Page 113 - Electrician - TT (Volume 1)

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