Page 113 - Electrician - TT (Volume 1)
P. 113

ELECTRICIAN - CITS




           •  Metal casing of apparatus such as heater, refrigerator electric drills should be connected to earth
           •  Frame of generator, motor, transformers should be earthed by two      separate connections with earth
           •  Neutral conductor of 3 phase 4 wire system & middle conductor of 2 phase 3 wire system should be earthed
              by not less than two separate earth connections at the generating station substation
           •  Steel transmission tower lines or poles carrying overhead conductors should be earthed at every mile (1.61km)
              at 4 points
           •  Stay wire for overhead lines should be earthed by connecting at least one strand to the earth wire
           Factors influencing earth resistance
           •  Condition of soil

           •  Temperature of soil
           •  Moisture content of soil
           •  Size and spacing of earth electrode
           •  Depth at which electrode is embedded
           •  Material of conductor

           •  Quality of coal and dust and charcoal in the earth electrode pit
           Methods for improve earth resistance
           •  Increase the pit depth
           •  Increase the plate area
           •  Increase the number of electrodes in parallel

           •  Pouring of fresh salt water (copper sulphate solution –copper plate earthing) through pipe over coal bed
           Earth resistance tester working principle
           Used for earth resistance measurement
           •  It’s a special type of Megger with rotating current reverser and rectifier with commutator made up of L shaped
              segment
           •  They mounted on the shaft of the hand driven generator
           •  Each commutator has 4 fixed brushes
           •  One pair of each set of brushes is positioned that they make contact alternately with one segment and then
              with the other as the commutator rotates
           •  2 nd pair of each set of brushes is positioned on the commutator so that continuous contact is made with one
              segment whatever the position of the commutator
           •  It has 4 terminals p1, p2, c1, c2
           •  P1, c1 shorted &connected to earth electrode
           •  P2, c2connected to auxiliary electrode P, C

           •  Indication depends on the ratio of the Voltage across the pressure coil and the current through the coil
           •  Deflection indicates the earth resistance directly
           •  It’s a PMMC instrument can operate only in DC
           •  By include reverser & rectifier possible to made measurements with AC flow through soil
           •  Use of AC eliminates unwanted effects due to production of back emf in the soil due to electrolytic action
           •  Instrument is free from effect of AC &DC present in soil.







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                                    CITS : Power - Electrician & Wireman - Lesson 20-25
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