Earthing

EARTHING:Connecting the electrical equipment to the general mass of the earth which has a very low resistance is called earthing. The earth can absorb millions of amperes of current, as it is a good conductor containing soluble salts and moisture. It also acts as a good insulator when containing silica and alumina.When a pipe is driven into the earth, the earth surrounding the pipe can be considered to be consisting of concentric cylinders of earth which will be bigger in size and area, as they are away from the pipe. The current can travel into the earth with large area having little resistance.3 M length of electrode will have contact with the earth area of 3 M in radius. Hence to have better effect 3 M pipes should be fixed at a distance of 6 M (i.e.) twice the distance of pipe lengths.

Moisture content is one of the controlling factors in earth resistivity. Above about 20% moisture the resistivity is very little affected, while below about 20%, the resistivity increases very abruptly with decrease in moisture. Hence average moisture content may be 16% to 18% by weight of dry soil.(i)As we go deeper, the earth contains more moisture and soluble salts and hence lesser resistance.(ii)If the diameter of earth pipe is increased from 12 mm to 25 mm, there will be 10% reduction in resistance. But the weight and cost will be fourfold.6.Temperature co-efficient of resistivity for soil is negative, but is negligible above freezing point. At about 20oC the resistivity change is 9% per degree Celsius. Below 0oC the resistivity rises enormously. Hence in such areas pipe should be driven below the frost line.

90% of resistance between pipe and earth lies with a radius of about 2 Metres from the pipe/rod. Hence for improving earth resistivity by artificial treatment of soil, excavation of one metre diameter around top of the rod/pipe to 30 CM deep and applying the artificial agent like Sodium Chloride, Calcium Chloride, Sodium Carbonate or Copper Sulphate and watering sufficiently, will be effective.

EARTHING IN SUB STATION:Earthing is important not only for the proper functioning of the electrical system, but also for safety of the operating personnels and equipment.The object of a good earthing system is to provide under and around the station, a surface with zero earth potential. The provision of such a surface under and around the sub station ensures that the sub station operation is not subjected to shock on injury on the occurrence of short circuit or development of other abnormal condition in the equipment handled by him.

3.The primary requirement of a good earthing system in a sub station are:(a)It should stabilize circuit potential with respect to ground and limit the overall potential rise.(b)It should protect men and materials from injury or damage due to over voltage.(c)It should provide a low impedance path to fault currents to ensure prompt and consistent operation of protective devices.(d)It should keep the maximum voltage gradient along thesurface inside and around the substation within safe limitsduring ground fault.

FACTORS WHICH DETERMINE RESISTIVITY OF SOIL: (a)Type of Soil (b)Moisture Content (c)Chemical Composition of Salt dissolved in the contained water. (d)Concentration of Salts (e)Temperature of Materials (f)Grain size and distribution of grain size (g)Closeness of packingTYPE OF SOILRESISTIVITY IN Ohm-Cm 1. Loamy Garden Soil500 5,000 2. Clay800 5,000 3. Clay, Sand and Gravel Mix4,000 25,000 4. Sand and Gravel6,000 10,000 5. Slates, Slab Sand Stone1,000 50,000 6. Crystalline Rocks20,000 1,00,000

Where there is any option, a site should be chosen in the order of preference shown below:(a) Wet marshy ground(b)Clay loamy soil, arable land, clayey soil, clayey soil or room mixed with small quantity of sand(c)Clay and loom mixed unit varying proposition of sand, growl and stores(d)Damp and wet sand pit

IMPROVING EARTH RESISTANCE: Powdered charcoal and salt put surrounding the earth pipe. Calcium chloride, magnesium sulphate may be administered. If salt is put corrosion of pipes will be earlier. Use of Bentonite Powder in Soil Treatment:Bentonite is a clay with outstanding electrical properties. It swells to several times its original volume when suspended in water; it binds the water of crystallization and the water absorbed during the mixing process is retained over a long period. Bentonite suspension in water when used to surround the earth electrode virtually increases the electrode surface area.Use of bentonite around the earth electrode results in reduction of ground resistance by about 25-30.Bentonite has a tremendous capacity to absorb water and retain it over along period. The best results are obtained by mixing bentonite with a very large quantity of water, say in the ratio of 1 : 4.

A cover of bentonite power for 300 mm dia. Around the electrode would be adequate in most of the cases to obtain a significant reduction in earth resistance.Even during the summer months, bentonite suspension retains the moisture where as the natural soil dries up.Bentonite or similar material may be used to advantage in rocky terrain.

Use of Fly Ash in Soil Treatment:Recent CPRI studies reveals that Fly Ash from Thermal Stations has equivalent chemical composition and hence can be used for the electrical installations in areas of high ground resistivity. Fly Ash can also be used as a chemical treatment material to reduce soil resistivity when compared to bentonite and salt charcoal combination.

CORROSION:On an average steel corrodes about six times faster than copper when placed in soil, the extent of corrosion depending upon the properties of the soil. Even though soils have conflicting properties, a fair degree of correlation has been found between electrical resistivity of soil and corrosiveness. The generally accepted correlation between resistivity of soil and its corrosivity is indicated in the Table below:Range of Soil Resistivity Class of Soil (ohm meter)Less than 25Severally Corrosive 25 50Moderately Corrosive 50 100Mildly Corrosive Above 100Very Mildly CorrosiveProvide Cathodic Protection.Use cement conducting corrosion resistent coating on steel (e.g. Zinc Coating)Use steel conductor with large cross section having allowance for corrosion.

EARTH CONNECTIONS:Main joints should not have been less than 150 sq mm.Other joints should not have been less than 65 sq mm.Joints are braced or welded. Welds should be treated with Barium chromate and welded surface painted with red lead and Aluminium paint and afterwards coated with Bitumen.

Combined earth resistance:Power House:0.5 ohmMajor S.S.:1.0 ohmOther S.S.:2.0 ohmDistribution Transformer:5.0 ohm

EARTHING IN SERVICE CONNECTIONS:At the place of availing service connection one earth terminal should be provided for the use of the consumer. That terminal should be copper plate of 32 x 3 mm size with provision for three 15 mm bolt & nuts.Neutral wire should be connected to one bolt, service connection bearer wire should be connected to the second bolt and consumer earth wire to the third bolt. Consumer earth wire should be taken to all the third pins of the power pin sockets in the wiring.Bearer or earth return wire should not be taken as neutral wire. No separate earth pit is to be maintained at consumers premises, where only light and fans are connected, provided our Neutral is pucca Multiple earthed Neutral system. If however equipments, or home need appliances are there, earch connections should be made to a separate earth pit, the leads taken through PVC pipe and any earth connection being in accessible to touch.

TOUCH POTENTIAL:If a man touches the metal part of an equipment during fault, the maximum potential difference that will exist between hand and foot is called TOUCH POTENTIAL. STEP POTENTIAL:The maximum potential difference between legs of a man when standing on the ground the pace being 1 meter is called STEP POTENTIAL. TRANSFER POTENTIAL:This is the maximum potential difference between the equipment other than electrical equipment, like water line, fencing etc. to the ground not connected directly to the equipment or earthing system.

Determine the tolerable touch and step potentials:These potentials very depending on the body weights, thickness and resistivity of surface layer and duration of shock currents and the same for persons with average weight of 50 Kgs are given below:E touch =[1000 + 1.5 Cs (hsK) ps]0.116 / tsF Step =[1000 + 6 Cs (hsK) ps]0.116 / tsWhere C = 1 for crushed rock having resistivity equal to that of soil. If crushed rock resistivity does not equal that of soil, reference may be made to fig. 1 for obtaining the value of Cs.ps=Resistivity of surface layer in Ohm-meter p=Resistivity of soil in Ohm-meter. K=p ps / p + ps ts=Duration of shock current flow in seconds. hs=Surface layer thickness in meter.

EFFECT OF BODY CURRENT:Common physiological effect due to current on body (1) Perception (2) Muscular contraction (3) Unconsciousness (4) Fibrillation of heart and burning1 m A:recognized threshold of perception 1 mA 6 mA:let go current 9 mA 25 mA:Current may be painful :further higher current result in muscular contraction and makes breaking difficult 60-100 mA:ventricular Fibrillation or stoppage of heart functioning may occur.SAFE BODY CURRENT:I=0.165 for t < 3 sec. And t =9 m.A. for t > 3 sec where t is the time duration of stock which may vary from 8 m sec. to 3 seconds.As per revised IEEE Std 80I=0.116 / t for body weight 55 kg. =0.165 / t for body weight 70 kg.

The human body can withstand (lk) fibrillation currents for a time t. lk (tolerable current) is inversely proportional to square root of duration of shock or exposure (t)lk x t.5 = 0.116 or lk = 0.116/t.5.The fibrillation limit of withstandable current is 116 milliamps if duration of shock (t) is one second.

Resistivity of soil at site 50100150Max tolerable step/touch potential for 1 sec, rs = 3000 & hs = 100 mmSteptouch (4a)(4b) VoltVolt122938612503911271396Potential out of col (4) actually applied to human body after the potential drop in top layer (100 mm) stoneSteptouch (5a)(5b) VoltVolt64.670.265.771.166.872.0

RYBN Earth (Ground)Earth PipeHTLTDistributionTransformerRemote EarthRSR1 ContactR ElectrodeR Soil ContactIF = V( R1 Contact + RS + R Line +R Soil Contact + R Electrode + R2 Contact )R LineR2 Contact

RBY180 Amps240 V240 VN180 Amps180 Amps240 V

RBY180 Amps190 V280 VN100 Amps100 Amps280 VN

245 V235 VN260VN lN l is THE SHIFTED NEUTRAL and N is THE ACTUAL NEUTRAL N N l IS THE VOLTAGE IN THE NEUTRAL DUE TO SHIFTINGNN

255 V175 VNB90 AmpsY190 Amps300 VN lR160 AmpsPHASOR DIAGRAM OF TRANSFORMER WITHOUT NEUTRAL EARTHING IN UNBALANCED LOADN l is THE SHIFTED NEUTRAL and N is THE ACTUAL NEUTRAL N N l IS THE VOLTAGE IN THE NEUTRAL DUE TO SHIFTINGN

10 TON PULL10 TON PULL10 TON PULLMECHANICAL ANALOGY FOR BALANCED LOAD WITHOUT NEUTRAL EARTHING Spring Tension 240 Kg/CM 2 Spring Tension 240 Kg/CM 2 Spring Tension 240 Kg/CM 2HingeHingeHinge

30 TON20 TON10 TONMECHANICAL ANALOGY FOR UNBALANCED LOAD (EQUIVALENT TO NEUTRAL NOT EARTHED ) Spring Tension 180 Kg/CM 2 Spring Tension 250 Kg/CM 2 Spring Tension 300 Kg/CM 2HingeHingeHinge

30 TON PULL Load20 TON PULL Load10 TON PULL LoadMECHANICAL ANALOGY FOR UNBALANCED LOAD WITH NEUTRAL EARTHING

Initial Tension of Spring 240 Kg/CM 2Initial Tension of Spring 240 Kg/CM 2Initial Tension of Spring 240 Kg/CM 2Crow Bar Driven in to the soilHingeHingeHinge

DANGERS OF IMPERFECT EARTHING:If the Transformer neutral is not earthed properly, in the event of an earth fault in the system a condition known as arcing ground will occur resulting in high voltages.If lightning arrester earth is not proper, in the event of lightning discharge, the lightning arrester will become in-effective and this may cause injury elsewhere in the equipment.In case AB switch handle is not earthed properly, during the operation of the AB switch, the touch voltage limits may get exceeded and cause injury to the operator.In effective earthing in a distribution system may, not only result in life hazards but also may affect metering.

PRECAUTIONS:Always see that earthing requirement are observed without any compromise.Never use an earth return to serve as neutral when tapping single phase supply.Consumer neutral must always be maintained pucca and proper linkage with the system neutral ensured.Remember that earth wire will carry current during fault conditions and hence adequate size of the wire should be used for earthing.Remember the removal of the earth or improper maintenance of the earth system will cause single voltages at some points which may cause overfluxing conditions on the feeding Power Transformer and cause irrepairable damages inside the Power Transformers.

PRESENTATION ON VAR COMPENSATION AND PF IMPROVEMENT

VI ( Load Current ) I L (Inductive Current) LaggingI R INPHASE WITH VOLTAGEjINPHASE and OUT OF PHASE LOAD CURRENTS I TO VOLTAGE VI RI L

INPHASE & OUT OF PHASE LOAD CURRENTS AFTER INTRODUCTION OF CAPACITOR IL OUT OF PHASEDUE TO INDUCTANCEI R INPHASEEICI loadIL

Esq1q2I cERERKWIIIRIRIXIXKvarIMPROVEMENT OF RECEIVING END VOLTAGE DUE TO INSTALLATION OF CAPACITORS PF IMPROVEMENT

Useful Power OR Active Power (KW)REATIVE POWER (KVAR)Apparent Power (KVA)jPHASOR DIAGRAM OF PLANT OPERATING AT A LAGGING POWER FACTORPOWER FACTOR ( COSF)= (Useful Power / Apparent Power) = KW / KVA

Power factor correction by adding leading KVARj2j1Useful Power (KW)Resulting Effective KVARInitial Load KVARLeading KVAR AddedReduced Apparent PowerApparent Power (KVA) Before Correction

Additional MWACTIVE POWER (MW)RPAPPARENT POWER (MVA)Additional MVAREACTIVE POWERjj21PHASOR DIAGRAM FOR CAPACITY RELEASE DUE TO SHUNT CAPACITORS

MECHANICAL ANALOGY FOR POWER FACTOR AND REACTIVE POWERjDUE TO ANGLE j ADDITIONAL 2 TON IS REQUIRED FOR MOVING THE TRAIN CARPOWER FACTOR =10/12 = 0.83POWER FACTOR = 10/10 = 1

SPEED 60 KM/Hr.10 HP12 HPSPEED 60 KM / Hr.jReactive Power MECHANICAL ANALOGY FOR REACTIVE POWERPOWER FACTOR =10/12= 0.83POWER FACTOR =10/10= 1

WHICH IS BENEFICIAL TO E.B & CONSUMER..COCA COLA FETCHES REVENUE50 %68 %PF: 0.707PF: 0.9550 %32 %FOAM FETCHES NO REVENUE THIS IS SIMILAR TO TRANSFORMERS LOADED TO FULL CAPACITY OF Kvar & KW ( 100 KVA 133 Amps)66.5 A66.5 A42.5 A90.5 ABOTH CUPS ARE FULL WITH FOAM AND COCA COLAEquivalent to Kvar (Reactive Power )Equivalent to KW ( Active Power )

THANK YOU