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 ofthe 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 ofelectricity. 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 eve ryday 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.
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
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
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.
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
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
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.
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.
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
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
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
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
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
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
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.
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
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