Insulating materials of Cables

  The satisfactory operation of a cable depends to great extent upon the the characteristics of insulation used. Therefore, the proper choice of insulating material for cable is of considerable importance. In general, the insulating material used in cable should have following properties.

(1).Higher insulation resistance to avoid the leakage current.

(2). High dielectric strength to avoid the electrical breakdown of the cable.

(3).High mechanical strength to withstand to mechanical handling of cables.

(4). Non hygroscopic i. e., it should not absorb moisture from air or soil. The moisture tends to decrease the insulation resistance and hastens the breakdown of cable. In case the insulating material is hygroscopic, it 'must be enclosed in a waterproof covering like lead shealth.

(5). Non. Inflammble.

(6).Low cost so as to make the underground system a viable proposition.

(7). Uneffected by acids and alkalies to avoid any chemicals action.

NO one insulating' materials possess all the above mentioned properties. Therefore, the type of insulating material to be used depends upon the purpose for which the cable is required and quality of insulation to be aimed at. The principal insulating material used in cables are rubber, vulcanised India rubber, impregnated paper, varnished cambric and polyvinyl chloride.

 

1. 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 between 2 and 3,directric strength is about 30kV/mm and resistivity of insulation is 10^17ohm cm. Although pure rubber has reasonably high insulating properties, it suffers from some major drawbacks viz., readily absorb moisture, maximum safe temperature is low (about 38 centigrade)., 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.

 

2. Vulcanised India Rubber (V. I. R). It is prepared by mixing pure rubber with mineral matter such as zine 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 150 centigrade. The Whole process is called vulcanisation and the product is obtained is known as vulcanised India rubber. Vulcanised India rubber has greater mechanical strength, durability and wear

resistant property than pure rubber. It main draw back is that Sulphur reacts 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

3. Impregnated paper. It consist of chemically pulped paper made from wood chipings and impregnated with some compound such as paraffinic or napthenic 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 it if is impregnated with suitable compound, it absorbs moisture and thus lowers the insulation resistance of 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 laying, it ends are temporarily covered with wax or tar.

Since the paper insulated cables have the tendency to absorb the 'moisture, they are used where the cable route has a few joints. For instance, they can profitably used for distribution at low voltages in congested areas where the joints are generally provided only at terminal apparatus. However, for small installation, where the length are small and the joints are required at the number of places, vir cables Will the cheaper and durable than paper insulated cables.

 

 

4. Varnished cambric. I 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 on to the conductor in the form of tape and it's surfaces are coated with petroleum jelly compound to allow for the sliding of one turn over another as the cable is bent,. As the vernished cambric is hygroscopic, therefore, such cables are always provided with metallic shealt. Its dielectric is about 4kV/mm and permittivity is 2.5 to 3.8.

5. Polyvinyl chloride (PVC). 3503 This insulating 'material is a synthetic compound. It is obtained from polymerisation of acetylene and is in the form of white powder. For obtaining this material as a cable insulation, it is compounded with certain material known as plastifcizers which are liquid with high boiling point. The plastifcizers form 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.