How does quality measuring work? An overview of wire and electrical cable tests
Wires and electrical cables are crucial for the transmission of mechanical forces, electrical energy, and electronic data. For this reason, they are used in all areas of an industrialized society. Common market outlets are, for example, the energy, automotive, and transportation sectors, as well as construction engineering.
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As defective wires and electrical cables can cause costly or even life-threatening damages, they must pass various quality tests before, during, and after production. These checks ensure that quality standards and customer specifications are met. Additionally, quality measuring helps detect weaknesses in production processes and avoid scraps.
Principles of wire and cable quality measuring
Quality measurement aims to assess the behavior of materials, semi-finished products, or ready-to-use products under various conditions. The results are used to determine if a measured object is suitable for some applications and meets required safety and quality standards. The International Organization for Standardization (ISO) and the American Society for Testing and Materials (ASTM) are two examples of standardization bodies that set the regulations of quality testing methods for several materials, including wires and cables. These methods can be categorized as either destructive or non-destructive testing (NDT), with the former involving the destruction of the test object during the process and the latter preserving its conditions without causing any damage.
Given the broad spectrum of applications of wires and electrical cables, they must meet various physical requirements. Therefore, in addition to standard methods, specific quality tests may also be necessary. It’s crucial to note that some quality measurements must be carried out even before wire drawing and further processing. For example, it’s vital to check the chemical composition of the wire alloy and the presence of inclusions, shrinkage cavities, segregations, gas bubbles, and cracks in the microstructure of the wire rod.
A standard quality testing method: the tensile test
A prime example of a standard quality testing method is the tensile test. Its functional principle can be seen in many other mechanical tests too. Its primary purpose is to find out the maximum load a test object can bear without being plastically deformed, and its breaking point. The tests are normally carried out at room temperature. They can, however, also be performed under special conditions, such as higher or lower temperatures or in a corrosive environment. The process is performed using a specific tensile testing machine or a universal testing machine equipped with two crossheads (one fixed and the other one movable, operated by one or two hydraulically or electrically driven spindles).
Let’s see how it works: a mechanical load acts on a test object, in many cases a standardized machined sample, in a specific direction and at a specific speed. The sample piece is clamped between the crossheads. During the operation, the movable crosshead moves at a constant velocity in one direction, applying a tensile force. As a result, the test object is elongated until it breaks or tears. The elongation is measured by an extensometer. Nowadays, it’s common to use digital optical extensometers, enabling non-contact measuring. Tensile tests are also used in a non-destructive way: in these cases, the piece is only subject to a tensile force until a certain load is reached. In each case, the force and elongation are continuously measured, recorded, and shown on the PC monitor of the testing machine as a force-displacement curve or as a stress-strain curve when referring to the load per area of the test object. Through the recorded curve, it is possible to draw conclusions about the mechanical characteristic values of the item, such as tensile and yield strength, as well as its physical behavior (for example, ductility or brittleness). Usually, the measured values are also recorded in a document.
Tensile and other tests on wires
As wires are usually rather thin, they cannot be machined for testing. Therefore, it is necessary to use cut-off lengths, which require a particular clamping or gripping device. Simply clamping wire sections between jaws can cause measuring errors. In addition to the tensile test, there are further methods to assess the mechanical properties of wires, such as the bending, torsional, shear, and creep behavior, fatigue strength, and hardness. With specially developed test methods, it is possible to also measure other wire characteristics and geometrical properties, such as roundness (ovality), surface quality, resistance to chemicals, and corrosion resistance. Wire conductors for electrical cables must also have certain electrophysical properties that depend on the material and its purity.
SQM wire surface defect detector by CERSA-MCI
Quality testing on electrical cables
Electrical cables encompass a wide variety of types, categorized as low, medium, high, or extra high voltage power cables, data, signal, automotive, and special cables. Additionally, there are fiber optical cables and hybrid cables that incorporate both optical fibers and copper wires within the same sheath. As a consequence of this diversity, there is a need for a wide range of testing methods. Some of these are non-destructive and are carried out during manufacturing, including overall dimension and voltage testing.
Others require destructive testing on samples, which means the cable samples are intentionally burned, exposed to extremely high temperatures, or damaged in a number of ways to understand the limits of their performance beyond normal operating conditions. These evaluations encompass factors such as tensile strength and elongation, impact resistance at cold temperature, flame resistance, and assessment for acid gas emissions. Samples of electrical cables are also subject to aging simulation to predict how cables are likely to perform over time. These can involve tests for UV stability, abrasion resistance, and continuous flexing.
Testing the geometrical properties
Cables must be precisely shaped in order to be joined with end components or connecting elements. Also, the installation space is usually limited. Therefore, it is paramount to continuously measure the diameter and the roundness (or ovality) of the cable coating and detect geometrical defects or cracks in it during the extrusion process. This is done by laser-based measuring instruments that are installed on the production line and work with one or more measuring axes. These instruments give real-time information on the geometry of the manufactured cable and enable immediate adjustments of the production parameters if necessary. In this way, it’s possible to minimize material waste. In addition to the ovality, X-ray measuring instruments can also measure the cable concentricity and the wall thickness of its insulation or sheath during the extrusion process.
Testing the insulation and electrical properties
The cable insulation is a protective layer of insulating materials that encases electrical conductors and cables. It ensures the safe and efficient transmission of energy or information within the cable. Various plastics are used as insulating and sheathing materials. Their composition depends on the specific requirements of the cable application. Important criteria can be tensile strength, elongation at break, abrasion behavior, dielectric constant, resistivity, breakdown voltage, as well as resistance against temperature, flame, radiation (for example UV light), chemicals, oil, and corrosion. A crucial quality aspect of cable insulation is having a pure and flawless insulation material. The contamination, for example by metallic particles or other impurities, can cause damage to production systems, such as to the crosshead of the extruder, or lead to cable failure over time. Therefore, the insulation material has to be inspected and analyzed for different types of defects before entering into the extruder. While optical inspection systems can only check the surface of the pellets or flakes, X-ray scanners can provide a deeper examination and discover inhomogeneity, contamination, defects, and differences within the material. Further essential criteria are good insulating properties and dielectric strength. During the extrusion or rewinding process, the cable insulation is inspected by spark testers to detect possible insulation faults. The cable runs through the tester where the insulation is exposed to a selected voltage. If there are any faults or imperfections in the insulation, they will disrupt the electrical field and create a current flow, signaling that the cable is defective. Spark testers are usually fitted with counters indicating the number of faults. There are various test and measurement methods, including high voltage, low voltage, and DC testing to verify the complete functionality of cables or detect any faults. While traditional tests provide “go / no go” results, cable diagnosis methods enable a deeper understanding of the cable's current condition. With some tests, it is even possible to locate the precise position of the insulation defect before failure.
Testing the fire behavior
A fire in a cable installation can cause serious damage. Burning cables will rapidly cease to function, and cable trays or ladders full of cables can burn intensely. Depending on the cable type, its polymers, and its additives, it can produce smoke and toxic gases. Therefore, standardized testing procedures have been developed for assessing fire behavior. Testing criteria are insulation integrity, flammability, heat release, flame propagation, flaming droplets released by the cable during combustion, smoke generation, smoke amount and density, the toxic gases content (such as halogen hydracids), as well as the smoke acidity and corrosiveness, among others. In the European Construction Products Regulation (CPR) for fire safety in buildings, cables are classified into seven classes according to their flame spread and heat release.
Literature
Günter Beyer: The Global Cable Industry: Materials, Markets, Products. Weinheim, April 2021. https://onlinelibrary.wiley.com/doi/book/10.1002/9783527822263
Wolfgang Grellmann, Sabine Seidler: Polymer Testing. Munich, 2022. https://www.hanser-fachbuch.de/fachbuch/artikel/9781569908068 978-1-56990-806-8
Roger N. Wright: Wire Technology. Process Engineering and Metallurgy. Oxford, Second Edition 2016. https://www.researchgate.net/publication/303434475_Wire_Technology_Process_Engineering_and_Metallurgy_Second_Edition https://books.google.de/books/about/Wire_Technology.html?id=t7Z0BgAAQBAJ&redir_esc=y
Horst Czichos, Tetsuya Saito, Leslie Smith: Springer Handbook of Metrology and Testing. Heidelberg, 2011. https://link.springer.com/book/10.1007/978-3-642-16641-9
The information has been compiled by Dipl.-Ing. Konrad Dengler, technical journalist and translator specialized in industrial topics.