Insulation Resistance Test and Polarization Index Test
Insulation Resistance Test and Polarization Index Test
Insulation Resistance Test and Polarization Index Test
Test
Under Electrical Switchgear
In insulation resistance test, a high DC voltage is applied across, conductor and ground more
specifically. The voltage is applied across the insulator. Due to this applied high DC voltage there
will be a current through the electrical insulator. Big dividing the applied voltage by this current we
get the actual resistive value of the insulator.
Say the applied high voltage is V and corresponding current through the insulator is I. Hence as per
Ohms law the value of insulation resistance is
This test is generally done by means of megger.
Megger gives required direct (dc) voltage across the insulator an it also shows the resistive value of
insulator directly in Mohm range.The megger are generally of 500 V, 2.5 KV an 5 KV.
500 V megger are used for insulation test upto 1.1 KV rated insulation. For high voltage
transformer, other HV equipment and machines, 2.5 or 5 KV megger are used.
As all insulators are dielectric in nature they have always a capacitive property. Due to that, during
application of voltage across the electrical insulator, initially there will be a charging current. But
after some time when the insulator is totally charged, the capacitive changing current becomes
zero and then only resistive conductive current presents in the insulator. That is why it is always
recommended to do insulation resistance test at least for 1 minute as it is proved that charging
current totally becomes zero after 1 minute.
Only measuring insulation resistance by megger for 1 minute does not always give reliable result. As
the resistive value of an electrical insulator also varies with temperature. This difficulty is partially
solved by introducing polarity index test or in short PI value test. The philosophy behind PI test is
discussed below.
Actually when a voltage is applied across an insulator there will be a leakage current from line to
ground. Although this leakage current is very small is in millampere or sometimes in micro ampere
range, but it has mainly four components.
1) Capacitive component.
2) Resistive or conductive component.
3) Surface leakage component.
4) Polarization component.
Let us discuss one by one.
Capacitive Component
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.
Polarization Component
Every insulator is hygroscopic in nature. Some contaminant molecules and mainly moisture in
insulator are very polar. When an electric field is applied across insulator the polar molecules align
themselves along the direction of electric field. The energy required for this alignment of polar
molecules, comes from voltage source in form of electric current. This current is called polarization
current. It continues until all the polar molecules allied themselves along the direction of electric
field.
It takes around 10 minutes to align the polar molecules along electric field and that is why if we
take megger result for 10 minutes, there would be no effect of polarizing in megger result.
So when we take megger value of an insulator for 1 minute, the results reflects, the IR value which
is free from effect of capacitive component of leakage current. Again when we take megger value
of an insulator for 10 minutes, the megger result shows the 12 value, free from affects of both
capacitive component and polarization component of leakage current.
Polarization index is the ratio of megger value taken for 10 minutes to the megger value taken for 1
minute.
The significance of polarization index test. Let I is the total initial current during polarization index
test or PI test.
IC is the capacitive current.
IR is resistive or conductive current.
IS is surface leakage current.
IP is polarization current of the insulator.
Value of insulation resistance test or IR value test, i.e.
value megger reading just after 1 minute of the test, is-
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