which need values as low as 5 ohms or less many times as low as 1 ohm.

Measuring earth resistance with fall of potential method
Measuring earth resistance with fall of potential method (photo credit: eblogbd.com)

For these special cases, establish a maintenance program for sensitive electronic loads to measure ground resistance semi-annually, initially, using a ground resistance meter. Ground resistance should be measured at least annually thereafter.

When conducting these measurements, appropriate safety precautions should be taken to reduce the risk of electrical shock.

Record the results for future reference. Investigate significant changes in ground resistance measurements compared with historical data, and correct deficiencies with the grounding system. Consult an electrical design professional for recommendations to reduce ground resistance where required.

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5. Ground Rods

The NEC permits ground rods to be spaced as little as 6 feet apart, but spheres-of-influence of the rods verlar.

Recommended practice is to space multiple ground rods a minimum of twice the length of the rod apart. Install deep-driven or chemically-enhanced ground rods in mountainous or rocky terrain, and where soil conditions are poor. Detailed design of grounding systems are beyond the scope of this document.

Earthing electrode
Earthing electrode

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6. Ground Ring

In some cases, it may be advisable to install a copper ground ring, supplemented by driven ground rods, for new commercial and industrial construction in addition to metal water piping, structural building steel, and concrete-encased electrodes, as required by Code.

Grounding rings provide a convenient place to bond multiple electrodes of a grounding system, such as multiple Ufer grounds, lightning down-conductors, multiple vertical electrodes, etc.

Install ground rings completely around buildings and structures and below the frost line in a trench offset a few feet from the footprint of the building or structure. Where low, ground impedance is essential, supplement the ground ring with driven ground rods in a triplex configuration at each corner of the building or structure, and at the mid-point of each side.

The emergency generator connected to the ring-ground, and additionally grounded to reinforcing rods in its concrete pad
The emergency generator connected to the ring-ground, and additionally grounded to reinforcing rods in its concrete pad (photo credit: psihq.com)

The NEC-minimum conductor size for a ground ring is 2 AWG, but sizes as large as 500 kcmil are more frequently used. The larger the conductor and the longer the conductor, the more surface area is in contact with the earth, and the lower the resistance to earth.

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7. Grounding Electrode System

Grounding electrode system bus
Grounding electrode system bus (photo credit: electrical-contractor.net)

Bond all grounding electrodes that are present, including metal underground water piping, structural building steel, concrete-encased electrodes, pipe and rod electrodes, plate electrodes, and the ground ring and all underground metal piping systems that cross the ground ring, to the grounding electrode system.

Bond the grounding electrodes of separate buildings in a campus environment together to create one grounding electrode system.

Bond all electrical systems, such as power, cable television, satellite television, and telephone systems, to the grounding electrode system. Bond outdoor metallic structures, such as antennas, radio towers, etc. to the grounding electrode system. Bond lightning protection down-conductors to the grounding electrode system.

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8. Lightning Protection System

Copper lightning protection systems may be superior to other metals in both corrosion and maintenance factors. NFPA 780 (Standard for the Installation of Lightning Protection Systems) should be considered as a minimum design standard.

Building lightning protection system
Building lightning protection system (photo credit: Schneider Electric)

A lightning protection system should only be connected to a high quality, low impedance, and robust grounding electrode system.

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9. Surge Protection Devices (SPD) (formerly called TVSS)

The use of surge protection devices is highly recommended. Consult IEEE Standard 1100 (The Emerald Book) for design considerations. A surge protection system should only be connected to a high quality, low impedance, and robust grounding electrode system.

Surge protection device - Single line diagram
Surge protection device – Single line diagram (credit: Schneider Electric)

Generally, a surge protection device should not be installed downstream from an uninterruptible power supply (UPS). Consult manufacturers’ guidelines.

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Reference // Recommended Practices for Designing and Installing Copper Building Wire Systems – Copper Development Association Inc.

About Author //

author-pic

Edvard Csanyi

Edvard - Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV high power busbar trunking (<6300A) in power substations, buildings and industry fascilities. Designing of LV/MV switchgears.Professional in AutoCAD programming and web-design.Present on

14 Comments


  1. ZUBER KHAN
    Sep 05, 2016

    Powerlines are running parallel to Buried Cross Country Coated Pipelines and are experiencing effect of aC induce votlage interference. We are installing copper grounding conductors of 25 sq mm cross section in carborneous coke backfill upto 150m long to get grounding resistance below 1 Ohms depending upon soil resistivity and connecting the two ends of copper grounding conductor ring to a cross country buried pipelines through a polarization cell to ground the buried coated pipeline to mitigate the effects of induce AC voltage on buried coated pipeline. After few months of installation we have found that the grounding copper conductor has oxidized and turning into green powder and copper ring got into various pieces. Please advice if copper is reacting with carbon backfill or high current discharge through grounding system is oxidizing the copper grounding ring.


  2. kamal
    Aug 01, 2016

    that my job and i have more idea in this thank u


  3. Ian A. Casey
    Jul 29, 2016

    Excellent article!

    So IG goes from isolated receptacle ground without bonding to anything back to service panel and connects to its own insulated, non bonded ground bus and the to independante grounding rod?

    SG is ground for all the mechanal bonds at receptacle, conduits, structure, neutral-ground and es to its separate ground rod so that both IG and SG never physically, mecanically bond?

    Correct?


  4. Ian A. Casey
    Jul 29, 2016

    As always, Edvard, another informative article!

    Am I correct to understand that for IG the ground pin is totally isolated at the receptacle and that conductor runs directly to the service panel?
    The SG conductor bonds/connects all metal parts/strutures and electrically connects at the service panel.
    That at the service panel there would be two grounding buses:
    a. One that bonds nuetral-ground-metal parts, etc and the SG conductor then will go to a grounding electrode.
    b. The other would be completely be isolated from metal parts, the grounded neutral and will bond the IG to its independant grouding electrodes.

    Therefore, there would not be a physical, a mechanical bond of these two grounds, correct?

    Would such cabling of L-N-G-IG conducters be applicable or necessary in an office, bank, or school?
    I see the sense in hospital and some industrial applications.

    Thanks


  5. Daniel
    Nov 12, 2015

    Señor Edvar, no soy Ingeniero, en mi país existe también un titulo menor y es de Tecnologo Eléctrico, desde hace un par de semanas que encontré su blog sigo y estudio de cerca sus aportes son muy bien explicados ademas de ajustados a la experiencia en campo, gracias por sus aportes son claros y me han despejado dudas.
    Saludos desde Sur America


  6. Karma Tenzin
    Aug 30, 2015

    I will read all the articles of Edvard hereon.


  7. CEPRO
    May 28, 2015

    Thanks for the insight.


  8. gilbert l. florida
    Apr 06, 2015

    Edvard,
    You are great ! I like all your EE articles issued. I am certainly sure that these helps engineers, professors,students, practitioner, EE project management team in the world .
    I am an experienced electrical engineer too and there are informative articles i read and even down load them for references.
    Sir Edvard appreciate If you could issue ieee standard 1100 ( the emerald book ) as well I think this would give more information to all ee practitioners.
    Man you are great! and more power for you. You help many people…
    Thanks a lot …


  9. Hossein
    Mar 26, 2015

    hi
    I am electrical power eng.
    I like your work and am intresting to have your technical subjects please.


  10. Bedekar Prakash
    Mar 12, 2015

    Please specify whether any special earthing is required for tuned filters and active filters in harmonic mitigation system. Can this earthing be connected to common grid earthing?


  11. Bob Kohli
    Mar 11, 2015

    Picture 4 could you have used the term fall of potential & talked about how you use Ohm law to teach the reason you use this way of of testing younger generation only learn what us older ones make them aware of just my thought kohli


    • John James Stockton
      Mar 23, 2015

      Mr. Kohli, if he used the term fall of potential would you think that everyone else will know meaning of that term. I think he explained it in a simple way so anyone can understand and that is a start. The reader will do his own research later on. This is still an informative and helpful site.


  12. Bob Kohli
    Mar 11, 2015

    In the picture with # 5 you use a green with yellow that is O K But why did you use a picture with a coiled conductor if by chance the voltage applied
    to this conductor was above , 600v or less than the current could be of a maganatude that some counter E M F could cause the voltage to rise on the other part of the system & not let the earthing / grounding do what it is to do just my thought B kohli at orkohli@tds.net


  13. hamdy anwar
    Mar 09, 2015

    in other subject (bonding earthing and grounding) you explain the difference bet. grounding and earthing.
    grounding is related to powr system common zero point which can be neutral in star connection.
    but earthing is for dead parts connection to the earth
    here you use term grounding for lightening system which is not power system or neutral point.
    thanks in advanc
    hamdy anwar

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