The megger , or megohmmeter , is a high range ohmmeter.
Used to measure insulation resistance and other high resistance values.
This principle forms the basis of insulation testing instrument known as
Also used for ground, continuity, and short circuit testing of electrical
Designed to cover a wide range(more than 1000megohms) of
Megger Instruments Limited was based at AVO’s Dover site, in Archcliffe Road, Dover,
CT17 9EN, Kent, England, which is also the present location of the UK branch of the
In 1991, due to the popularity of the AVO products the company changed the name
to AVO( Multimeter with Amps, Volts and Ohms) Megger Instruments Limited.
In 2000 the company became known as AVO International Holdings Limited and in
2002 as AVO International Limited.
Finally the whole Megger group changed its name to Megger Group Limited in 2002, as
it is known today.
Megger Group Limited is a company that manufactures electric test equipment and
measuring instruments for electrical power applications
The Dover site employs around 230 people. In total (including the sales channels)
Megger Group Limited employs around 1500 people worldwide.
• Storage of test results.
• Recall of test results to instrument display.
• Connect to IBM compatible PC for
storage of results.
• Variable RCD test current for
• Designed and manufactured in
accordance with International Safety
• Continuity tests with 200 mA short
• Prospective Earth Fault and Short
• Supply voltage, polarity and
• Complete with Carry Case, Test
Leads and Certificate of Test.
The instrument works on the principle of ratio-meter/ohmmeter . The required deflection torque is
produce by both voltage and current(in the cross-coil) . Due to the interaction between the
magnetic field produce by voltage and current , the deflecting torque is produce . The required coil
are so positioned that the deflecting torque is proportional to the ratio V/I.
*Torque of coil B proportional to I2 cos(90- Ө )
*Torque of coil A proportional to 1 cos Ө
I1 cos Ө = I2 cos (90- Ө )
I1 cos Ө = I2 sin Ө
= I1/I2 which is equal to R/R1
*If R1 fixed then the scale calibrated R directly.
*The two coil comes in position of equilibrium when two torque equal.
Permanent magnet DC
Measures resistance between
zero to infinity
• Permanent magnet DC generator: The
megger has a small permanent magnet DC generator. The
generator is capable of generating either 250V or 500V at
full speed. This generator generates a constant voltage
and thus avoids the effect of capacitive currents.
• Measures resistance between zero to
infinity: A megger can measure all resistance lying
between zero and infinity essentially it is a high
resistance measuring device.
ERRORS OF MEGGER
ERRORS READING VALUE DUE TO LOW BATTERY
ERRORS DUE TO CHANGE IN TEMPERATURE
ERRORS DUE TO SENSITIVITY
ERRORS DUE TO CHANGE IN HIGH VOLTAGE
1.Measurement of high value resistances.
2.Used to measure the electrical resistance of insulators.
3.Testing of electrical components, machinery.
4.Installations for winding
Megger is a VERY USEFUL INSTRUMENT which is use to determine all the resistances
between Zero and Infinity.
Actually zero is the 1st mark and 10k ohm is the 2nd mark so it is impossible to accurately
measure small resistances with the help of a Megger
This instrument is very simple to operate, portable and independent of the external
A text Book of Electrical Technology By B.L. THERAJA
Electrical and Electronic Measurements And Instrumentation By A.K.Sawhney