insulating materials

1. CHAPTER 4
INSULATING MATERIALS
Introduction to Insulating Materials:
The Electrical Insulating Material/insulating materials are the materials that inhibit heat transmission, electric
current, or noise.
The importance of the insulating materials is ever-increasing in day by day as there is an innumerable number of
types of insulators available in the market. The selection of the right type of insulating matter is very important
because the life of the equipment depends on the type of material used.

23. 1. Large insulating resistance.
2. High dialectic strength.
3. Uniform viscosity—it gives uniform electrical and thermal properties.
4. Should be uniform throughout—it keeps the electric losses as low as possible and
electric stresses uniform under high voltage difference.
5. Least thermal expansion.
6. When exposed to arcing should be non-ignitable.
7. Should be resistance to oils or liquids, gas fumes, acids and alkalies.
8. Should have no deteriorating effect on the material, in contact with it.
9. Low dissipation factor (loss tangent).
10. High mechanical strength.
A good insulating material should possess the following characteristics:
2. Characteristics of a Good Insulating Material

24. 11. High thermal conductivity.
12. Low permittivity.
13. High thermal strength.
14. Free from gaseous insulation to avoid discharges (for solids and gases).
15. Should be homogeneous to avoid local stress concentration.
16. Should be resistant to thermal and chemical deterioration.

25. 3. Properties of Insulating Materials
The properties of insulating materials are enumerated and discussed as under:
1. Electrical Properties
2. Thermal Properties
3. Chemical Properties
4. Mechanical Properties.
1. Electrical Properties of Insulating Materials:
I) Insulation Resistance:It may be defined as the resistance between two conductors (or systems of
conductors) usually separated by insulating materials. It is the total resistance in respect of two parallel
paths, one through the body and other over the surface of the body.
Insulation resistance is affected by the following factors:
1. It falls with increase in temperature.
2. The resistivity of the insulator is considerably lowered in the presence of moisture.
3. It decreases with the increase in applied voltage.

26. a) Resistivity:
This is usually measured as insulation resistance. This term when applied to insulating materials needs
qualification to indicate whether it refers, to volume or surface.
b) Volume Resistivity:
Volume resistivity is the resistance between opposite faces of a cube of unit dimensions; it is usually
expressed in mega ohm-centimetres. The volume resistivity of most insulating materials is affected by
temperature, the resistivity decreasing with an increase of temperature, i.e., the temperature co-efficient
of resistivity is negative.
c) Surface Resistivity:
Surface resistivity is the resistance between the opposite sides of a square of unit dimension on the
surface of the materials, it is usually expressed in mega ohms per centimetre square. The surface
resistivity of any square on the surface of materials however, is independent of the size of the square
provided that the surface resistivity is uniform over the whole surface.

27. d) Insulation Resistance of a Cable:
In a cable useful current flows along the axis of the core but there is always present some leakage of current.
This leakage is radial i.e., at right angles to the flow of the useful current. The resistance offered to this radial
leakage of current is called “insulation resistance” of the cable. If the length of the cable is greater, the
leakage area is also greater meaning thereby that more current will leak. In other words insulation resistance
is decreased. Hence the insulation resistance is inversely proportional to the length of the cable.

28. ii. Dielectric Strength:
If the voltage across an insulating materials is increased slowly the way in which the current
increases depends upon the nature and condition of the material as illustrated schematically in
Fig. below.
For material I, the current
increase very slowly and
approximately linearly with
voltage until a large, sharp
increase result in what can
be described disruptive
dielectric breakdown.
In contrast, for material II the
current increases more rapidly
until current “runway” occurs. It
can be shown that the voltage
at which current “run way”
occurs depends upon the rate
at which the voltage is
increased, so that a more
definite though arbitrary, value
of dielectric breakdown may
be obtained.
The potential gradient at which
breakdown occurs is termed as
dielectric strength. It is easily
calculated for uniform fields by
dividing the breakdown voltage by
insulation thickness.

29. The dielectric strength of an insulating material decreases with the length of time that voltage is applied.
Moisture, contamination, elevated temperatures, heat ageing, mechanical stress, and other factors may
also markedly decrease dielectric strength to as little as 10% of the short time values at
standard laboratory condition.
Dielectric failure that occurs along the interface between a solid insulating material and air, or a liquid
insulating material is termed “surface breakdown”.
iii. Power Factor:
Power factor is a measure of the power loss in the insulation and should be low. It varies with the
temperature and usually increases with the rise in temperature of the insulation. A rapid increase indicates
danger.

30. The dielectric constant is the ratio of the permittivity of a substance to the permittivity of free space. It
is an expression of the extent to which a material concentrates electric flux, and is the electrical
equivalent of relative magnetic permeability.
iv. Dielectric Constant (Permittivity):
Dielectric, insulating material or a very poor conductor of electric current. When dielectrics are placed in an electric
field, practically no current flows in them because, unlike metals, they have no loosely bound, or free, electrons that may
drift through the material. Instead, electric polarization occurs. The positive charges within the dielectric are displaced
minutely in the direction of the electric field, and the negative charges are displaced minutely in the direction opposite to
the electric field. This slight separation of charge, or polarization, reduces the electric field within the dielectric.
A dielectric material is a poor conductor of electricity but an efficient supporter of electrostatic fields.

31. As the dielectric constant increases, the electric
flux density increases, if all other factors remain
unchanged. This enables objects of a given size,
such as sets of metal plates, to hold their electric
charge for long periods of time, and/or to hold
large quantities of charge. Materials with high
dielectric constants are useful in the manufacture
of high-value capacitors.

33. v. Dielectric Loss:
The dielectric losses occur in all solid and liquid dielectrics due to:
I) A conduction current: The conduction current is due to imperfect insulating qualities of the dielectric
and is calculated by the application of ohm’s law- it is in phase with the voltage and results in a power
(I²R) loss in the material which is dissipated as heat.
(ii) Hysteresis: Dielectric hysteresis is defined as the lagging of the electric flux behind the electric force
producing it so that under varying electric forces a dissipation of energy occurs, the energy loss due to
this cause being called the dielectric hysteresis loss.
The dielectric loss is affected by the following factors:
(i) Presence of humidity … it increase the loss
(ii) Voltage increase … it causes high dielectric loss
(Hi) Temperature rise … it normally increases the loss
(iv) Frequency of applied voltage … the loss increases proportionally with the frequency of applied voltage.

34. 2. Thermal Properties of Insulating Materials:
i. Specific Heat & Thermal Conductivity:
Thermal conductivity describes the ability of a material to conduct heat, and the specific heat capacity
tells how much heat energy is absorbed or released depending on the temperature difference and
mass.
ii. Thermal Plasticity:
Pressure on the wires of a wound coil varies under operating conditions because of the expansion and
contraction of the parts caused by variations in temperature.
iii. Ignitability:
Insulating materials exposed to arcing should be non-ignitable. In case they are ignitable, they should
be self-extinguishing, resistant to cracking or carbonisation of the material.
iv. Softening Point:T he softening point is the temperature at which a material softens beyond some
arbitrary softness.The softening point of solid insulating material should be above the temperature
occurring in practice.

35. v. Heat Ageing:
Ageing is, in effect, the wearing out of an insulating material by reducing its resistance to mechanical injury.
It increase rapidly with temperature, approximately doubling for each increase of 10°C to 16°C, depending
upon the material.
vi. Thermal Expansion:
Thermal expansion is important because of the mechanical effects caused by thermal expansion due to
temperature changes. In insulating materials it should be very small.
3. Chemical Properties of Insulating Materials:
i. Resistance to External Chemical Effect
Insulating materials should be resistant to oils or liquids, gas fumes, acids and alkalies. The materials should not undergo
oxidation and hydrolysis even under adverse conditions.
ii. Resistance to Chemicals in Soils
Cables laid in the soil can deteriorate by the action of chemicals in soils. The suitability of insulating
materials for such conditions can be decided by a long experience.
iii. Effect of Water
Water directly lowers electrical properties, such as electrical resistance and dielectric strength.

36. 4. Mechanical Properties of Insulating Materials
i. Density
ii. Viscosity
iii. Moisture Absorption
iv. Hardness of Surface
v. Surface Tension
vi. Uniformity