The Process
A conductor becomes a core when it has been coated with an Insulating Material. The purpose of the Insulating material apart from colour identifying the individual cores is to prevent the flow of current from core to core and core to screen. A non conductive Insulation material is employed to totally isolate each conductor.
The radial thickness of the Insulating material is conditioned by the intended operating voltage of the Insulated core and the higher the voltage the thicker the insulation needed. All cores have undergone a high voltage spark test during manufacture to check the integrity of the Insulating layer.
As you would expect and as with all components in a cables construction there are numerous criteria that will influence the choice of Insulating material, these include operating temperature range, flexibility, Mutual capacitance levels, Environmental operating conditions Fire, Smoke, Zero halogens, and a whole range of Fluids. And finally, the intended performance of the cable i.e. Power or Coaxial cables.
Whilst covering the Materials and Extrusion process needed to insulate the Conductors we should include the Bedding and final sheathing in this section.
The bedding is just that and serves as a barrier between the cores at the heart of the cable from usually a Braiding or Armouring operation covered later. As such the materials used, whilst having to comply with the main characteristics of the Insulation and /or the sheathing materials tend to be of a lower quality grade of Compound.
The outer sheath or Jacket provides protection during and after installation from mechanical abrasion and also climatic influences. Both thermoplastic and thermosetting compounds can be used. The choice would depend on the working environment and the overall performance requirements of the finished cable.
Finally the extrusion process whilst being very similar in principal across the range of materials available does require some variations to the equipment needed and therefore specialisation of the manufacturing process, particularly for Silicone, PTFE and Irradiated Compounds.
Materials
Most frequently used Insulation materials are thermoplastic and thermosetting compounds applied by extrusion. The thermoplastic materials are relatively cost effective and suitable for most hazard free installations, whereas thermosetting materials cover a greater temperature range and can be strengthened to match specific installation characteristics. A few of the main insulating types are given below.
PVC- Polyvinyl chloride is the most widely used low voltage insulation material, inherently flame retardant, easy to use and handle, with good chemical and oil resistance. Though not halogen free, the acidic emission levels and smoke generation can be limited and the general temperature range of –10 to 70Deg c can be reduced or exceeded by using special grades.
PE – Polyethylene is an excellent water resistant material, having electrical properties superior to PVC and temperature range of –55 to 70 Deg C. Used mainly in telecommunications, data transfer and instrumentation cables. Heat deformation issues limit its use for power applications, although halogen free the compound is not flame retardant.
XLPE – Cross-linked PE offers good water resistance and excellent electrical properties with improved heat deformation characteristics where operating temperatures of 90 Deg C increases the current carrying capacity. XLPE is now the most popular Insulation material for power cable applications.
XLOHLS – Cross-linked zero halogen low smoke with superior flame retardancy when compared to XLPE.
The list goes on with Silicone, PTFE, Irradiated Cross-linked materials all offering a combination of important characteristics suited to more focused operating conditions such as higher temperature, flexibility, and weight and size reductions required by highly specified applications.
The range of materials available is endless, as in this area of the specification, the chemical combinations used to satisfy the needs of the cables operation can be tailored specifically. However as always there is a cost/performance consideration to be made when Public safety is paramount e.g. LUL Low Smoke Zero Halogen cables for the Underground. Offshore oil rigs where cables need to be resistant to unique Mud/oil combinations, biologically safe materials on Hospital equipment, Low temperature operation on Military Equipment, suitability for use in retractable coiled leads. The list of available compounds is large and cost will be dependent upon the Quantity of the cable required, to justify the use of the more specialised materials. In many cases it is often possible to find a good compromise without dramatically interfering with the integrity of the cable.
