1. Rubber
2. Vulcanized
Indian Rubber
4. Varnished
Cambric
5. PVC
Rubber:
Rubber may be obtained from milky sap of tropical trees or it may be produced from oil products. It has relative permittivity varying between 2 and 3, dielectric strength is about 30 kV/mm and resistivity of insulation is 1017Ù cm. Although pure rubber has reasonably high insulating properties, it suffers from some major drawbacks viz., readily absorbs moisture, maximum safe temperature is low (about 38ºC), soft and liable to damage due to rough handling and ages when exposed to light. Therefore, pure rubber cannot be used as an insulating material.
It
is prepared by mixing pure rubber with mineral matter such as zinc oxide, red
lead etc., and 3 to 5% of sulphur. The compound so formed is rolled into thin
sheets and cut into strips. The rubber compound is then applied to the
conductor and is heated to a temperature of about 150ºC. The whole process is
called vulcanization and the product obtained is known as vulcanized India
rubber.
Vulcanized
India rubber has greater mechanical strength, durability and wear resistant
property than pure rubber. Its main drawback is that sulphur reacts very
quickly with copper and for this reason, cables using VIR insulation have
tinned copper conductor. The VIR insulation is generally used for low and
moderate voltage cables.
It
consists of chemically pulped paper made from wood chippings and impregnated
with some compound such as paraffinic or naphthenic material. This type of
insulation has almost superseded the rubber insulation. It is because it has
the advantages of low cost, low capacitance, high dielectric strength and high
insulation resistance. The only disadvantage is that paper is hygroscopic and
even if it is impregnated with suitable compound, it absorbs moisture and thus
lowers the insulation resistance of the cable. For this reason, paper insulated
cables are always provided with some protective covering and are never left
unsealed. If it is required to be left unused on the site during lying, its
ends are temporarily covered with wax or tar. Since the paper insulated cables
have the tendency to absorb moisture, they are used where the cable route has a
few joints. For instance, they can be profitably used for distribution at low voltages
in congested areas where the joints are generally provided only at the terminal
apparatus. However, for smaller installations, where the lengths are small and
joints are required at a number of places, VIR cables will be cheaper and
durable than paper insulated cables.
It
is a cotton cloth impregnated and coated with varnish. This type of insulation
is also known as empire tape. The cambric is lapped on to the conductor in the
form of a tape and its surfaces are coated with petroleum jelly compound to
allow for the sliding of one turn over another as the cable is bent. As the
varnished cambric is hygroscopic, therefore, such cables are always provided
with metallic sheath. Its dielectric strength is about 4 kV/mm and permittivity
is 2.5 to 3.8.
This
insulating material is a synthetic compound. It is obtained from the
polymerization of acetylene and is in the form of white powder. For obtaining
this material as cable insulation, it is compounded with certain materials
known as plasticizers which are liquids with high boiling point. The
plasticizer forms a gell and renders the material plastic over the desired
range of temperature. Polyvinyl chloride has high insulation resistance, good
dielectric strength and mechanical toughness over a wide range of temperatures.
It is inert to oxygen and almost inert to many alkalis and acids. Therefore,
this type of insulation is preferred over VIR in extreme environmental
conditions such as in cement factory or chemical factory. As the mechanical
properties (i.e., elasticity etc.) of PVC are not so good as those of rubber,
therefore, PVC insulated cables are generally used for low and medium domestic
lights and power installations.
