What is Electrical Grounding

Published on February 2017 | Categories: Documents | Downloads: 60 | Comments: 0 | Views: 241
of 29
Download PDF   Embed   Report

Comments

Content

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 Europe where 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 driven rod 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 be installed 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. Driven grounding 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, the electric 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. However some 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 usually provide 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 that may prove helpful in selecting proper electrode usage.

Driven Rod Resistanceto-Ground (RTG) Corrosion Resistance Increase in RTG in Cold Weather Increase in RTG over Time Electrode Ampacity Installation Cost Poor Poor

Advanced Driven Rod

Grounding Plate

Concrete Encased Electrode

Building Foundation

Water Pipe

Electrolytic Electrode

Average

Poor

Average

Above Average

Poor to Excellent**

Excellent

Good

Poor

Good *

Good *

Varies

High

Highly

Slightly

Highly Affected

Slightly Affected RTG typically

Slightly Affected RTG typically

Minimally Affected RTG typically unaffected Poor to Excellent** Average Below Average* 10-15 years

Minimally Affected

Affected Affected RTG typically unaffected Average

RTG Worsens

RTG Increases

RTG Improves

unaffected unaffected Average * Below Average Above Average * Average

Poor

Average Below Average

Excellent

Average Excellent

Poor

Life Expectancy

Poor 5–10 years

Average 15–20 years

Poor 5-10 years

Average * Above 15-20 years Average * 20-30 years

Excellent 30-50 years

* High-current discharges can damage foundations when water in the 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.

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.
Resistivity of Sample in Ohmmeters Dry 140 x 106 4,000 40 x 106 2 x 106 1.5 x 106 to 4.5 x 106 2.6 x 106 to 3 x 106 7 x 106 2 x 106 to 30 x 106 1 x 106 to 1 x 109 1,300 1,200 5,000 10,000 5,000 10,000 2,000 to 3,000 10,000 to 6 x 106 21 to 100 Wet

Type of Surface Material Crusher granite w/ fines Crusher granite w/ fines 1.5” Washed granite – pea gravel Washed granite 0.75” Washed granite 1-2” Washed granite 2-4” Washed limestone Asphalt Concrete Soil Types or Type of Earth Bentonite Clay Wet Organic Soils Moist Organic Soils Dry Organic Soils Sand and Gravel Surface Limestone Limestone Shale‟s Sandstone Granites, Basalt‟s, etc. Decomposed Gneiss‟s Slates, etc.

Average Resistivity in Ohm-meters 2 to 10 20 to 1,000 10 to 100 100 to 1,000 1,000 to 5,000 50 to 1,000 100 to 10,000 5 to 4,000 5 to 100 20 to 2,000 1,000 50 to 500 10 to 100

How To Do Grounding System Testing
The 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 earthelectrode 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 the grounding 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 grounding system (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 Xaxis 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. 2. 3. The amount of amperage running through the tested system must be below the equipment manufacturer‟s limits. The test signal must be injected at the proper location, so that the signal is forced through the grounding system and into the earth. 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 nonnegligible and can be greater than the ground resistance of a very low resistance grounding system, which can therefore not be measured accurately. The ground resistance of a large grounding system at 60 Hz can be significantly lower than at 1.7 kHz.

4.

Many erroneous tests have been conducted where the technician only measured metallic loops and not the true 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. These new 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 and electric 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 nondwellings 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 the neutral 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 Television Equipment, 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 o

Grounding electrode system. Grounded interior metal water piping system within 5 feet of point of entrance to the building.

o o o o

Power service accessible grounding means external to the building. Metallic power service raceway. Service equipment enclosure. 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 copper conductor. 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 within 20 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, drinking fountains, 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 noncurrentcarrying 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 equipmentgrounding 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 branchcircuit 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, the overcurrent 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. Wiring daisy-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 equipmentgrounding 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 that creates 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 other sections 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

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 equipment ground. 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 lowmagnitude 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 ground loops. 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

Sponsor Documents

Or use your account on DocShare.tips

Hide

Forgot your password?

Or register your new account on DocShare.tips

Hide

Lost your password? Please enter your email address. You will receive a link to create a new password.

Back to log-in

Close