Page 47 - Electrician - TT (Volume 1)

ELECTRICIAN - CITS

Medium resistance
• Above 1 ohm up to 100,000 ohms (100 k ohms or 0.1 m ohms)
Example-resistance of bulb, heater, motor starters
Method used for the measurement

• Ammeter voltmeter method
• Wheat stone bridge
• substitution method
• ohmmeter method

High resistance
• 0.1 M ohm (100 k ohms) and above
• Example- insulation resistance, carbon composition resistor
Methods used for measurement
• Direct deflection method
• loss of charge method

• meg ohm bridge
• megger
Wheat stone bridge (Fig 20)
• Measurement of medium resistance

• Comparison type instrument
• Null type instrument so accuracy (0.1%) is high
For balanceI1 P=I2 R
• For galvanometer current to be zero I1=I3=E/P+Q, I2=I4=E/R+S
• P/P+Q=R/R+S

• QR=PS
• R=S P/Q
• R=Unknown resistance
• S=Standard arm

• P, Q=Ratio arm

Fig 20

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CITS : Power - Electrician & Wireman - Lesson 5 - 12