This document discusses insulation resistance, which is the ability of electrical insulation to resist direct current. It defines the four components of current drawn by insulation when DC is applied: absorption, conduction, capacitance charging, and surface leakage currents. It also discusses the importance of polarization, factors that affect insulation resistance like temperature and moisture, recommended polarization index values, and minimum insulation resistance values according to IEEE and IS standards.

1. Concept of Insulation Resistance
By: HARESH YADAV
• BASICS OF INSULATION RESISTANCE
• IMPORTANCE OF POLARIZATION
INDEX
• IEEE STD 43 -2000 & IS 732
• IR & P.I MEASUREMENT
EQUATIONS

2. WHAT IS INSULATION RESISTANCE ???
• The capability of the electrical insulation of a winding to resist direct current
(Flow of Electrons)
4 COMPONENTS OF CURRENT DROWN BY INSULATION WHEN DC APPLIED
• Absorption (Polarization) current (Ia)
• conduction (Leakage) current (Ig)
• Capacitance Charging Current (Ic)
• surface leakage current (IL):

3. Equivalent Circuit & Test connections

4. Absorption(Polarization): The absorption current,
corresponding to the additional energy necessary for the
molecules of the insulating material to reorient themselves
under the effect of the electrical field applied. This current falls
much more slowly than the capacitance charging current,
sometimes requiring several minutes to reach a value close to
zero.
Leakage current: The leakage current or conduction current.
This current characterizes the quality of the insulation. leakage
current that flows through the insulation volume itself is of
primary importance. This is the current that is used to evaluate
the conditions of the insulation system under test.
surface leakage current: The surface leakage current is due
to the conduction on the surface of the insulation where the
conductor emerges and points of ground potential. Clean the
contaminated surface before testing.
Capacitance Current: The capacitance charging current
necessary to charge the capacitance of the insulation being
tested. This is a transient current which starts relatively high
and falls exponentially towards a value close to zero once the
circuit being tested is charged electrically. After a few seconds
or tenths of seconds, this current becomes negligible
compared with the current to be measured.

6. Voltage Range for I.R Measurement (IEEE 43-2000)

7. Polarization Index:
• The polarization index (P.I.) is traditionally defined as the ratio
of the 10 min insulation resistance (IR10) to the 1 min
insulation resistance (IR1), tested at a relatively constant
temperature..
• In older insulation materials, such as asphaltic-mica, the
absorption currents often take 10 min or more to decay to
nearly zero
• Modern types of film coated wire, as well as epoxy-mica or
polyester-mica insulated stator windings, may approach a
constant value of insulation resistance in 4 min or less.
• If the winding is wet or dirty, a low steady value will usually be
reached 1 min or 2 min after the test voltage is applied.
• There is no standard for what time intervals the IR values are
to be recorded. Different organizations use different ratios.
• it usually takes several minutes after application of the electric
field for the molecules to become reoriented, and, thus, for
the current-supplied polarizing energy to be reduced to almost
zero

8. Factors Affecting Insulation Resistance:
• Surface Condition:Dust (or salts) on insulation surfaces, which are ordinarily
nonconductive when dry, may become partially conductive when exposed to moisture or
oil, and, thus, can lower the insulation resistance. Cleaning & Drying To be done to
Restore the Value.
• Effect of moisture: Older winding insulation systems are hygroscopic (easily absorb
water) and moisture may be drawn into the body of the insulation from the humid
ambient air. Absorbed moisture increases the conduction current (IG) and significantly
lowers the insulation resistance.
• Effect of temperature: in insulators, an increase in temperature supplies thermal energy,
which frees additional charge carriers and reduces resistivity. The insulation resistance
value of a winding depends upon the winding temperature and the time elapsed since
the application of the voltage.
• Effect of Voltage: The value of insulation resistance may decrease somewhat with an
increase in applied voltage; however, for insulation in good condition and in a thoroughly
dry state, substantially the same insulation resistance will be obtained for any test voltage
up to the peak value of the rated voltage.

9. Recommended P.I Values: (IEEE 43-2000)

10. Minimum I.R Values: (IEEE 43-2000)

11. Minimum I.R Value Equation: (IS-732)
• The insulation resistance of each phase
winding against the frame and between the
windings shall be measured.
• Mega-ohmmeter of 500 V or 1000 V rating
shall be used. Star points should be
disconnected.
• Minimum acceptable value of the insulation
resistance varies with the rated power and
the rated voltage of the motor.
• If the resistance is measured at a
temperature different from 25’C, the value
shall be corrected to 25°C.

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