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Issued by : EBG - Electrical Systems & Equipment, LARSEN & TOUBRO LIMITED, Powai Works, Mumbai 400 072

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April-June 06.p65 3/6/07, 10:23 PM1

Earthing of Electrical & Electronic Systems

- Mr. Rajan Jategaonkar

In a broad sense, earthing can becategorised as per the referencemade. Following are the meaningscommonly referred as :a) Common negativeb) Distribution Earthc) A conductive rod driven into the

ground

A simple working definition of “Earth“in Electrical & Electronic System is"the Zero Volt reference in thesystem".

As per NEC (National ElectricalCode), it is defined as "a conductingconnection, whether intentional oraccidental, between an electricalcircuit or equipment and the Earth orsome conducting body that serves inplace of the earth".

The scientific meaning of the “Earth”can be made as "the place to whichthe electrons go at the end of theiruseful lives". It should be noted thatthe term “Ground“ and “Earth“ is usedsynonymously here. Earth is a fairlygood conductor and has enormouslyhigh capacity to dissipate current /charge in it. After understanding thedefinition and the meaning of the“Earth“, let us understand how itsproperties are utilized as a apart ofElectrical System.

Fig.1 shows the connection of theSingle Phase power outlet.

It indicates the top terminal as “E“,which is considered as the absolute

zero voltage. Normalcolor code for thiswire is Green –Yellow. The terminalon the right side is“L”, a Line terminal.The terminal on theleft is “N”, a Neutralterminal (Neutral is a

circuit conductor that carries a currentin the normal operation and in most ofthe cases is connected to the Earth).

The concept of Earth can be moreclearly studied by considering thecommonly discussed subject of“Neutral to Ground (Earth)“ voltagecase of three-phase system.

Following are the points to beconsidered with respect to the abovetopic :a) What are the Neutral to Ground

voltagesb) Why do they occurc) What is the necessity to eliminate

this voltage

Let's refer to the Fig.2 for the threephase system with load connected.Whenever there is an imbalance inthe system, either due to source orthe load, current will flow in theNeutral. (When the system is inbalanced condition, there is no currentflowing through the Neutral).

The current flowing through the Neutralconductor will develop a voltage andwhen it is measured w.r.t. to Earth, itgives “Neutral to Earth“ voltage. ThisNeutral to Earth voltage can causesignificant disruption to the operationof the logic based circuit if the isolation

is not provided at the power supplylevel. This will be of more concernwhen the short circuit occurs. Thiswill lead to a large value of currentflowing through the neutral, which willcreate sufficient voltage that can belethal to the system.

As the current or impedance of thecircuit increases, “Neutral to Earthvoltage“ also increases. The impedanceis a function of frequency. Thecommon method of reducing thisvoltage is by having a oversizedNeutral conductor. This will reduce theimpedance & hence Neutral to Earthvoltage.

Following are the types of Earthingsystems used :

The IEC 60364 distinguishes threefamilies of Earthing arrangements usingthe two letter codes TN, TT & IT.

The first letter indicates the connectionbetween the Earth & the power supplyequipment (Generator & Transformer).

T Direct connection of a point withearth

I No point is connected with earth(isolation), except via a highimpedance

Fig.2

FEATURE

Fig.1

April-June 06.p65 3/6/07, 10:23 PM2

The second letter indicates theconnection between earth & theelectrical device being supplied.

T Direct connection with earth,independent of any other earthconnection in the supply system.

N Connection to earth via supplynetwork.

In TN earthing system, one of thepoints in the generator or transformeris connected with earth. This isnormally the Star point in a ThreePhase system. The body of theelectrical device is connected withearth via this earth connection at theTransformer. The system can run adifferent Neutral conductor & Earthconductor or a combined one fromsource to load. Shown in Fig. (3).

In a TT Earthing system, theprotective earth connection of theequipment used is provided by a localconnection to a earth, independent ofany earth connection at the source.

In this type, the equipment isconnected with minimum noise carriedin the system. Each equipment canhave high quality of Earth and isindependent of other equipment. Thisis shown in Fig. (4).

In IT earthing system, the sourcehas no connection to earth or it hasonly high impedance connection. Inthis type, direct personnel safety islacking. Also residual current devicesare far less likely to detect aninsulation fault. This type of systemexperiences larger transient over

voltages than the other types ofearthing system. This is shown inFig (5).

Why the earthing of the Electrical/ Electronic system should beconsidered?a) Proper grounding of the metallic

enclosures - Since it is a part ofElectrical system, it may happenthat the Live part of the system istouching the enclosure and thesame is in contact with theoperating personnel. If theenclosure is not earthed, then theelectric potential may be harmful

to the personneltouching it. Even asmall amount ofcurrent, say 0.1Amp for onesecond can befatal. A smallvoltage of 100Volts is enough forcreating the 100

mA of current through the body, ifthe skin is moist, which can proveto be lethal.

b) Protection against direct lighteningstrokes - Elevated structuresneeds lightening rods connectedinto the grounding system. Thecurrents generated during lightningis of the order of kiloamperes.Hence, the conductor used shouldbe of appropriate size andgrounded with a electrode takendeep into the earth so as todissipate the energy released after

the lightening strikes.

c) Protection against theinduced lightening voltages -It is of particular importancewhen the aerial distribution ofpower is in use. In this case,the arresters may be requiredto be placed at strategiclocations throughout the plant.

d) Earthing in Electronic Systems -One of the disturbances affectingthe working of electronic instrument/ system is due to theelectromagnetic interference.These disturbances have differentparameters such as:

a) High Frequency signalsb) In the vicinity of High Voltage

Phenomenac) Fast switching of the power

devices (Breakers, SCRs).

These disturbances severelyaffect the working of digitalsystems. e.g. the signalscarried by the communicationcables.

One of the possible solutions is touse Shielded Twisted Pair wire. Apartfrom neutralizing of capacitanceeffect due to the structure of the cable,effect of Electromagnetic Interferenceis avoided /reduced by a metal shieldsurrounding the signal carrying cable.It is recommended to earth one of theends of the metal shield.

In case both the ends of the shieldget grounded, a ground loop is formedwhich generates its own voltage, whichin turn disturbs the working of thesystem. This is shown in Fig. (6).

One of the important aspect whichshould be taken care of in the earthingsystem is when different instruments/ voltages with different earth locationsare interconnected. This forms theground loops and generates thecirculating currents. In the electricalsystem, an unwanted current thatflows in a conductor connecting twopoints that are expected to be at thesame potential, i.e. Ground, but areactually at different potentials. Theground loop is also a common wiringconditions where aground current maytake more than one paths to return tothe grounding electrode. i.e. when morethan one computer is connected witha common ground (with aux. supply).There is also a shieldedcommunication cable, installed withinthe computers. If this shield is roundedat both the ends (Transmitter &Receiver end), then it will also createa separate return path and hence forma Ground loop. This is shown in thefig (7).

Fig.4

Fig.3

Fig.5

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The principle involved in betterquality Earthing:

The low impedance earth electrodeimproves considerably the protectionof the electrical installation. The qualityof the earth electrode (Resistance aslow as possible) depends essentiallyon two factors :a) Installation methodb) Nature of the earth

The electrode should be buried aroundthe perimeter of the excavation madefor the foundation. It is important thatthe bare conductor should makeintimate contact with the soil.

The resistance R of such an electrodeis given in ohms byR = 2p/ LWhere,L= Length of the buried conductor inmeters

P = Soilresistivity inOhm-cm

If number ofsimilar rodsareconnectedin parallel,then thecombinedresistor isgiven by :

R = p/ NL

These earthing rods are made up ofCopper or Copper clad steel andextended from the installation by aGalvanized steel. When more thanone rod is used then the spacingbetween the rods should be morethan 2 to 3 times of the length of therod.

Resistivity of the soil in Ohm-cm variesas per its type i.e. for sandy clay it is50 to 200; for Limestone, it varies from1000 to 5000. It is lowest for moistsoil.

It is desirable to have low earthresistance. Practically, resistanceof earth can be improved by usingthe following measures :

1) Increase the length of electrode inthe earth i.e. driving a longer rod

deeper into the earth decreasesits resistance. In general, doublingthe rod length reduces theresistance by 40%.

2) Use of multiple electrodes - Twowell-spaced rods driven into theearth provide parallel paths. Theyare, in effect, two resistances inparallel.

3) Treatment of the soil - Chemicaltreatment of the soil is a good wayto reduce the earth's resistancewhen it is not possible to drive theelectrode deeper. One importantpoint to be considered in thisprocess is that the chemical usedin the soil treatment should nothave corrosive effect on the earthelectrode. Magnesium Sulfate orCopper Sulfate is a suitable non-corrosive material. This treatmentoffers the advantage of reducingthe reasonable variations inresistance that results fromperiodic wetting and drying of thesoil.

It can be concluded that for havingthe best quality earthing for theelectrical/electronic installation, it isrequired to have :a) Minimum soil resistivityb) Minimum contact resistance

between earth conductor andEarthing rod

c) Shortest path from the installationto the Earth

d) Proper size of the Earth conductore) S e p a r a t eEarth for the powerequipment and theelectronic systemsworking on micro-controllers basedcircuitsf) Shielding ofthe cables at oneend only, insteadof both the ends.

Fig.6

For further details on this subject, please contact:EBG - Electrical Systems & Equipment, Larsen & Toubro Limited, Saki-Vihar Road, P.O. Box 8901, Powai, Mumbai 400 072

Fax: 022-5505 1553 * E-mail: kotnisnd@Intebg.com

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April-June 06.p65 3/6/07, 10:24 PM4