Megger is an electrical
device which is used to measure the Insulation Resistance by performing
Insulation Resistance Test (IR Test). Basically Megger injects High DC Voltage across
the Insulator and ground due to which leakage current flows through the
Insulator to the ground. By measuring this Leakage Current, Megger calculates
the Insulation Resistance.

Suppose the DC Voltage
applied by the Megger = V

And Leakage Current through
the Insulator = I

Therefore from Ohm’s Law,

Insulation Resistance = V/I
ohm

A typical Megger is shown in
figure below.

Meggers are generally of
Rating 500 V, 2.5 kV and 5 kV. In modern Megger as shown in figure above these
range can be selected. 500 V is used to measure the Insulation resistance of
control cable for Insulator which can withstand up to 1.1 kV voltage. For High
Voltage Transformer / High Voltage Machine or Equipment, 5 kV range is selected
to perform Insulation Resistance Test. Megger has the provision to directly read the Insulation Resistance during Insulation Resistance Test.

All Insulators are supposed
to be pure capacitor having very few capacitance so as to take minimum charging
current. When an Insulator is connected across a DC voltage it takes leakage
current. Leakage current can be divided into following types:

·
Capacitive
Charging Current

·
Resistive
or Conductive Current

·
Surface
Leakage Current

·
Polarization
Current.

**Capacitive Charging Current:**

When a DC voltage is
applied across in insulator, because of its dielectric nature there will be an
initial high charging current through the insulator from line to
ground. Although this current decays exponentially and becomes zero.
Generally this current exists for initial 10 seconds of the test. But
it takes nearly 60 seconds to decay totally. That is why it is always
recommended to do Megger or Insulation Resistance Test at least for 1
minute as it is proved that charging current totally becomes zero
after 1 minute. Thus after 1 minute leakage current measured by Megger won’t
include the charging current due to Capacitance of Insulator.

**Resistive or Conductive Current:**

This current is
purely conductive in nature, flows through the insulator as if the insulator is
purely resistive. This is direct flow of electrons. Every insulator should have
this component of electric current. Resistive or Conductive leakage current
through the Insulator will be more if moisture and contamination in Insulator
is high.

The resistive or conductive
component of insulator leakage current remains constant throughout
the test.

**Surface Leakage Current:**

Due to dust, moisture and
other contaminants on the surface of the insulator, there is one small
component of leakage current through the outer surface of the
insulator. Therefore before conducting Megger or Insulation Resistance Test,
Insulator should be cleaned properly so as to eliminate the surface leakage component
of Leakage Current.

**Polarization Current:**

Because of presence of
impurities and moisture in Insulator, insulator becomes polar in nature.
Therefore when we apply high DC Voltage across the Insulator, the polar
molecules i.e. dipole try to align themselves in the direction of Electric
Field. During the period the molecules try to align along the Electric Field, a
current will flow through the Insulator because of movement of dipole along
their axis. This current is called Polarization Current and it lasts for short
time and as soon as the polar molecules align themselves along the Electric
Field, there will be no further movement of dipole and hence polarization
current will stop. It normally takes 10 minutes for Polarization Current to
become Zero.

Thus if we take the Megger
reading after 10 minute then Megger will not consider the Polarization Current
for the calculation of Insulation Resistance.

So when we take Megger value
of an insulator for 1 minute, the Insulation Resistance value will be free from
the effect of capacitive component of leakage current. Again if we take Megger value
of an insulator for 10 minutes, the Megger result shows the value,
free from effects of both capacitive component and polarization component of
leakage current.

**Polarization Index (PI):**

Polarization index is the
ratio of Megger value taken for 10 minutes to the Megger value
taken for 1 minute.

Therefore, PI = Megger value
after 10 minutes / Megger value after 1 minute

**Significance of PI Test:**

Let I = Total initial current during
Polarization Index Test or PI test.

I

_{C}= Charging current due to Capacitance of Insulator.
I

_{R}= Resistive or Conductive Current.
I

_{S}= Surface leakage current.
I

_{P}= Polarization Current of the Insulator.
Therefore,

I = I_{C} + I_{R} + I_{S}
+ I_{P} |

The value of Megger or
Insulation resistance Test / IR Test after 1 minute,

R

_{1minute}= V/( I_{R}+ I_{S}+ I_{P}) …….I_{C}= 0 after 1 minute
Similarly, Megger Value
after 10 minutes,

R

_{10minute}= V/( I_{R}+ I_{S}) ………….I_{P}= 0 after 10 minutes
So, from Polarization Index
Test or PI Test,

PI Value = R

_{10minute }/ R_{1minute}_{}

_{ }= (I

_{R}+ I

_{S}+ I

_{P})/( I

_{R}+ I

_{S})

= [1+ I

_{P}/( I_{R}+ I_{S})]
So,
PI Value = [1+ I_{P}/( I_{R}+
I_{S})] |

From the above it is clear
that, if the value of (I

_{R}+ I_{S}) >> I_{P}, the PI of insulator approaches to 1. Large value of I_{R}or I_{S}or both indicate unhealthiness of the insulation.
The value of PI becomes high
if (I

_{R}+ IS) is very small compared to I_{P}. This equation indicates that high Polarization Index of an insulator implies healthiness of insulator. For good insulator resistive leakage current is very very small.
The value of Polarization
Index of an insulator should be more than 2. If the value of Polarization
Index is less than1.5 then it means Insulator is unhealthy and shall not be
used.

**Thank you!**

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