A MEGOHMMETER IS A HIGH RESISTANCE METER DEDICATED TO MEASURE AN OHMIC VALUE UNDER A DIRECT VOLTAGE OF GREAT STABILITY.

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Mega ohm Meter (Megger)
Used for insulation testing

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Constructional Details
DC Generator
• Hand moved
• Speed constant
• Permanent magnet
• Test voltage 500 V, 1000 V, 2500 V (selectable)
• Handle, gears and centrifugally controlled switch
• Brush less ac generator with rectifier also used

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Coils and Magnet
• A permanent magnet with
special design
• Current coil (CC) on moving
former
• Two pressure coil (PC1 &
PC2) on the moving element
• A fixed C-shaped core
• Spatial angle between CC
and PC is 900
Constructional Details

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Constructional Details
Coil connections
• A pressure coil connected in series with a fixed
standard resistance
• Pressure coil along with fixed resistance is
connected in parallel to the DC generator
• Current coil in series with unknown resistance
(insulation) and a fixed standard resistance
•Guard rings prevents the error due to leakage
currents

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Constructional Details

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Constructional Details
Non-linear scale
• Torque produced by current coil will be in
opposition to that by voltage coil
• Pointer stabilizes when both torque are equal
• Non-linear scale (similar to inverse Tan)

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Factors Affecting Insulation Resistance
• Dirt, Moisture and temperature
• A thumb rule – insulation resistance becomes
twice for 10 0
C decrease in temperature
• Naturally deteriorates with time

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How to connect ?

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How to connect ?

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How to connect ?

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Precautions
• Instrument develops high voltages, hence the
possibility of getting shock if terminals are
touched during use
• A capacitive circuit should be discharge

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