Cabling - What’s in a Cable?
The wide range of cables on the market today reflects the multiplicity of cabling 
applications available to the electrical engineer. As such, there is a great degree of variation in the production process making cables highly customisable and suitable for many different applications depending on their unique requirements. Owen Dale of FS Cables explains
Generic cable elements such as conductors and insulation can differ greatly according to the application, dramatically altering the properties and performance of the cable itself. This means using different manufacturing processes according to the materials and components specified. Production techniques vary too, for example in the way the conductors are insulated and how the core bundle is created in the laying up process. This article takes a look at the manufacturing processes involved in producing a cable, from the initial drawing of the conductors, insulation materials used and the ‘laying-up' of cores.
Conductors
Conductors are single or multiple strands of highly conductive metal, usually copper. Other materials commonly used include aluminium and nickel. Their purpose is to carry an electric current for data or power between two points.
Copper
Using the example of copper, the raw ore once extracted from the earth is smelted into ingots which then undergo electrolysis to remove impurities. This involves attaching a negative charge to the ingot and submersing it in a tank of copper solution.
This has the twin effect of dissolving the copper and at the same time attracting it to reform on the positive element. The impurities drop to the bottom as they won't take the charge. While not all impurities are removed, this procedure can produce 99.99% pure copper. The purity of the copper has a huge effect on reducing resistance on the electrical charge passing along the conductor, greatly increasing its ability to carry a current. This is just as important for a signal as it is for a power or energy cable.
The pure copper is rolled out into rods and then drawn through a series of dies made from a very hard material such as ceramic or even diamond to make very thin strands. It is common to draw copper strands as small as 0.05mm diameter (a human hair is normally between 0.07mm to 0.1mm in diameter). These strands can either be used singly or in bunches to make a larger, more flexible conductor.
Depending on the application of the cable, the strands can be coated with an inert metal at this stage to reduce corrosion or enhance heat resistance. Tinning, the process of coating the conductors with a layer of tin either electronically or in molten form is very common for this process as it is low cost and relatively resistant to corrosion. Nickel plating is used where the cables will be working at temperatures between 200° - 400°C.
Stranding
Conductors are normally measured either by their diameter or cross-sectional area in mm². The method for measuring cross sectional area (CSA) is standard across Europe and is in accordance with BS6360 and IEC60028.
It is important to note the standards also specify the resistance of a conductor for a given stranding. This means that a conductor can be made up of fewer strands or smaller diameter strands but still conforms to the standard.
Another method is the American Wire Gauge size (AWG). This denotes the gauge size as a number i.e 24AWG followed by a number in brackets i.e (7) which shows the number of strands. It's worth mentioning, the higher the number the smaller the conductor, so a 24AWG conductor is smaller than a 20AWG. Full stranding charts and AWG / Metric conversion tables can be found on in our cabling guide - The Little Red Book.
Using the metric cross sectional area method, Class 5 stranding is the most common flexible grade, as it combines good flexibility with a reasonable cost. Generally, the more strands you have the more flexible the cable, but it also becomes more expensive. Classes 1 (solid) and 2 are widely used in fixed installations where the cable won't be moved or bent after it has been fitted.
Cores are normally made up from multiple strands by twisting or bunching the strands together. In larger or very finely stranded conductors the strands may be first twisted into groups and then twisted together to form a conductor similar in appearance to a rope. Conductors become ‘cores' when coated with an insulating material.
Insulation Materials
Insulation is a non-conductive material used to coat the conductor to keep the electricity flowing to its intended destination. In the case of a transformer or motor armature this may be a simple enamel varnish but in most cables and wires thermoplastic or elastomeric compounds are normally used.
With the exception of overhead power lines, conductors are normally insulated within a cable to ensure the electricity, whether it is data, signal or power, only goes where it is intended and doesn't jump from conductor to conductor. Insulation types vary enormously in range and application. The popular types are outlined below.
Thermoplastic compounds
These include PVC, Polyethylene and many Low Smoke Halogen Free (LSHF) materials. Thermoplastic compounds are defined as materials that can be melted in an extruder and, when cooled, reform with the original properties unaltered. The compounds are supplied in bulk in granules about the size of a match head. The advantage of these compounds is that they are relatively easy to work with and the equipment needed in manufacturing is generally fairly simple. It is also quite easy to change colours within a material type. However the cost of the compound varies significantly with high performance LSHF compounds costing up to five times that of basic PVC.
Elastomeric or Curing compounds
This group includes rubbers and materials that are altered after extrusion by a catalyst, for example by cross-linking the molecules to improve the performance of the material. XLPE is commonly used for signal and power cables.
In the case of silicone rubber, the silicone is squeezed out of the extruder cold, like a putty and then enters the curing process - normally a steam tunnel or through salts at very high temperatures.
Taping
Taping is not as popular as it used to be, although some products are still insulated by winding a tape around the conductor. Paper taping is used for some power cables and PTFE is used for heat resistant and high performance wires. One advantage of taping is the conductor is central or concentric within the insulation. In the case of PTFE the insulated wire is then subjected to extreme heat for a very short time (sintering) to fuse the edges of the tape together and stop it unwinding. Mica tape is often used to ensure circuit integrity during a fire. Mica is a naturally occurring substance which is bonded to an inert substrate that is wrapped around the conductor prior to the insulation being applied.
Lapping or braiding
These are now mainly used for heat resistant cables working over 250°C. Glass or ceramic fibres are wound around the conductor, normally in two layers in opposite directions with a glass fibre braid overall to hold it all together. These cables are stable at high temperatures (up to 750°C) but are not suitable at normal ambient temperatures where there may be moisture. Most of the products are silicone varnished to ease handling during installation but the varnish burns off at a high temperature.
As you can see there are many different types of insulation each with a specific job. PVC and XLPE are the most common and offer great all round properties in terms of flexibility and cost. Other materials are crucial for high performance applications such as fire alarm cables where it is important a cable can carry on functioning even in the event of fire.
When choosing insulation it is important to establish what the cable or wire is expected to do. Temperature, voltage, electrical characteristics, flexibility, performance in the event of fire and other physical factors all need to be taken in to account when specifying. With literally hundreds of different grades of PVC, eighty-plus grades of silicon and practically every other compound being able to be split into subgroups, there are insulations to meet practically every need.
The Laying Up Process
The laying up process is used to create the core bundle once the conductors have been insulated.
In a simple cable two cores can be laid next to each other and have a jacket extruded over them. However, in many two core cables and all multicore cables it is normal practice to twist the cores together. This has a number of benefits. Firstly, it improves flexibility with each core taking the same amount of stress as the others. Additional benefits include easier production (by keeping the cores together as they pass through the jacketing extruder) and better cable uniformity.
Multicore and multipair cables
Multicore cables have a number of cores twisted together to form the core bundle. The ideal core bundle is circular and to achieve this, dummy or blind cores may be added. These cores contain no metallic elements and just act as fillers. In cables with 12 cores or more, the cores will be arranged in layers with each layer twisted in opposite directions. This ensures the finished cable has the best chance of remaining circular even if driven over or trodden on. Cores can also be arranged into pairs, triples or quads. This is common for data, signal and instrumentation applications. By twisting the cores to form pairs the problem of crosstalk is reduced. Crosstalk occurs where the signal on one circuit leaks over onto another circuit. Analogue signals in particular can be affected by this.
By creating a twisted pair, the cores are in limited contact with adjacent cores and are exposed to equal levels of electromagnetic interference (EMI). The process of twisting cores into pairs is relatively fast with speeds of up to a few hundred metres a minute. The pairs are then laid up in the same way as with multicore cables. On high performance data cables the twist rate may vary, for example in Cat 5 cable the twist rate of pairs varies between 18mm and 22mm. On telephone cables this may extend to one complete twist every 80mm or so which is hardly noticeable.
Termination can be easier and faster on twisted paired cables. Core identification is normally achieved by either colouring each core differently (colour coding) or printing a number on each core. On twisted pair cables colour coding is very popular. Number coding on tapes wrapped around each pair, or a combination of colour and number coding are also sometimes used.
Talcum
Having laid the cores or pairs into a bundle the next stage is to add a jacket. There is one vital element prior to extrusion and this is to create a barrier between the core bundle and the jacket. A barrier is particularly important because if the insulation and jacket are of similar compounds the jacket could stick to the insulation.
The most common barrier is talcum or French chalk which is held in a trough through which the core bundle is pulled before going into the extruder. Talcum has the advantage of being both cheap and effective.
The disadvantage of talcum is that it is difficult to detect if it has run out or fails to cover the core bundle properly. This can result in a cable that is very difficult to strip and has reduced flexibility. Also, in a small number of applications the talcum could contaminate the surrounding equipment, for example with medical equipment or in clean rooms.
Other release agents can be used, including silicone oils, but these are more expensive than talcum and can be difficult to apply. Other methods include tapes of polyester or mylar and the non-woven polyester or fleece types. The latter are ideal for constantly flexing cables as they allow movement. They are sometimes used between layers in core bundles or either side of braid screens. Popular in robotics and audio cables, they are increasingly used in more general cables where flexibility and ‘stripability' are important.
This article was originally printed in the FS Cables quarterly newsletter Wired, available either in print or electronically by calling 01727 840 841 or emailing wired@fscables.com. The next instalment of ‘What's in a Cable' will be published in the winter edition of Wired, printed in December.
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