Earthing Practice

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    04-Oct-2015

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Earthing Practice

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  • EARTHING PRACTICES & Basics of HV ProtectionM.Arunachalam,M.E.,Former Chief Engineer/TNEBDon't be afraid to give your best to what seemingly are small jobs. Every time you conquer one it makes you that much stronger. If you do the little jobs well, the big ones tend to take care of themselves.

  • WHAT IS EARTHING? Earthing means an Electrical connection to the general mass of earth to provide safe passage to fault current to enable to operate protective devices and provide safety to personnel.

  • Objectives of EarthingTo ensure that no part of equipments, other than live parts, assume dangerous potential.To allow sufficient current to flow safely for proper operation of protective devices.To suppress dangerous potential gradients on the earth surface which may cause incorrect operation of control & protective devices and also may cause shock or injury to personnel.Provide stability of voltage, prevent excessive voltage peaks during disturbances and protect against lightning surges.

  • Basic ObjectivesIt should stabilize circuit potential with respect to ground and limit the overall potential rise.It should protect men and materials from injury or damage due to over voltage or touching It should provide a low impedance path to fault currents to ensure prompt and consistent operation of protective devices. It should keep the maximum voltage gradient along the surface inside around the substation within safe limits during ground fault.It should protect under ground cables from over all ground potential and voltage gradient during ground fault in the system.

  • Classification of Earthing Grounding is defined as a conducting connection, whether intentional or accidental between an electrical circuit or equipment and the earth; or to some conducting body that serves in the place of earth Earthing can be broadly classified as 1. System Earthing System earthing associated with the electrical circuit2. Equipment Earthing Equipment earthing is connecting the non-current carrying metal parts of equipment to the ground

  • Classification of Earthing1. Neutral grounding Solidly grounded systemResistance/Reactance grounding2. Equipment earthing3. Reference grounding 4. Discharge grounding

  • Discharge groundingFor a 22 KV system the BIL is a 150 kvSuppose a lightning discharge 10 KAResistance shall be < 150 / 10 ohms = 15 ohmsGeneral rule is Tower foot resistance shall be less than 0.02 *E ,where E is the system voltage in KV

  • Resistance to Earth Earth resistance of an electrode is made-up of:Resistance of the (metal) Electrode Contact resistance between electrode and the soil, and Resistance of the soil from the electrode surface outward in the geometry setup for the flow of current from the electrode to the infinite earth.Soil resistively is the single factor, which dominates in the arrival of earth resistance of an electrode.

  • Soil ResistivitySoil resistivity is largely depends upon (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 packing

  • Type of Soil - Resistivity1. Loamy Garden Soil500 5,002. Clay800 5,000 3. Clay, Sand and Gravel 4,000 25,000 4. Sand and Gravel 6,000 10,0005. Slates, Slab Sand Stone1,000 50,0006. Crystalline Rocks 20,000 1,00,000

  • THREE MAIN TYPES OF EARTH RESISTANCE MEASUREMENTS

    Measurement of Soil resistivity Measurement of Ground Resistance Measurement of Earth Surface Potentials

  • Typical values of resistivity for various types of soils

    Sl. No.Nature of soil Range of Resistivity1Red loamy soil40-200 -m2Red sandy soil200-2000 -m3Laterite soil300-2600 -m4Shallow black soil20-100 -m5Medium black soil50-300 -m6Deep black soil50-250 -m7Mixed red & black soil50-250 -m8Coastal alluvium 300-1300 -m9Laterite gravelly 200-1000 -m

  • Resistance of Rods EmbeddedGrounding Resis.exhibits drooping trend due to changing climatic condition (Restvty-200/169/35.18).Resist.of rod embedded in bentonite falls more rapidly with increase in size from 250 to 300mm. The degree of variation of resis.is drastically reduced or almost nullified during rainy & flood season.

  • Current Reaction1 milliamp Just a faint tingle.5 milliamps Slight shock felt. Disturbing, but notpainful Most people can let go. however strong involuntary movements can cause injuries.625 milliamps (women) Painful shock. Muscular control is lost. This is the range where freezing currents start930 milliamps (men). It may not be possible to let go.50150 milliamps Extremely painful shock, respiratory arrest (breathing stops), severe muscle contractions . Flexor muscles may cause holding on; extensor muscles may cause intense pushing away. Death is possible. Contd..Effects of Electrical Current* on the Body

  • Current Reaction

    1,0004,300 milliamps Ventricular fibrillation (heart pumping action (14.3 amps) not rhythmic) occurs. Muscles contract; nerve damage occurs. Death is likely.

    10,000 milliamps Cardiac arrest and severe burns occur. Death is (10 amps) probable15,000 milliamps Lowest over current at which a typical fuse (15amps) circuit breaker opens a circuit

    *Effects are for voltages less than about 600 volts. Higher voltages also cause severe burns.Differences in muscle and fat content affect the severity of shock.Effects of Electrical Current* on the Body

  • Effect of Magnitude of Current:

    EFFECT dc current in mAac current in mAMenWomenMen WomenNo sensation on hand10.60.40.3Slight tingling-perception threshold 5.23.51.10.7Shock-not painful and muscular control not lost 961.81.2Painful shock-painful but muscular control not lost624196Painful shock let - go threshold765116.010.5Painful and severe shock-muscular contractions, breathing difficult906023 15

  • Effect of duration of CurrentIB= 0.116/t Current which can be tolerated by a person of 50 Kg. Wt.IB= 0.157/t Current which can be tolerated by a person of 70 Kg. Wt.Valid for 0.03 < t < 3 seconds.

    Effect of frequencyThe tolerable currents mentioned above are for 50 60 Hz. At high frequencies (3000 10000 Hz) still higher currents can be tolerated.

  • Protective distanceskVmax distance 11on trsf tank 22on trsf tank 33 3m 44 5m 66 6m 88 6m 1327m 27518m 40024m

  • TN-C (terre neutre commune):Good enough for old loads, too poor for ITWhich low voltage network configuration is adequate?D Y

  • TN-S (terre neutre spar):Good for IT, if not absolutely necessaryD YWhich low voltage network configuration is adequate?

  • TN-C-S (terre neutre commune spar):Insufficient compromiseD YWhich low voltage network configuration is adequate?

  • TT (terre terre):Expensive copper savingsD YRemember:Copper wire 1 m long, 1 mm thick:17.5 m(10-3)Wire of garden soil 1 m long, 1 mm thick:17.5 M(106)Which low voltage network configuration is adequate?

  • IT (isol terre):Advantage: No failure after first earth fault.But is IT good for IT?D YWhich low voltage network configuration is adequate?

  • In a TN-C system, problems arise because data streams and working currents mix!

    Others who have drawn attention to this source of data transmission errors include NKL ...... insurance com-panies, experts, consultants and many others besides

  • Working currents have no place in earthing systems and protective conductors

    WRONG! RIGHT!

  • The Electric Utility Basic Protection

  • Faults can be the result of external or internal influences (e.g., lightning strikes, resulting in an overload.)Since the damage a fault can cause is mainly dependent on its duration, it is necessary for the protection device to operate as quickly as possible.Protection engineering is thus an extremely important part of the secondary electrical plant and of decisive significance for the reliable operation of electrical power system

  • Among the most important consequences of a fault are:- damage to plant due to the dynamic effects of the fault current- damage to plant due to the thermal effects of the current- loss of system stability- loss of supply to loads, also during downtime for repairs- danger to life

  • Among the most important consequences of a fault are:- damage to plant due to the dynamic effects of the fault current- damage to plant due to the thermal effects of the current- loss of system stability- loss of supply to loads, also during downtime for repairs- danger to life

  • SELECTIVITY:

    Ability to isolate only the defective plant from the rest of the system. The methods of isolations are:Time grading, i.e. the protection device nearest the fault trips the fastest and all the others between it and the power source successively slower HOW ?

  • By a definite minimum time or a time inversely proportional to the level of fault current e.g., Over current & distance protection amplitude and/ or phase comparison of the currents at both sides of the protected unit e.g., pilot wire and differential protection

  • S