Earthing, Grounding and Bonding

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10/16/2013 1 EARTHING, EARTHING, EARTHING, EARTHING, GROUNDING GROUNDING GROUNDING GROUNDING AND AND AND AND BONDING BONDING BONDING BONDING PDF Created with deskPDF PDF Writer - Trial :: http://www.docudesk.com

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Transcript of Earthing, Grounding and Bonding

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EARTHING, EARTHING, EARTHING, EARTHING,

GROUNDING GROUNDING GROUNDING GROUNDING

AND AND AND AND

BONDINGBONDINGBONDINGBONDING

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Introduction:

• One of the most misunderstood and confused

concept is difference between Bonding,

Grounding and Earthing. Bonding is more

clear word compare to Grounding and

Earthing but there is a micro difference

between Grounding and Earhing.

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• Ground or earth in a mains electrical wiringsystem is a conductor that provides a lowimpedance path to the earth to preventhazardous voltages from appearing onequipment.

• The Earthing is to reference our electrical sourceto earth (usually via connection to some kind ofrod driven into the earth or some other metalthat has direct contact with the earth).

• The grounded circuits of machines need to havean effective return path from the machines to thepower source in order to function properly (Hereby Neutral Circuit).

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EARTHING• Earthing means connecting the dead part (it means the

part which does not carries current under normalcondition) to the earth for example electricalequipment’s frames, enclosures, supports etc.

• The purpose of earthing is to minimize risk of receivingan electric shock if touching metal parts when a fault ispresent. Generally green wire is used for this as anomenclature.

• Under fault conditions the non-current carrying metalparts of an electrical installation such as frames,enclosures, supports, fencing etc. may attain highpotential with respect to ground so that any person orstray animal touching these or approaching these willbe subjected to potential difference which may resultin the flow of a current through the body of the personor the animal of such a value as may prove fatal.

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• To avoid this non-current carrying metal parts of

the electrical system are connected to the general

mass of earth by means of an earthing system

comprising of earth conductors to conduct the

fault currents safely to the ground.

• Earthing has been accomplished through bonding

of a metallic system to earth. It is normally

achieved by inserting ground rods or other

electrodes deep inside earth.

• Earthing is to ensure safety or Protection of

electrical equipment and Human by discharging

the electrical energy to the earth.

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• Equipment Protection Vs Human Safety:

• Earthing is to protect the circuit elements whenever high voltage ispassed by thunders or by any other sources while Grounding is thecommon point in the circuit to maintain the voltage levels.

• Earth is used for the safety of the human body in fault conditionswhile Grounding (As neutral earth) is used for the protection ofequipments.

• Earthing is a preventive measure while Grounding is just a returnpath

• The ground conductor provides a return path for fault current whena phase conductor accidentally comes in contact with a groundedobject. This is a safety feature of the wiring system and we wouldnever expect to see grounding conductor current flow duringnormal operation.

• Do not Ground the Neutral Second time When It is groundedeither at Distribution Transformer or at Main service Panel ofConsumer end.

• Grounding act as neutral. But neutral cannot act as ground.

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GROUNDING• Grounding means connecting the live part (it means the

part which carries current under normal condition) to theearth for example neutral of power transformer.

• Grounding is done for the protections of power systemequipment and to provide an effective return path from themachine to the power source. For example grounding ofneutral point of a star connected transformer.

• Grounding refers the current carrying part of the systemsuch as neutral (of the transformer or generator).

• Because of lightening, line surges or unintentional contactwith other high voltage lines, dangerously high voltages candevelop in the electrical distribution system wires.Grounding provides a safe, alternate path around theelectrical system of your house thus minimizing damagefrom such occurrences.

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• Generally Black wire is used for this as a nomenclature.

• All electrical/electronic circuits (AC & DC) need areference potential (zero volts) which is called groundin order to make possible the current flow fromgenerator to load. Ground is May or May not beearthed. In Electrical Power distribution it is eitherearthed at distribution Point or at Consumer end but itis not earthed in Automobile( for instance all vehicles’electrical circuits have ground connected to the chassisand metallic body that are insulated from earththrough tires). There may exist a neutral to groundvoltage due to voltage drop in the wiring, thus neutraldoes not necessarily have to be at ground potential.

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• In a properly balanced system, the phase

currents balance each other, so that the total

neutral current is also zero. For individual

systems, this is not completely possible, but

we strive to come close in aggregate. This

balancing allows maximum efficiency of the

distribution transformer’s secondary winding

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Balancing the Load Vs Safety:

• Ground is a source for unwanted currents andalso as a return path for main current sometimes. While earthing is done not for return pathbut only for protection of delicate equipments. Itis an alternate low resistance path for current.

• When we take out the neutral for a three phaseunbalanced connection and send it to ground, itis called grounding. Grounding is done to balanceunbalanced load. While earthing is used betweenthe equipment and earth pit so as to avoidelectrical shock and equipment damage.

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Micro Difference between earthing & Grounding:

• There is no major difference between earthingand Grounding, both means “Connecting anelectrical circuit or device to the Earth”. Thisserves various purposes like to drain awayunwanted currents, to provide a referencevoltage for circuits needing one, to leadlightning away from delicate equipment. Eventhough there is a micro difference betweengrounding & earthing.

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System Zero Potential Vs Circuit Zero Potential:

• Earthing and Grounding both is refer to zeropotential but the system connected to zero potential isdiffer than Equipment connected to zero potential .If aneutral point of a generator or transformer isconnected to zero potential then it is knownas grounding. At the same time if the body of thetransformer or generator is connected to zero potentialthen it is known as earthing.

• The term “Earthing means that the circuit is physicallyconnected to the ground and it is Zero Volt Potential tothe Ground (Earth) but in case of “Grounding” thecircuit is not physically connected to ground, but itspotential is zero(where the currents are algebraicallyzero) with respect to other point, which is also knownas “Virtual Grounding.”

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Conclusion:

• Ground is a source for unwanted currents and

also as a return path for main current. While

earthing is done not for return path but only

for protection of delicate equipments. It is an

alternate low resistance path for current.

Earth is used for the safety of the human body

in fault conditions while Grounding (As neutral

earth) is used for the protection of

equipments.

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BONDING

• Bonding is simply the act of joining two electricalconductors together. These may be two wires, a wireand a pipe, or these may be two Equipments.

• Bonding has to be done by connecting of all the metalparts that are not supposed to be carrying currentduring normal operations to bringing them to the sameelectrical potential.

• Bonding ensures that these two things which arebonded will be at the same electrical potential. Thatmeans we would not get electricity building up in oneequipment or between two different equipment. Nocurrent flow can take place between two bondedbodies because they have the same potential.

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• Bonding, itself, does not protect anything. However, if oneof those boxes is earthed there can be no electrical energybuild-up. If the grounded box is bonded to the other box,the other box is also at zero electrical potential.

• It protects equipment & Person by reducing current flowbetween pieces of equipment at different potentials.

• The primary reason for bonding is personnel safety, sosomeone touching two pieces of equipment at the sametime does not receive a shock by becoming the path ofequalization if they happen to be at different potentials.

• Bonding to electrical earth is used extensively to ensurethat all conductors (person, surface and product) are at thesame electrical potential. When all conductors are at thesame potential no discharge can occur.

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Good Earthing Means

Good Earthing must have low impedance

enough to ensure that sufficient current can

flow through the safety device so that it

disconnects the supply ( <0.4 sec ). Fault

current is much more than the full load

current of the circuit which melts the fuse.

Hence, the appliance is disconnected

automatically from the supply mains.

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Qualities Of Good Earthing

• Must be of low electrical resistance

• Must be of good corrosion resistance

• Must be able to dissipate high fault current

repeatedly

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Purpose of Earthing

• To save human life from danger of electrical shock or death by blowing a fuse i.e. To provide an alternative path for the fault current to flow so that it will not endanger the user

• To protect buildings, machinery & appliances under fault conditions ie. To ensure that all exposed conductive parts do not reach a dangerous potential.

• To provide safe path to dissipate lightning and short circuit currents.

• To provide stable platform for operation of sensitive electronic equipments i.e. To maintain the voltage at any part of an electrical system at a known value so as to prevent over current or excessive voltage on the appliances or equipment .

• To provide protection against static electricity from friction

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Methods of Earthing

• Conventional Earthing

• Maintenance Free Earthing

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Conventional Earthing

• The Conventional system of Earthing calls for

digging of a large pit into which a GI pipe or a

copper plate is positioned in the middle layers

of charcoal and salt.

• It requires maintenance and pouring of water

at regular interval.

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CONVENTIONAL EARTHING

FIGURE:.

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Maintenance Free Earthing

• It is a new type of earthing system which is Readymade, standardized and scientifically developed.

Its Benefits are

• MAINTENANCE FREE: No need to pour water at regular interval- except in sandy soil.

• CONSISTENCY: Maintain stable and consistent earth resistance around the year.

• MORE SURFACE AREA: The conductive compound creates a conductive zone, which provides the increased surface area for peak current dissipation. And also get stable reference point.

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Maintenance Free Earthing(Contd.)

• LOW EARTH RESISTANCE: Highly conductive.

Carries high peak current repeatedly.

• NO CORROSION:

• LONG LIFE.

• EASY INSTALLATION.

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Methods of Conventional Earthing

1. Plate Earthing

2. Pipe Earthing

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Earthing ElectrodeThe resistance of a ground

electrode has 3 basic

components:

A) The resistance of the ground

electrode itself and the connections

to the electrode.

B) The contact resistance of the

surrounding earth to the electrode.

C) The resistance of the surrounding

body of earth around the ground

Electrode. It consist of three basic components:

1. Earth Wire

2. Connector

3. Electrode

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Plate Earthing• In this method a copper plate of 60cm x 60cm x

3mm or a GI plate is used for earthing. The plate is

placed vertically down inside the ground at a depth

of 3m and is embedded in alternate layers of coal

and salt for a thickness of 15 cm. In addition, water

is poured for keeping the earth electrode

resistance value well below a maximum of 5 ohms.

The earth wire is securely bolted to the earth plate.

A cement masonry chamber is built with a cast iron

cover for easy regular maintenance.

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Pipe earthing• Earth electrode made of a GI (galvanized) iron pipe of

50mm in diameter and length of 3m (depending on the

current) with 6 mm holes on the surface is placed upright at

a depth of 3 m in a permanently wet ground. To keep the

value of the earth resistance at the desired level, the area

(15 cms) surrounding the GI pipe is filled with a mixture of

salt and coal. The efficiency of the earthing system is

improved by pouring water through the funnel periodically.

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• The application of

bentonite as electrical

grounding improvement

material (GIM) has been

investigated. Bentonite is a

type of clay which has high

tendency to absorb and

retain water, and swells.

This property makes it

desirable for applications in

grounding system

improvement as they could

result in lowering as well as

minimizing the fluctuation

of ground resistance over a

long period of time.

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SOIL RRESISTIVITY

• Soil resistivity measurements have a threefold purpose.

• First, such data are used to make sub-surface geophysicalsurveys as an aid in identifying ore locations, depth tobedrock and other geological phenomena.

• Second, resistivity has a direct impact on the degree ofcorrosion in underground pipelines. A decrease in resistivityrelates to an increase in corrosion activity and thereforedictates the protective treatment to be used.

• Third, soil resistivity directly affects the design of agrounding system, and it is to that task that this discussionis directed. When designing an extensive grounding system,it is advisable to locate the area of lowest soil resistivity inorder to achieve the most economical groundinginstallation.

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SOIL RRESISTIVITY

• The resistivity does not change with length

and other physical characteristics.

• Resistance (R) = V/I

• It will change if the measurement geometry or

volume of material changes. Therefore, it is

nota physical property.

• Resistivity is the resistance per unit volume.

• R = ρl/A ρ=RA/l

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Effects of Soil Resistivity on Ground Electrode

Resistance

• Soil resistivity is the key factor that determineswhat the resistance of a grounding electrodewill be, and to what depth it must be driven toobtain low ground resistance. The resistivity ofthe soil varies widely throughout the worldand changes seasonally. Soil resistivity isdetermined largely by its content ofelectrolytes, which consist of moisture,minerals and dissolved salts. A dry soil hashigh resistivity if it contains no soluble salts

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• Resistivity is defined as the voltage measured

across a unit length (V/m) divided by the

current flowing through the unit cube’s cross

sectional area (A/m2).

• This result is units of Ω-m2/m or Ω-m

• Lower the soil resistivity value, the lower the

grounding electrode resistance will be.

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SOIL RESISTIVITY MEASUREMENTS• Resistivity measurements are of two types; the 2-

point and the 4-point method. The 2-pointmethod is simply the resistance measuredbetween two points. For most applications themost accurate method is the 4-point method (fig-1) as the name implies, requires the insertion offour equally spaced and in-line electrodes intothe test area. A known current from a constantcurrent generator is passed between the outerelectrodes. The potential drop (a function of theresistance) is then measured across the two innerelectrodes..

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• In the figure

P1, P2 = Potential electrodes

C1, C2 = Current electrodes

A = Distance between electrodes

B = Depth of electrodes

A = 20 X B or B = 1/20 * A

ρ ( Ω – m) = 2 π A R ( A in meters)

• This value is the average resistivity of the

ground at a depth equivalent to the distance

“A” between two electrodes.

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Max. Value of Earth Resistance to

be achievedEquipment to be Earthed Max. Value of Earth Resistance to be

achieved in Ohms

Large Power Stations 0.5

Major Substations 1.0

Small Substations 2.0

Factories Substations 1.0

Lattice Steel Tower 3.0

Industrial Machine and Equipment 0.5

* The Earth Resistance depends upon the moisture content in the soil.

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