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2.4.2 Working Safety Clearances125 220170 320325 630380 750450 900550 1100650 1300750 1500850 1600950 19001050 2400Safety clearance consists of ground clearance and section clearance. The groundclearance is the minimum clearance from any point on or about the permanent equipmentwhere a man may be required to stand (measured from the position of feet) to the nearestpart not at earth potential of an insulator supporting a line conductor and the same hasbeen taken as 2.59 meters (i.e. 8.5 feet), which is the dimensions for a tall man with armsoutstretched below the conductor.The section clearance is the minimum clearance from any point on or about thepermanent equipment where a man may be required to stand (measured from the positionof feet) to the nearest unscreened live conductor in the air. The section clearance systemupto 132 kV 650 kV BIL may be determined by adding 2.5 meters to minimum phase toground clearance of 1.3 which works to 3.8 meters for 132 kV system.Height of Bus Bars Above Ground Within Sub-Station PromisesThe minimum conductor clearance from ground is obtained by adding ground clearance,earth clearance and height of bus bar supporting clamps on the post insulator. Inconsideration to it, minimum height of bus bar for 132 kV may be about 365 mm whichmay be raised to about 450 mm to correspond to the terminal height of the 132 kV circuitbreakers.Conductor Clearance from Roadways Within Sub-Station PromisesMinimum clearance between overhead conductors and roadways within sub-stationpremises is computed to be as “Ground clearance plus 625 mm. This dimension providesfor a truck with a man standing on its top 130 + 625 meter = 755 meters app.2.4.3 Minimum and Safety clearances recommended in Central Board of Irrigation andPower manualClearances from the point of view of system reliability and safety of operating personnelrecommended for sub station upto 245 kv are given in table 2.7. These include the1-9
minimum clearances from live parts to earth, between two live parts of different phasesand sectional clearances between live parts of different phases and sectional clearancesbetween live parts and work section required for maintenance of an equipment. Besides,it is also necessary that sufficient clearance to ground is also available within thesubstation so as to ensure safety of the personnel moving about within the switchyard.HighestSystemVoltage (kV)Lightningimpulse voltage(kVp)Table 2.7Minimum clearancesBetween phase& earth (mm)Betweenphase (mm)36 170 320 320 280072.5 325 630 630 3100123 4505509001100900110034003700145 550650110013001100130037003800245 9501050190021001900210043004600Notes:Safetyclearances (mm)i) Safety clearances are based on the insulation height of 2.44 m which is the heightof lowest point on the insulator where it meets the earthed metal.ii) The distances indicated above are not applicable to equipment which has beensubjected to impulse test since mandatory clearances might hamper the design ofthe equipment, increase its cost.iii) The values in table refer to an attitude not exceeding 1000 m and take intoaccount the most unfavorable conditions which may result from the atmosphericpressure variation, temperature and moisture. A correction factor of 1.25 % per100 m is to be applied for increasing the air clearance for altitude more than 1000m and upto 3000 m.iv) No safety clearance is required between the bus-bar isolator or the bus-barinsulator. However, safety clearance is necessary between the section isolator orthe bus-bar itself and the circuit breaker.v) For the purpose of computing the vertical clearance of an overhead strungconductor the maximum sag of any conductor shall be calculated on the basis ofthe maximum sag in still and the maximum temperature as specified.vi) As an alternative to maintain safety clearances in some substation earthedbarriers are used to ensure safety of the maintenance personnel. The use ofearthed barriers is quite common at lower voltages of 36 kV 72.5 kV. In case ofpaucity of space and if 2.44 m clearance is not available then localized earthedfencing with clearance can be considered by the designer.Following are the normally adopted spacing for the strung bus:1-10
Page 2 and 3: Section -1Guide for Design of Outdo
Page 4 and 5: The terms atmospheric overvoltages
Page 6 and 7: Table 2.31. Basic Data Example 1.Hi
Page 8 and 9: Spark gaps specified for fitting to
Page 12 and 13: Highest System Voltage rating kV Sp
Page 14 and 15: 2.6 Insulator TypeTypes of insulato
Page 16 and 17: Bending .. .. 1265 kg/sq.cm.Shear .
Page 18 and 19: v) Installations in difficult site
Page 20 and 21: During maintenance, it is essential
Page 22 and 23: Areva Compact Air Insulated Substat
Page 24 and 25: The bus bar sizes should meet the e
Page 26 and 27: disconnect switch contacts in such
Page 28 and 29: (iv)Electro magnetic type interlock
Page 30 and 31: ) Temperature rise of 11 kV Class C
Page 32 and 33: 4.6 TestsThe CTs shall be provided
Page 34 and 35: 10. 1.2/50 micro seconds impulse 32
Page 36: 7.0 Lightning ArrestorsLightning ar
Page 39 and 40: and this aspect has to be kept in m
Page 41 and 42: Fig. 12.1: Layout of 12 kV Agnoor S
Page 43 and 44: Fig. 12.4 Layout of 72.5 kV Switchy
Page 45 and 46: 464006800 7600 3000LA30002000 2000X
Page 47 and 48: Fig. 12.6:145 kV Outdoor Switchyard
Page 49 and 50: Fig. 12.7: 245 kV Outdoor Switchyar
Page 51 and 52: 2-20
Page 53 and 54: 3.0 Transformer RatingThe full load
Page 55 and 56: OFAFOil Immersed with forced oil ci
Page 57 and 58: The transformer may be operated wit
Page 59 and 60: 8.10 Parallel OperationThe transfor
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(a)(b)33 kV Transformers (MVA)1.001
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(e)(f)(g)(h)(i)(j)(k)(l)(m)(n)(o)(p
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18.0 Oil Containment and Fire Prote
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) The limits of temperature rise me
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Example Calculation -1Generator Tra
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TRANSFORMER RATING AND CHARACTERSTI
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3.0 Type of Circuit BreakerFollowin
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iii) during arcing of SF6 breaker,
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3.4 Protection Classes for Switchge
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The rated voltage is expressed in k
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5.7 Rated Short Circuit Making Curr
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Effect of circuit conditions on rec
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5.17 Rated time quantities and oper
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25405017.5 812.516254024 812.516254
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component of fault current in addit
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Steps involved in applying method a
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to earthxvii. Difference in the ins
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Datai) Sub transient reactance of g
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Annexure-1Current waveAA = envelop
Page 99:
Annexure-3Fig. 3.3 Arc extinction4-
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