1. What are different types of insulating material?
2. List the important properties of cable insulation.
3. What are thermoplastic and thermoset materials?
4. What is the function of Conductor / Insulation Screen and material used?
   

Insulation is provided to electrically isolate the individual cores from one another. The applied insulation must perform adequately in the specified temperature range, and its dielectric strength should be sufficient to sustain the electrical stresses.

Cable insulation should have:

• High Dielectric Strength
• Low dielectric constant
• Good Mechanical properties
• Resistance to Ageing
• High temperature withstandability

Insulating materials are classified as thermoplastic or thermoset. Thermoplastic materials lose their form upon heating. Thermoset materials maintain their form in spite of heating. Various types of insulating materials used are : PVC, XLPE, Rubber, Impregnated Paper, etc.

Polyvinyl Chloride (PVC)

Polyvinyl chloride (PVC or vinyl) is a thermoplastic. PVC compound is the standard insulation for cables rated at 11000 volts or less & sheathing of entire range of cables. PVC compound is a mixture of PVC resin, plasticizer, fillers, stabilizers, lubricant, pigment. The quantity and type of each ingredient determines the properties. A broad range of electrical, physical and chemical properties is possible.

PVC has good electrical properties. It is tough and resistant to flame, moisture, and abrasion. Resistance to ozone, acids, alkalis, alcohol, and most solvents is also adequate. PVC can be made resistant to oils and gasoline.

PVC has the disadvantage of having a high dielectric constant and dissipation factor. Also plasticizer loss can cause hardening and cracking.

Crosslinked Polyethylene

Crosslinked polyethylene (XLPE) is a thermoset. It is produced by compounding PE (polyethylene) with a crosslinking agent, like organic peroxide. The molecules of polyethylene are "crosslinked", forming an interconnected network. The terms "cured" and "vulcanised" are also used for "crosslinked".

Elastomer

Elastomeric material are used for insulation and for sheaths. They are applied mainly where the product has to be particular flexible. A wide range of elastomers is nowadays available to the cable industry. This makes possible the manufacture of compounds with specific properties, such as abrasion and oil resistance, weather and heat resistance and flame resistance, combined with good overall electrical and mechanical charateristics.

The classical elastomeric material, natural rubber, has declined in significance in recent years. In its place, the synthetic elastomers produced by the co-polymerisation of ethylene and propylene, are constantly finding new areas of application in cable engineering. This co-polymers, are generally known as EPR.

Rubber was the first insulant to be used in Electric cable manufacture but gave way to other insulants like paper, PVC, XLPE etc. Rubber is still considered the preferred insulation for flexible cables and cables where very small bending diameter is desired.

Paper

Paper tapes of specific thickness and suitable widths are lapped around the conductor. The thickness of built up insulation depends on the rated voltage of the cable. Paper insulated cores are dried and impregnated using mass impregnating non-draining compound.

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Function of Conductor / Insulation Screen

The conductor shield is a layer of semi-conducting material. Semi-conducting materials do not conduct electricity well enough to be a conductor but will not hold back voltage.

It "smoothes" out the surface irregularities of the conductor. The conductor shield makes the voltage on the inside of the insulation the same. Industry specifications define the performance of conductor shield.

Good insulation shields are extruded in tandem with the insulation.

The insulation shield consists of two components. These components are the extruded (auxiliary) shield and the metallic (primary) shield.

The extruded shield consist of a semi-conducting layer similar to the conductor shield. It makes the voltage on the outside of the insulation the same.

The primary shield can consist of metal tape, drain wires or concentric neutral (CN) wires. Grounding the primary shield makes the voltage on the outside of the insulation ground. The Copper of the shield is usually bare, but may be coated with lead or tin. Some primary shields consists of drain wires and tape. Aluminium and lead can also be used as the shield.

Concentric neutral wires serve a two-fold purpose. They function as the metallic component of the insulation shield and as a conductor for the neutral return current. Their cross sectional area must be sized in order to function as the neutral conductor.

Cables with paper Insulation
The inner conducting layer consists of several layers of semi-conducting paper. The outer conducting layer consists of metallised paper.

Cables with PVC Insulation
The inner conducting layer consists of PVC compound having high carbon black content. For the outer conducting layer, a cover of conducting tapes is preferred.

Cables with PE or XLPE Insulation
Because of higher sensistivity to partial discharge the reliable well adhesive gap and cavity free bonding to conducting layers is of greatest significance. The inner conducting layer consists of polymer compound made conductive by adding carbon black. And the outer conducting layer is formed by semi-conducting compound along with semi-conducting tapes.

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