what is electrical grounding

7/27/2019 What is Electrical Grounding

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What Is Electrical Grounding?ELECTRICAL GROUNDING or "Grounding" originally began as a safety measure used to help prevent

people from accidentally coming in contact with electrical hazards. Think of your refrigerator. It is a metal

box standing on rubber feet with electricity running in and out of it. You use magnets to hang your child's

latest drawing on the metal exterior. The electricity running from the outlet and through the power cord to

the electrical components inside the refrigerator are electrically isolated from the metal exterior or chassis of

the refrigerator.

If far some reason the electricity came in contact with the chassis, the rubber feet would prevent the

electricity from going anywhere and it would sit waiting for someone to walk up and touch the refrigerator.

Once someone touched the refrigerator the electricity would flow from the chassis of the refrigerator and

through the unlucky person possibly causing injury.

Grounding is used to protect that person. By connecting a wire from the metal frame of the refrigerator to

the ground, if the chassis inadvertently becomes charged for any reason, the unwanted electricity will travel

down the wire and out safely into the earth; and in the process, trip the circuit-breaker stopping the flow of

electricity. Obviously, that wire has to connect to something that is in turn connected to the earth or ground

outside. Typically this connection is a grounding electrode.

The process of electrically connecting to the earth itself is often called "earthing", particularly in Europewhere the term "grounding" is used to describe the above ground wiring. The term "Grounding" is used in

America to discuss both earthing and grounding.

While electrical grounding may have originally been considered only as a safety measure, with today's

advances in electronics and technology, electrical grounding has become an essential part of everyday

electricity. Computers, televisions, microwave ovens, fluorescent lights and many other electrical devices,

generate lots of "electrical noise" that can damage equipment and cause it to work less efficiently. Proper

grounding can not only remove this unwanted "noise", but can even make surge protection devices work

better.



What Are Some Different Types of Grounding Electrodes?Grounding is the process of electrically connecting any metallic object to the earth by the way of an earth

electrode system. The National Electric Code requires that the grounding electrodes be tested to ensure that

they are under 25-ohms resistance-to-ground (Earth). It is important to know that aluminum electrodes are

not allowed for use in grounding.

Driven Rod

The standard driven rod or copper-clad rod consists of an 8 to 10 foot length of steel with a 5 to 10-mil

coating of copper. This is by far the most common grounding device used in the field today. The driven rod

has been in use since the earliest days of electricity with a history dating as far back as Benjamin Franklin.

Driven rods are relatively inexpensive to purchase, however ease of installation is dependent upon the type

of soil and terrain where the rod is to be installed. The steel used in the manufacture of a standard drivenrod tends to be relatively soft. Mushrooming can occur on both the tip of the rod, as it encounters rocks on

its way down, and the end where force is being applied to drive the rod through the earth. Driving these

rods can be extremely labor-intensive when rocky terrain creates problems as the tips of the rods continue

to mushroom. Often, these rods will hit a rock and actually turn back around on themselves and pop back

up a few feet away from the installation point.

Because driven rods range in length from 8 to 10 feet, a ladder is often required to reach the top of the rod,

which can become a safety issue. Many falls have resulted from personnel trying to literally „whack‟ these

rods into the earth, while hanging from a ladder, many feet in the air.



The National Electric Code (NEC) requires that driven rods be a minimum of 8 feet in length and that 8 feet

of length must be in direct contact with the soil. Typically, a shovel is used to dig down into the ground 18

inches before a driven rod is installed. The most common rods used by commercial and industrial

contractors are 10 ft in length. Many industrial specifications require this length as a minimum.

A common misconception is that the copper coating on a standard driven rod has been applied for electrical

reasons. While copper is certainly a conductive material, its real purpose on the rod is to provide corrosion

protection for the steel underneath. Many corrosion problems can occur because copper is not always the

best choice in corrosion protection. It should be noted that galvanized driven rods have been developed to

address the corrosion concerns that copper presents, and in many cases are a better choice for prolonging

the life of the grounding rod and grounding systems. Generally speaking, galvanized rods are a better choice

in all but high salt environments.

An additional drawback of the copper-clad driven rod is that copper and steel are two dissimilar metals.

When an electrical current is imposed, electrolysis will occur. Additionally, the act of driving the rod into the

soil can damage the copper cladding, allowing corrosive elements in the soil to attack the bared steel and

further decrease the life expectancy of the rod. Environment, aging, temperature and moisture also easily

affect driven rods, giving them a typical life expectancy of five to 15 years in good soil conditions. Driven

rods also have a very small surface area, which is not always conducive to good contact with the soil. This is

especially true in rocky soils, in which the rod will only make contact on the edges of the surrounding rock.

A good example of this is to imagine a driven rod surrounded by large marbles. Actual contact between the

marbles and the driven rod will be very small. Because of this small surface contact with the surrounding

soil, the rod will increase in resistance-to-ground, lowering the conductance, and limiting its ability to handle

high-current faults.

Advanced Driven Rods

Advanced Driven Rods are specially engineered variations of the standard driven rod, with several key

improvements. Because they present lower physical resistance, advanced rods can now go into terrain

where only large drill-rigs could install before and can quickly be installed in less demanding environments.

The modular design of these rods can reduce safety-related accidents during installation. Larger surface

areas can improve electrical conductance between the soil and the electrode.



Of particular interest is that Advanced Driven Rods can easily beinstalled to depths of 20 ft or more, depending upon soil conditions.

Advanced Driven Rods are typically driven into the ground with a standard drill hammer. This automation

dramatically reduces the time required for installation. The tip of an Advanced Driven Rod is typically made

of carbide and works in a similar manner to a masonry drill bit, allowing the rod to bore through rock with

relative ease. Advanced Driven Rods are modular in nature and are designed in five foot lengths. They have

permanent and irreversible connections that enable an operator to install them safely, while standing on the

ground. Typically, a shovel is used to dig down into the ground 18 inches before the Advanced Driven Rod is

installed. The Advanced Driven Rod falls into the same category as a driven rod and satisfies the same codes

and regulations.

In the extreme northern and southern climates of the planet, frost-heave is a major concern. As frost sets in

every winter, unsecured objects buried in the earth tend to be pushed up and out of the ground. Drivengrounding rods are particularly susceptible. Anchor plates are sometimes welded to the bottom of the rods

to prevent them from being pushed up and out of the earth by frost-heave. This however requires that a

hole be augured into the earth in order to get the anchor plate into the ground, which can dramatically

increase installation costs. Advanced Driven Rods do not suffer from frost-heave issues and can be installed

easily in extreme climes.

Grounding Plates



Grounding plates are typically thin copper plates buried in direct contact with the earth. The National Electric

Code requires that ground plates have at least 2 ft2 of surface area exposed to the surrounding soil. Ferrous

materials must be at least .20 inches thick, while non-ferrous materials (copper) need only be .060 inches

thick. Grounding plates are typically placed under poles or supplementing counterpoises.

As shown, grounding plates should be buried at least 30 inches below grade level. While the surface area of

grounding plates is greatly increased over that of a driven rod, the zone of influence is relatively small as

shown in “B”. The zone of influence of a grounding plate can be as small as 17 inches. This ultra-small zone

of influence typically causes grounding plates to have a higher resistance reading than other electrodes of

similar mass. Similar environmental conditions that lead to the failure of the driven rod also plague the

grounding plate, such as corrosion, aging, temperature, and moisture.

Ufer Ground or Concrete Encased Electrodes

Originally, Ufer grounds were copper electrodes encased in the concrete surrounding ammunition bunkers.

In today‟s terminology, Ufer grounds consist of any concrete-encased electrode, such as the rebar in a

building foundation, when used for grounding, or a wire or wire mesh in concrete.

Concrete Encased Electrode

The National Electric Code requires that Concrete Encased Electrodes use a minimum No. 4 AWG copper

wire at least 20 feet in length and encased in at least 2 inches of concrete. The advantages of concrete

encased electrodes are that they dramatically increase the surface area and degree of contact with the

surrounding soil. However, the zone of influence is not increased, therefore the resistance to ground is

typically only slightly lower than the wire would be without the concrete.



Concrete encased electrodes also have some significant disadvantages. When an electrical fault occurs, theelectric current must flow through the concrete into the earth. Concrete, by nature retains a lot of water,

which rises in temperature as the electricity flows through the concrete. If the extent of the electrode is not

sufficiently great for the total current flowing, the boiling point of the water may be reached, resulting in an

explosive conversion of water into steam. Many concrete encased electrodes have been destroyed after

receiving relatively small electrical faults. Once the concrete cracks apart and falls away from the conductor,

the concrete pieces act as a shield preventing the copper wire from contacting the surrounding soil, resulting

in a dramatic increase in the resistance-to-ground of the electrode.

There are many new products available on the market designed to improve concrete encased electrodes.

The most common are modified concrete products that incorporate conductive materials into the cement

mix, usually carbon. The advantage of these products is that they are fairly effective in reducing the

resistivity of the concrete, thus lowering the resistance-to-ground of the electrode encased. The most

significant improvement of these new products is in reducing heat buildup in the concrete during fault

conditions, which can lower the chances that steam will destroy the concrete encased electrode. Howeversome disadvantages are still evident. Again, these products do not increase the zone-of-influence and as

such the resistance-to-ground of the concrete encased electrode is only slightly better than what a bare

copper wire or driven rod would be in the ground. Also a primary concern regarding enhanced grounding

concretes is the use of carbon in the mix. Carbon and copper are of different nobilities and will sacrificially

corrode each other over time. Many of these products claim to have buffer materials designed to reduce the

accelerated corrosion of the copper caused by the addition of carbon into the mix. However, few

independent long-term studies are being conducted to test these claims.

Ufer Ground or Building Foundations

Ufer Grounds or building foundations may be used provided that the concrete is in direct contact with the

earth (no plastic moisture barriers), that rebar is at least 0.500 inches in diameter and that there is a direct

metallic connection from the service ground to the rebar buried inside the concrete.



This concept is based on the conductivity of the concrete and the large surface area, which will usuallyprovide a grounding system that, can handle very high current loads. The primary drawback occurs during

fault conditions, if the fault current is too great compared with the area of the rebar system, when moisture

in the concrete superheats and rapidly expands, cracking the surrounding concrete and the threatening the

integrity of the building foundation. Another drawback to the Ufer ground is they are not testable under

normal circumstances as isolating the concrete slab in order to properly perform resistance-to-ground

testing is nearly impossible.

The metal frame of a building may also be used as a grounding point, provided that the building foundation

meets the above requirements, and is commonly used in high-rise buildings. It should be noted that many

owners of these high-rise buildings are banning this practice and insisting that tenants run ground wires all

the way back to the secondary service locations on each floor. The owners will have already run ground

wires from the secondary services back to the primary service locations and installed dedicated grounding

systems at these service locations. The goal is to avoid the flow of stray currents, which can interfere with

the operation of sensitive electronic equipment.

Water Pipes

Water pipes have been used extensively over time as a grounding electrode. Water pipe connections are not

testable and are unreliable due to the use of tar coatings and plastic fittings. City water departments have

begun to specifically install plastic insulators in the pipelines to prevent the flow of current and reduce the

corrosive effects of electrolysis. The National Electric Code requires that at least one additional electrode be

installed when using water pipes as an electrode. There are several additional requirements including:

10 feet of the water pipe is in direct contact with the earth

Joints must be electrically continuous

Water meters may not be relied upon for the grounding path Bonding jumpers must be used around any insulating joints, pipe or meters

Primary connection to the water pipe must be on the street side of the water meter

Primary connection to the water pipe shall be within five feet of the point of entrance to the building

The National Electric Code requires that water pipes be bonded to ground, even if water pipes are not used

as part of the grounding system.

Electrolytic Electrode



The electrolytic electrode was specifically engineered to eliminate the drawbacks of other grounding

electrodes. This active grounding electrode consists of a hollow copper shaft filled with natural earth salts

and desiccants whose hygroscopic nature draws moisture from the air. The moisture mixes with the salts to

form an electrolytic solution that continuously seeps into the surrounding backfill material, keeping it moist

and high in ionic content.

The electrolytic electrode is

installed into an augured hole and backfilled with a special highly conductive product. This specialty product

should protect the electrode from corrosion and improve its conductivity. The electrolytic solution and the

special backfill material work together to provide a solid connection between the electrode and the

surrounding soil that is free from the effects of temperature, environment, and corrosion. This active

electrode is the only grounding electrode that improves with age. All other electrode types will have a

rapidly increasing resistance-to-ground as the season‟s change and the years pass. The drawbacks to these

electrodes are the cost of installation and the cost of the electrode itself.

Earth-Electrode Comparison Chart

The following chart compares the various types of electrodes versus some important characteristics thatmay prove helpful in selecting proper electrode usage.



Driven

Rod

Advanced

Driven

Rod

Grounding

Plate

Concrete

Encased

Electrode

Building

Foundation

Water

Pipe

Electrolytic

Electrode

Resistance-

to-Ground

(RTG)

Poor Average Poor AverageAbove

Average

Poor to

Excellent**Excellent

Corrosion

Resistance Poor Good Poor Good * Good * Varies High

Increase in

RTG in Cold

Weather

Highly

Affected

Slightly

Affected

Highly

Affected

Slightly

Affected

Slightly

Affected

Minimally

Affected

Minimally

Affected

Increase in

RTG over

Time

RTG

Worsens

RTG

typically

unaffected

RTG

Increases

RTG

typically

unaffected

RTG

typically

unaffected

RTG

typically

unaffected

RTG

Improves

Electrode

Ampacity Poor Average Average Average *

Above

Average *

Poor to

Excellent**Excellent

Installation

Cost Average Excellent

Below

Average

Below

AverageAverage Average Poor

Life

Expectancy

Poor

5–10

years

Average

15–20

years

Poor

5-10 years

Average *

15-20

years

Above

Average *

20-30 years

Below

Average*

10-15

years

Excellent

30-50 years

* High-current discharges can damage foundations when water inthe concrete is rapidly converted into steam.

** When part of extensive, bare, metallic, electrically continuous

water system.

How To Do Electrical Grounding System Design

Grounding System Design & Planning

A grounding design starts with a site analysis, collection of geological data, and soil resistivity of the area.

Typically, the site engineer or equipment manufacturers specify a resistance-to-ground number. The

National Electric Code (NEC) states that the resistance-to-ground shall not exceed 25 ohms for a single

electrode. However, high technology manufacturers will often specify 3 or 5 ohms, depending upon the

requirements of their equipment. For sensitive equipment and under extreme circumstances, a one (1) ohm

specification may sometimes be required. When designing a ground system, the difficulty and costs increase

exponentially as the target resistance-to-ground approaches the unobtainable goal of zero ohms.

Data Collection

Once a need is established, data collection begins. Soil resistivity testing, geological surveys, and test

borings provide the basis for all grounding design. Proper soil resistivity testing using the Wenner 4-point

method is recommended because of its accuracy. This method will be discussed later in this chapter.

Additional data is always helpful and can be collected from existing ground systems located at the site. For

example, driven rods at the location can be tested using the 3-point fall-of-potential method or an induced

frequency test using a clamp-on ground resistance meter.

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Data Analysis

With all the available data, sophisticated computer programs can begin to provide a soil model showing the

soil resistivity in ohm-meters and at various layer depths. Knowing at what depth the most conductive soil is

located for the site allows the design engineer to model a system to meet the needs of the application.

Grounding Design

Soil resistivity is the key factor that determines the resistance or performance of an electrical grounding

system. It is the starting point of any electrical grounding design. As you can see in Tables 2 and 3 below,

soil resistivity varies dramatically throughout the world and is heavily influenced by electrolyte content,

moisture, minerals, compactness and temperature.

Type of Surface MaterialResistivity of Sample in Ohmmeters

Dry Wet

Crusher granite w/ fines 140 x 106 1,300

Crusher granite w/ fines 1.5” 4,000 1,200

Washed granite – pea gravel 40 x 106 5,000

Washed granite 0.75” 2 x 106 10,000

Washed granite 1-2” 1.5 x 106 to 4.5 x 106 5,000

Washed granite 2-4” 2.6 x 106 to 3 x 106 10,000

Washed limestone 7 x 106 2,000 to 3,000

Asphalt 2 x 106 to 30 x 106 10,000 to 6 x 106

Concrete 1 x 106 to 1 x 109 21 to 100

Soil Types or Type of Earth Average Resistivity in Ohm-meters

Bentonite 2 to 10

Clay 20 to 1,000

Wet Organic Soils 10 to 100

Moist Organic Soils 100 to 1,000

Dry Organic Soils 1,000 to 5,000

Sand and Gravel 50 to 1,000

Surface Limestone 100 to 10,000

Limestone 5 to 4,000

Shale‟s 5 to 100

Sandstone 20 to 2,000

Granites, Basalt‟s, etc. 1,000

Decomposed Gneiss‟s 50 to 500

Slates, etc. 10 to 100






How To Do Grounding System TestingThe measurement of ground resistance for an earth electrode system is very important. It should be done

when the electrode is first installed, and then at periodic intervals thereafter. This ensures that the

resistance-to-ground does not increase over time. There are two (2) methods for testing an existing earth-

electrode system. The first is the 3-point or Fall-of- Potential method and the second is the Induced

Frequency test or clamp-on method. The 3-point test requires complete isolation from the power utility. Not

just power isolation, but also removal of any neutral or other such ground connections extending outside thegrounding system. This test is the most suitable test for large grounding systems and is also suitable for

small electrodes. The induced frequency test can be performed while power is on and actually requires the

utility to be connected to the grounding system under test. This test is accurate only for small electrodes, as

it uses frequencies in the kiloHertz range, which see long conductors as inductive chokes and therefore do

not reflect the 60 Hz resistance of the entire grounding system.

Fall-of-Potential Method or 3-Point Test

The 3-point or fall-of-potential method is used to measure the resistance-to-ground of existing grounding

systems. The two primary requirements to successfully complete this test are the ability to isolate the

grounding system from the utility neutral and knowledge of the diagonal length of the grounding system

(i.e. a 10‟ x 10‟ grounding ring would have a 14‟ diagonal length). In this test, a short probe, referred to as

probe Z, is driven into the earth at a distance of ten times (10X) the diagonal length of the groundingsystem (rod X). A second probe (Y) is placed in-line at a distance from rod X equal to the diagonal length of

the grounding system.

At this point, a known current is applied across X & Z, while the resulting voltage is measured across X & Y.

Ohm‟s Law can then be applied (R=V/I) to calculate the measured resistance. Probe Y is then moved out to

a distance of 2X the diagonal length of the grounding system, in-line with X & Z, to repeat the resistance

measurement at the new interval. This will continue, moving probe Y out to 3X, 4X, ... 9X the diagonal

length to complete the 3–point test with a total of nine (9) resistance measurements.



Graphing & Evaluation

The 3-point test is evaluated by plotting the results as data points with the distance from rod X along the X-

axis and the resistance measurements along the Y-axis to develop a curve. Roughly midway between the

center of the electrode under test and the probe Z, a plateau or “flat spot” should be found, as shown in the

graph. The resistance of this plateau (actually, the resistance measured at the location 62% from the center

of the electrode under test, if the soil is perfectly homogeneous) is the resistance-to-ground of the tested

grounding system.

Invalid Tests

If no semblance of a plateau is found and the graph is observed to rise steadily the test is considered

invalid. This can be due to the fact that probe Z was not placed far enough away from rod X, and can usually

indicate that the diagonal length of the grounding system was not determined correctly. If the graph is

observed to have a low plateau that extends the entire length and only rises at the last test point, then this

also may be also considered invalid. This is because the utility or telecom neutral connection remains on the

grounding system.



Induced Frequency Testing or Clamp-On Testing

The Induced Frequency testing or commonly called the “Clamp-On” test is one of the newest test methods

for measuring the resistance-to-ground of a grounding system or electrode. This test uses a special

transformer to induce an oscillating voltage (often 1.7 kHz) into the grounding system. Unlike the 3-point

Test which requires the grounding system to be completely disconnected and isolated before testing, this

method requires that the grounding system under test be connected to the electric utilities (or other large

grounding system such as from the telephone company) grounding system (typically via the neutral return

wire) to provide the return path for the signal. This test is the only test that can be used on live or „hot”

systems. However, there are some limitations, primarily being:

1. The amount of amperage running through the tested system must be below the

equipment manufacturer‟s limits.

2. The test signal must be injected at the proper location, so that the signal is forced

through the grounding system and into the earth.

3. This instrument actually measures the sum of the resistance of the grounding system

under test and the impedance of the utility neutral grounding, including the neutral

wiring. Due to the high frequency used, the impedance of the neutral wiring is non-

negligible and can be greater than the ground resistance of a very low resistance

grounding system, which can therefore not be measured accurately.

4. The ground resistance of a large grounding system at 60 Hz can be significantly

lower than at 1.7 kHz.

Many erroneous tests have been conducted where the technician only measured metallic loops and not thetrue resistance-to-ground of the grounding system. The veracity of the Induced Frequency Test has been

questioned due to testing errors, however when properly applied to a small to medium sized, self-standing

grounding system, this test is rapid and reasonably accurate.

Test Application

The proper use of this test method requires the utility neutral to be connected to a wye-type transformer.

The oscillating voltage is induced into the grounding system at a point where it will be forced into the soil



and return through the utility neutral. Extreme caution must be taken at this point as erroneous readings

and mistakes are often made. The most common of these occur when clamping on or inducing the oscillating

voltage into the grounding system at a point where a continuous metallic path exists back to the point of the

test. This can result in a continuity test being performed rather than a ground resistance test.

Understanding the proper field application of this test is vital to obtaining accurate results. The induced

frequency test can test grounding systems that are in use and does not require the interruption of service to

take measurements.

Ground Resistance Monitoring

Ground resistance monitoring is the process of automated timed and/or continuous resistance-to ground

measurement. These dedicated systems use the induced frequency test method to continuously monitor the

performance of critical grounding systems. Some models may also provide automated data reporting. Thesenew meters can measure resistance-to-ground and the current that flows on the grounding systems that are

in use. Another benefit is that it does not require interruption of the electrical service to take these

measurements.

The 10 Worst Grounding Mistakes You'll Ever Make

Why common errors in residential, commercial, and industrial wiring can lead to fire andelectric shock hazards

Proper grounding and bonding prevent unwanted voltage on non-current-carrying metal

objects, such as tool and appliance casings, raceways, and enclosures, as well as facilitate the

correct operation of overcurrent devices. But beware of wiring everything to a ground rod

and considering the job well done. There are certain subtleties you must follow to adhere to

applicable NEC rules and provide safe installations to the public and working personnel.



Although ground theory is a vast subject, on which whole volumes have been written, let's

take a look at some of the most common grounding errors you may run into on a daily basis.

1. Improper replacement of non-grounding receptacles. Dwellings and non-

dwellings often contain non-grounding receptacles (Photo 1). It's not the NEC's

intent to immediately replace all noncompliant equipment with each new edition of

the Code. In fact, it's perfectly fine to leave the old “two prongers” in place. But

because an intact functioning equipment ground is such an obvious safety feature,

most electricians tend to replace these old relics whenever possible. There are several

ways you can complete this upgrade, many of which are erroneous and strictly against

the Code. For example, never apply the following non-NEC-compliant solutions:

o Hook up a new grounding receptacle on the theory that this is a step in the right

direction. This can lead future electricians and occupants to believe they are fully

protected by a non-functioning ground receptacle.

o Connect the green grounding terminal of a grounded receptacle via a short

jumper to the grounded neutral conductor. This practice is totally noncompliant

and dangerous because when a load is connected, voltage will appear on both theneutral and ground wires. Therefore, any noncurrent-carrying appliance or tool

case will become energized, causing shock to the user, who is typically partially

or totally grounded.

o Run an individual ground conductor from the green grounding terminal of a

grounded receptacle to the nearest water pipe or other grounded object. This



“floating ground” presents various hazards. It is likely that this ground rod of

convenience will have several ohms of ground resistance so that, in case of

ground fault within a connected tool or appliance, the breaker will not trip — and

exposed metal will remain energized.

o Run an individual ground conductor back to the entrance panel and connect it to

the neutral bar or grounding strip. This solution is somewhat better, but still

noncompliant. Any grounding conductor must be within the circuit cable or

raceway. One objection is that an individual conductor could be damaged or

removed in the course of work taking place in the future.

What are the correct ways to handle this type of situation, when you find yourself

working with non-grounded receptacles?

o The best approach is to run a new branch circuit back to the panel, verifying

presence of a valid ground. Because this procedure usually involves fishing cable

behind walls or, in some cases, removing and then replacing wall finish, it's not

always feasible unless a total rewiring job is being performed.

o Another possibility is to replace the two-prong receptacle with a GFCI. Hook up

the two wires and leave the grounding terminal unattached. Included with the

GFCI is a sticker that says, “No equipment ground.” This sticker must be in place

so that future electricians and users are not misled. The thinking behind this

strategy is that even though the tool or appliance case is not grounded, the GFCI

will provide enhanced safety. It's important to note that a GFCI functions

properly without the presence of a grounding conductor. The device compares

current flowing through the hot and neutral conductors and trips if a difference

of more than 5 milliamps is detected.

o Non-grounding receptacles are still manufactured. If replacement is necessary

(and acquiring a ground is not feasible), installation of a new non-grounding

receptacle is a way to go.

2. Installation of a satellite dish, telephone, CATV, or other low-voltage

equipment without proper grounding. If you look at a number of satellite dish

installations in your neighborhood, a certain percentage will inevitably not be

grounded at all. Of those that are grounded, there is still a high probability many are

not fully compliant. For example, the grounding electrode conductor could be too



long, too small, have unlisted clamps at terminations, have excess bends, or be

connected to a single ground rod but not be bonded to other system grounds.

For NEC purposes, a satellite dish is an antenna, and installation requirements are

found in Chapter 8, Communications Systems. Article 810, Radio and TelevisionEquipment, details the installation requirements. Part II deals with receiving

Equipment — Antenna Systems. This type of equipment, which includes the satellite

dish, must have a listed antenna discharge unit, which can be either outside the

building or inside between the point of entrance of the lead-in conductors and the

receiver — and as near as possible to the entrance of the conductors to the building.

The antenna discharge unit is not to be located near combustible material and

certainly not within a hazardous (classified) location.

The antenna discharge unit must be grounded. The grounding conductor is usually copper; however, you can use aluminum or copper-clad aluminum if it's not in contact

with masonry or earth. Outside, aluminum or copper-clad aluminum cannot be within

18 inches of the earth.

The grounding conductor can be bare or insulated, stranded or solid, and must be

securely fastened in place and run in a straight line from the discharge unit to the

grounding electrode (Photo 2). If the building has an intersystem bonding

termination, the grounding conductor is to be connected to it or to one of the

following:

o Grounding electrode system.

o Grounded interior metal water piping system within 5 feet of point of entrance to

the building.



o Power service accessible grounding means external to the building.

o Metallic power service raceway.

o Service equipment enclosure.

o Grounding electrode conductor or its metal enclosure.

If this grounding conductor is installed within a metal raceway, you must bond the

metal raceway to it at both ends. For this reason, if raceway is deemed necessary for

extra protection, UL-listed PVC (rigid non-metallic conduit) is generally used. The

grounding conductor must be no smaller than 10 AWG copper.

Where separate electrodes are used, you must connect the antenna discharge unit

grounding means to the premises power system grounding system by a 6 AWG copperconductor. Needless to say, grounding a satellite dish goes well beyond simply driving

a ground rod at the point of entrance.

Grounding for CATV is slightly different. Typically, CATV is brought into the building

via coaxial cable, which has a center conductor, insulating spacer, and outer electrical

shield. Because of the spacer, capacitive coupling is diminished so that the cable

provides a high-quality signal for data, voice, and video transmission. Improper

grounding of coaxial cable used for CATV is very common.

There is no antenna discharge unit as required for satellite dish installation. Instead,

the shield of the coaxial cable is connected to an insulated grounding conductor that is

limited to copper but may be stranded or solid. The grounding conductor is 14 AWG

minimum so that it has current-carrying capacity approximately equal to the outer

shield of the coaxial cable.

The major distinguishing characteristic is that for one- and two-family homes the

grounding conductor cannot exceed 20 feet in length and should preferably be shorter.

If a grounding electrode such as the Intersystem Bonding Termination is not within20 feet, it is necessary to drive a ground rod for that purpose. However, even after this

dedicated grounding means is established, in order to be NEC-compliant, the

installation must have a bonding jumper not smaller than 6 AWG or equivalent, which

is connected between the CATV system's grounding electrode and the power

grounding electrode system for the building. Omitting this jumper is a serious Code



violation, second only to no grounding at all. You must bond all system grounds,

antenna, power, CATV, telephone, and so on with a heavy bonding jumper.

3. Non-installation of GFCIs where required. Recent Code editions have

mandated increased use of GFCIs. In dwelling units, GFCIs are required on all 125V,single-phase, 15A and 20A receptacles in: bathrooms; garages; accessory buildings

with a floor at or below grade level not intended as a habitable room, limited to

storage, work and similar areas; outdoors; kitchens along countertops; within 6 feet of

outside edge of laundry, utility, and wet bar sinks; and boathouses. In other than

dwelling units, GFCIs are required on all 125V, single-phase, 15A and 20A receptacles

in bathrooms, kitchens, rooftops, outdoors, and within 6 feet of the outside edge of

sinks.

Other areas requiring the use of GFCIs include: boat hoists, aircraft hangers, drinkingfountains, cord- and plug-connected vending machines, high-pressure spray washers,

hydromassage bathtubs, carnivals, circuses, fairs (and the like), electrically operated

pool covers, portable or mobile electric signs, electrified truck parking space supply

equipment, elevators, dumbwaiters, escalators, moving walks, platform lifts/stairway

chairlifts, fixed electric space heating cables, fountains, commercial garages, electrical

equipment for naturally and artificially made bodies of water, pipeline heating,

therapeutic pools and tubs, boathouses, construction sites, health-care facilities,

marinas/boatyards, pools, recreational vehicles, sensitive electronic equipment, spas,

and hot tubs.

4. Improperly connecting the equipment-grounding conductor to the system

neutral. You must connect a grounded neutral conductor to normally noncurrent-

carrying metal parts of equipment, raceways, and enclosures only through the main

bonding jumper (or, in the case of a separately derived system, through a system

bonding jumper). Make this connection at the service disconnecting means, not

downstream. When you buy a new entrance panel, a screw or other main bonding

jumper is usually included in the packaging. Attached to it are instructions stipulating

that it is to be installed only when the panel is to be used as service equipment.

It's a major error to install a main bonding jumper in a box used as a subpanel fed by a

4-wire feeder. It's also wrong not to install it when the panel is used as service

equipment. Improper redundant connection of grounded neutral to equipment-

grounding conductors can result in objectionable circulating current and presence of



voltage on metal tool or appliance casings. You should connect grounded neutral and

equipment-grounding conductors at the service disconnect. Then separate them —

never to rejoin again. Additional optional ground rods may be connected anywhere

along the equipment-grounding conductor but never to the grounded neutral.

5. Improperly grounding frames of electric ranges and clothes dryers. Prior

to the 1996 version of the NEC, it was common practice to use the neutral as an

equipment ground. Now, however, all frames of electric ranges, wall-mounted ovens,

counter-mounted cooking units, clothes dryers, and outlet or junction boxes that are

part of these circuits must be grounded by a fourth wire: the equipment-grounding

conductor.

An exception permits retention of the pre-1996 arrangement for existing branch-

circuit installations only where an equipment-grounding conductor is not present.Several other conditions must be met. If possible, the best course of action is to run a

new 4-wire branch circuit from the panel. If you must keep an old appliance, be sure

to remove the neutral to frame bonding jumper if an equipment-grounding conductor

is to be connected.

6. Failure to ground submersible well pumps. At one time, submersible well

pumps were not required to be grounded because they were not considered accessible.

However, it was noted that workers would pull the pump, lay it on the ground, and

energize it to see if it would spin. If, due to a wiring fault, the case became live, theovercurrent device would not function, causing a shock hazard. The 2008 NEC

requires a fourth equipment-grounding conductor that you must now lug to the top of

the well casing. Many people assume that in a 3-wire submersible pump system one

wire is a “ground.” In actuality, submersible pump cable consists of three wires (plus

equipment-grounding conductor) twisted together and unjacketed. Yellow is a

common 240V leg, black is run, and red is start, which the control box energizes for a

short period of time. Prior to the new grounding requirement, everything was hot.

7. Failure to properly attach the ground wire to electrical devices. Wiringdaisy-chained devices in such a way that removing one of them breaks the equipment

grounding continuity is a common problem. The preferred way to ground a wiring

device is to connect incoming and outgoing equipment-grounding conductors to a

short bare or green jumper. The bare or green insulated jumper is then connected to

the grounding terminal of the device.



8. Failure to install a second ground rod where required. A single ground rod

that does not have a resistance to ground of 25 ohms or less must be augmented by a

second ground rod. Once the second ground rod is installed, it's not necessary for the

two to meet the resistance requirement. As a practical matter, few electricians do the

resistance measurement.

You cannot use a simple ohmmeter because that would require a known perfect

ground. Special equipment and procedures are needed, so it's common practice to

simply drive a second ground rod. You must locate them at least 6 feet apart. Greater

distance is even better (Figure). If both rods and the bare ground electrode conductor

connecting them are directly under the drip line of the roof, ground resistance will be

further diminished. This is because the soil along this line is more moist. Ground

resistance greatly increases when soil becomes dry.

9. Failure to properly reattach metal raceway that is used as an equipment-

grounding conductor. When equipment is relocated, replaced, or removed for

repair, many times equipment ground paths are broken. If these connections are not

fixed, there's an accident waiting to happen (Photo 3). Setscrews, locknuts, and



threads should be fully engaged and continuity tests performed before equipment is

put back into service. Dirt and corrosion can also compromise ground continuity.

NEC Article 250.4 requires that electrical equipment, wiring, and other electrically

conductive material likely to become energized shall be installed in a manner thatcreates a low-impedance circuit from any point on the wiring system to the electrical

supply source to facilitate the operation of overcurrent devices.

10. Failure to bond equipment ground to water pipe. Improper connections are

often seen in the field. Screw clamps and other improvised connections do not provide

permanent low impedance bonding. The worst method would be to just wrap the wire

around the pipe or to omit this bonding altogether.

In a dwelling, a conductor must be run to metallic water pipe, if present, and connected with

a UL-listed pipe grounding clamp (Photo 4). This bonding conductor is to be sized

according to Table 250.66, based on the size of the largest ungrounded service entrance

conductor or equivalent area for parallel conductors.

Understanding the Differences Between Bonding,

Grounding, and Earthing

The importance of bonding and grounding in commercial, industrial, and institutional

buildings cannot be overstated. The grounded circuits of machines need to have an effective

return path from the machines to the power source in order to function properly. In

addition, non-current-carrying metallic components in a facility, such as equipment

cabinets, enclosures, and structural steel, need to be electrically interconnected so voltage



potential cannot exist between them. The benefits for the building owner are many —

maximized equipment protection, elimination of shock hazard potential, increased process

uptime, and reduced costs through avoiding expensive machine servicing. However,

troubles can arise when terms like “bonding,” “grounding,” and “earthing” are interchanged

or confused in certain situations.

Earthing is the attachment of a bonded metallic system to earth, typically through ground

rods or other suitable grounding electrodes. The NEC prohibits earthing via isolated ground

rods as the only means of equipment grounding. Nevertheless, some manufacturers of

sensitive machinery actually encourage this practice in their installation manuals, in order

to reduce “no problem found” service calls associated with machine errors and rebooting.

An illustration

Understanding the differences between bonding/grounding and earthing is best illustrated

with an example. A manufacturer of molded components was replacing failed printed circuit

boards in a computerized numerically controlled (CNC) machine. After a thunderstorm, the

machine's self-diagnostic system occasionally registered a component problem. The

machine would not start, delaying the day's production cycle. Plant electronics technicians

identified and replaced failed circuit boards, then returned the CNC machine to operation.

However, each occurrence cost thousands of dollars in repairs and lost production.

Called upon to rectify the problem, personnel from the engineering services organization of

a major electrical distribution equipment manufacturer observed that although the plant

had grounded the CNC machine in accordance with the manufacturer's installation manual,

the ground was in clear violation of the NEC. This apparent contradiction demonstrates a

disturbing fact: Some grounding practices that are designed to decrease data errors in

sensitive machines can actually violate grounding codes and standards, causing equipment

damage and introducing safety hazards. It's also important to note that the conflicting

requirements can be overcome, but never by compromising employee safety.

Key concepts and terms

Understanding the difference between bonding/grounding and earthing requires implicit

understanding of several important concepts and terms, including those outlined below.

Safety grounding and machine operation



The problem experienced by the plant in the example is not uncommon. Manufacturers of

sensitive machines have discovered that isolated ground rods can decrease the number of

nuisance problems, such as rebooting, data errors, and intermittent shutdowns. This

decrease is due to the reduced amount of voltage transients or “noise” on the ground rod, as

compared to a common building grounding system. Because of the reduction in data errors

attributed to the ground rod, some manufacturers include isolated ground rods in their

installation instructions. Some even imply the machine warranty will not be honored if the

ground rod is not installed.

During thunderstorms or ground faults, however, an isolated ground rod becomes a

liability, creating shock hazard potential for employees and high potential rises on sensitive

machine components. Figure 1 (click here to see Fig. 1) illustrates the extremely large

transient voltages that can develop between driven ground rods due to lightning currents

and earth resistance. Although ground faults in the machine itself may not draw enough

current to trip overcurrent protective devices, they can create touch hazard potential for

employees.

Article 250.54 of the 2008 NEC specifically prohibits the use of isolated ground rods, or

earthing, as the sole means of equipment grounding, although some have used othersections of the NEC to justify this practice. The “NEC Handbook” provides the following

commentary associated with Art. 250.6 (Objectionable Currents):

“An increase in the use of electronic controls and computer equipment, which are sensitive

to stray currents, has caused installation designers to look for ways to isolate electronic

equipment from the effects of such stray circulating currents. Circulating currents on

http://ecmweb.com/images/901ecmGROUNDfig1.jpg








equipment grounding conductors, metal raceways, and building steel develop potential

differences between ground and the neutral of electronic equipment.

“A solution often recommended by inexperienced individuals is to isolate the electronic

equipment from all other power equipment by disconnecting it from the power equipmentground. In this corrective action, the equipment grounding means is removed or

nonmetallic spacers are installed in the metallic raceway system contrary to fundamental

safety grounding principles covered in the requirements of Art. 250. The electronic

equipment is then grounded to an earth ground isolated from the common power system

ground. Isolating equipment in this manner creates a potential difference that is a shock

hazard. The error is compounded because such isolation does not establish a low-impedance

ground-fault return path to the power source, which is necessary to actuate the overcurrent

protection device.”

Bonding/grounding vs. earthing

Isolated connections to earth are not required for sensitive machine operation. Issues crop

up when equipment bonding/grounding and earthing are confused. In the United States,

the term “grounding” is used to refer to at least five or more grounding-related systems,

including:

System type

This refers to the means by which power source voltage relationships are established.Power sources fall into four general categories: Transformers, generators, electric

utilities, and static power converters. These systems may be configured as wye or

delta, and the means by which they are interfaced with the grounding system

determines the system type. The most common 3-phase system type is the solidly

grounded wye, which is established by connecting a properly rated conductor (also

known as the main or system bonding jumper) from the X0 terminal of the source

(usually a transformer) to the grounding system.

Equipment grounding (bonding) Resolving the issue

The best means of equipment grounding is to route a grounding conductor, suitably

sized, along the same route as the power and neutral conductors, from source to

machine. The NEC does allow use of metallic conduit and other substitutes, but some

industry experts believe these systems are less effective and should be avoided.



Grounding electrode (earthing)

This term refers to the method by which the facility grounding system is connected

and referenced to earth. The most common grounding electrode for small facilities is a

metallic ground rod, but earthing systems for larger buildings can — and should — be

more elaborate and include the means by which to inspect and test these systems

periodically. A grounding electrode system that is buried in earth or encased in

concrete and then forgotten is often the source of increasing problems as the building

ages and the grounding electrodes deteriorate.

Lightning abatement

Some facilities use air terminals (also known as lightning rods) to direct lightning

strikes away from power equipment, but these devices are often connected to the

grounding system in such a way that they have the opposite effect — unintentionally bringing lightning energy into facility structural steel, low-voltage transformer

windings, and, subsequently, sensitive building loads.

Signal-reference grounding

Sensitive electronic machines rely on the grounding system for reference of low-

magnitude signals. Therefore, it's often crucial to provide multiple grounding paths,

rather than rely on a single equipment grounding conductor between the power source

and the sensitive load. This ensures that spurious voltages on the grounding system

are maintained well below the level at which they might be confused with sensitive

machine reference signals. The best guide for signal-reference grounding is IEEE

Standard 1100-2006, “Recommended Practice for Powering and Grounding Electronic

Equipment.”

Note that earthing is not required for sensitive machine operation. Modern aircraft, for

example, are packed with sensitive computers and electronic devices, which operate

correctly without an attachment to earth. They rely on a bonded metallic system — the

airplane framework, skin, structural supports, raceways, and grounding conductors — to

serve as the ground reference. If this bonded system rises in voltage with respect to earth, all

machines onboard experience the increase together. The net result is that the machines see

no voltage potential differences with respect to each other. Once the airplane lands, any

voltage potential between the plane and earth must be discharged by an electrode that

bypasses the rubber tires.



Resolving the issue

The immediate solution to the example plant's illegal ground rod (click here to see Fig.

2) was to remove the shock hazard. This was done by connecting a grounding conductor (1/0

copper) from the ground rod to the nearest part of the building grounding system — in this

case, the structural steel. This connection eliminated the shock potential during storms by

reducing the resistance between the ground rod and the building grounding system.

The next step was to eliminate the wiring errors and install a ground wire from the source to

the CNC machine (click here to see Fig. 3). The primary reason that the isolated ground rod

was effective in decreasing operating problems was the building's bonded system

experienced voltage transients, imposed on it due to wiring errors. One common error is the

improper connection of neutral wires to ground buses or ground wires to neutral buses. This

error allows neutral currents to flow on the bonded system, thereby creating voltage

transients. Neutral wires are only allowed to be connected to the bonded system at a service

entrance or at a step-down transformer (called a separately derived source by the NEC).

Notice in Fig. 2 that the plant had installed both a voltage regulator and a noise suppression

device ahead of the CNC machine. These devices are often applied to solve the nuisance

operating problems brought on by ground system transients. Suppression devices are not a

cure-all, however. In fact, they're sometimes unnecessary when wiring and grounding

problems are corrected first.
















Once the spurious ground rod had been connected to the rest of the bonded system,

operating issues had to be addressed, which involved correcting the wiring errors identified

in the site survey. For the example facility, these steps were adequate. For other situations,

you should refer to the following checklist:

1. Connect the ground rod to the bonded system and install a grounding conductor from

the power source to the sensitive load to eliminate the safety hazard and allow an

effective ground-fault return path.

2. Correct wiring and grounding errors on the power system serving the sensitive

machine.

3. Install a step-down transformer (i.e., a separately derived source) to serve only the

process machine. Derive a new neutral to the ground bonding point at the load side of

the transformer.

4. Any remaining operating problems are probably caused by communications groundloops. Ground loops, which are introduced by communication wiring between

sensitive machines fed from different power sources, may require more elaborate

correction schemes, such as optical isolation.



Taking the next step

In summary, the plant in the example had installed a CNC process machine in accordance

with the manufacturer's recommendations. Unfortunately, those recommendations

included the requirement for a separate ground rod to serve as the only means of equipment

grounding. While this practice may reduce data errors in sensitive process machines, it

violates the NEC, creates a shock hazard for employees, and causes a potential difference

that may damage sensitive electronic components.

Electrical engineers and contractors can help customers avoid situations like this by

providing proactive counsel in this area. The best place to start is to gather as much

information as possible — from the 2008 NEC, seminars/conferences, trusted electrical

equipment manufacturers, and online sources. With that knowledge in hand, you have yet

another reason to call on a customer and resolve an issue of critical importance.

These definitions are:

• Grounded - Connected to earth

• Bonded - The permanent joining of metallic parts to form an electricallyconductive path that

ensures electricalcontinuity and the capacity to conductsafely any current likely to be imposed

what is electrical grounding

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