R.N.G.P.I.T, Bardoli �Electrical Engineering Department

Subject: EMMI

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Prepared By:

Dr. Shaikh Mohammed Suhel

Prepared By:

• Name: Dr. Shaikh Mohammed Suhel

(Assistant Professor RNGPIT, Surat)

• Former Assistant Professor in SCET, SNPIT&RC, VIT
• Qualification: PhD (Power- Electronics & Drives, NIT, Surat), M.Tech (Industrial Electronics, NIT-Surat), Gate, B.E. (Electrical Engineering., VNSGU-Surat).
• Experience: 13 Years.

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CH: Resistance Measurements

• This Lecture contain
• Various methods for measurement of High resistance(>100kΩ)

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• By Dr. Shaikh Mohammed Suhel

• Measurement of high resistance:
• Example: Insulation resistance of cable, leakage resistance of capacitor, surface and volume resistivity of insulating material

(1)Direct deflection method

(2)Loss of charge method

(3)Megha ohm bridge

(4)Meggere

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• Direct deflection

method:

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• This is a rough method for measurement of insulation resistance of cables & insulating materials
• Highly sensitive & high resistance (around 1kΩ) is used in series with the resistance to be measured and battery supply

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Surface leakage current flows through guard wires and hence not measured by guard wires.

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• Switch is connected in parallel with galvanometer to protect with high inrush current during starting

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• Cable without metal sheath:

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• Entire cable except the ends is immersed in metal tank having water for about 24 hours and temp is kept constant (at about 20⁰C.

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• Insulating material in sheet form:

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• Specimen of insulating material is provided with tin foil or colloidal graphite electrode.
• Voltage resistivity is obtained from G₁ and Surface resistivity from G₂
• The resistivity of thin layer of dielectric material is different from volume resistivity due to adherent humidity layer, contamination, chemical alteration, absorption of gases or structural modification

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• Insulating material in sheet form:

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• Loss of charge method:

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• The voltage across the capacitor at any instant t :

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• Loss of charge method:

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• This method requires a capacitor of a very high leakage reactance as high as the resistance being measured.
• In the account of leakage resistance of capacitor and resistance of voltmeter R₁ resistance is considered that represents leakage resistance & resistance of voltmeter

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• The test is then repeated with R disconnected

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• Mega ohm bridge method:

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• Used to measure from 0.1 MΩ to 10⁶ M Ω with accuracy of about 3% for lower part of range and about 10 % above 10,000 M Ω

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• Leakage current

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R_L>>R

Req=R

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• High resistances are provided with three terminals

A, B : Main Terminals

G: Guard terminal

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• If high resistance of 100 M Ω is measured with normal whetstone bridge:

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• Then, R= 100MΩ and R_AG=R_BG=100MΩ Then Req= R\\(R_AG+R_BG)=67MΩ.
• Resistance measured is 67MΩ instead of 100MΩ. 33% error is introduced.

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• To avoid the resistance bridge is modified.
• Then, R_AG is connected in parallel with P. As, P< (1/10) R_AG Req=P.
• R_BG is connected across galvanometer and hence balance is not disturbed as R_BG>> R_galvanomter esistance
• Error is very less and accuracy of up to 3% can be obtained.

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• Mega ohm meter or Megger:

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• Used to measure insulation resistance of machines, insulators, bushings etc.

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• Insulation resistance of machine decreases with time due to electrical and mechanical stress, absorption of moisture, dust etc.
• Periodically insulation resistance of material is checked to avoid short circuit due to insulation failure.

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• Traditional deflection type (hand operated) megger is a permanent magnet crossed coil shunt type ohm meter.

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• Permanent magnet DC generator is used to generated the required voltage.

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• The voltage rating can be (100, 200,500,1000,1500,2500V) and depends on the voltage rating of equipment.

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• Principle :
• Hand driven, through gear arrangement and through a centrifugally controlled clutch switch which slips at pre defined speed so that constant voltage is produced.

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Control Coil

Deflecting

Coil

• Principle :
• The torque produced by both coils are opposite and align in a position where torque produce by them is equal.
• i.e., T depends on rate of current through coils => independent of voltage
• Surface leakage current is diverted by the guard wire arrangement.

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End