Posted 17 July 2019
As a regular feature in upcoming editions of Wired, we are going to take a look at the manufacturing processes involved in cable production. There is a great degree of variation in the cable production process. This makes cables highly customisable and suitable for many different applications depending on their unique requirements.
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.
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.
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 that 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 in the catalogue.
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. We will look at cores and insulation materials in more detail in the next edition of 'Wired'