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9 Recommended Practices for Grounding

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

Basis for safety and power qualityGrounding and bonding are the basis upon which safety and power quality are built. The grounding system providesa low-impedance path for fault current and limits the voltage rise on the normally non-current-carrying metalliccomponents of the electrical distribution system.

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

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

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

Recommended practices for grounding //

1. Equipment Grounding ConductorsThe IEEE Emerald Book recommends the use of equipment-grounding conductors in all circuits, not relying on araceway system alone for equipment grounding. Use equipment grounding conductors sized equal to the phaseconductors to decrease circuit impedance and improve the clearing time of overcurrent protective devices.

Equipment grounding conductor

Bond all metal enclosures, raceways, boxes, and equipment grounding conductors into one electrically continuoussystem. 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-sizegrounding conductor is recommended for power quality considerations.

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2. Isolated Grounding SystemAs permitted by NEC 250.146(D) and NEC 408.40 Exception, consider installing an isolated grounding system toprovide a clean signal reference for the proper operation of sensitive electronic equipment.

Isolated grounding is a technique that attempts to reduce the chances of noise entering the sensitive equipmentthrough 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 itis usually not connected to the metallic enclosure. Instead it is insulated from the enclosure, and run all the waythrough to the ground bus of the service equipment or the ground connection of a separately derived system. Isolatedgrounding systems sometimes eliminate ground loop circulating currents.

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

Isolated grounding system for branch circuits (photo credit:

iaeimagazine.org)

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3. Branch-Circuit GroundingReplace branch circuits that do not contain anequipment ground with branch circuits with anequipment ground. Sensitive electronic equipment, suchas computers and computer-controlled equipment,require the reference to ground provided by anequipment grounding conductor for proper operation andfor protection from static electricity and power surges.

Failure to utilize an equipment grounding conductormay cause current flow through low-voltage control orcommunication circuits, which are susceptible tomalfunction and damage, or the earth.

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

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4. Ground ResistanceMeasure 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 manyindustrial cases, particularly where electronic loads are present, there are requirements which need values as low as5 ohms or less many times as low as 1 ohm.

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

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

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

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5. Ground RodsThe NEC permits ground rods to be spaced as little as 6 feet apart, but spheres-of-influence of the rodsverlar.

Recommended practice is to space multiple ground rods a minimum of twice the length of the rod apart. Install deep-

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

driven or chemically-enhanced ground rods in mountainous or rocky terrain, and where soil conditions are poor.Detailed design of groundingsystems are beyond the scope of thisdocument.

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6. Ground RingIn some cases, it may be advisableto install a copper ground ring,supplemented by driven groundrods, for new commercial andindustrial construction in addition tometal water piping, structural buildingsteel, and concrete-encasedelectrodes, as required by Code.

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

Install ground rings completelyaround buildings and structures andbelow the frost line in a trench offseta few feet from the footprint of thebuilding 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 orstructure, and at the mid-point of each side.

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

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7. Grounding Electrode SystemBond 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 undergroundmetal 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 groundingelectrode system.

Bond all electrical systems , such as power, cable television, satellite television, and telephone

Earthing electrode

The emergency generator connected to the ring-ground, andadditionally grounded to reinforcing rods in its concrete pad (photo

credit: psihq.com)

systems, to the grounding electrode system. Bondoutdoor metallic structures, such as antennas, radiotowers, etc. to the grounding electrode system. Bondlightning protection down-conductors to the groundingelectrode system.

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8. Lightning Protection SystemCopper lightning protection systems may be superior toother metals in both corrosion and maintenance factors.NFPA 780 (Standard for the Installation of LightningProtection Systems) should be considered as a minimumdesign standard.

A lightning protection system should only be connected to ahigh quality, low impedance, and robust groundingelectrode system.

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9. Surge Protection Devices (SPD) (formerlycalled TVSS)The use of surge protection devices is highly recommended.Consult IEEE Standard 1100 (The Emerald Book) for designconsiderations. A surge protection system should only beconnected to a high quality, low impedance, and robustgrounding electrode system.

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

Building lightning protection system (photo credit: Schneider Electric)

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 CopperDevelopment Association Inc.

9 Recommended Practices for GroundingBasis for safety and power qualityRecommended practices for grounding //1. Equipment Grounding Conductors2. Isolated Grounding System3. Branch-Circuit Grounding4. Ground Resistance5. Ground Rods6. Ground Ring7. Grounding Electrode System8. Lightning Protection System9. Surge Protection Devices (SPD) (formerly called TVSS)