Driving Safety: Innovations in testing techniques for cables and conductors

Cables and conductors play a crucial role in the transmission of el­ec­trical power and data signals, whether in residential, commercial, or industrial facilities. Therefore, the reliability and safety of cables are of utmost importance, making testing an essential step in ensuring their optimal performance, durability and adherence to sa­fe­ty standards.

Testing of cables and conductors serves mul­tiple purposes that are essential for all manufacturers, utilities and end-us­ers. It helps manufacturers ensure that ca­b­les and conductors meet specific qu­ality standards and performance criteria, thereby minimising the risk of faulty or substandard components. Testing also ensures that these cables can withstand sp­ecified voltages, currents and environmental conditions, reducing the risk of system failures, downtime, signal degradation, overheating and fires. In addition to ensuring security, testing assesses the performance characteristics of cables and conductors. These include electrical re­sistance, insulation integrity, capacita­nce and attenuation. To this end, various tests such as tan delta tests, type tes­ts and surface condition tests are conducted to as­sist utilities in selecting the most suitable cables for their applicatio­ns, ultimately optimising system performance.

Testing parameters

Several testing methods are employed to evaluate the quality and performance of cables and conductors. These testing methods assess various parameters in such as resistance, voltage and impedance. The insulation resista­nce test measures the resistance bet­ween a conductor and its insulation. It identifies any weaknesses or defects that could lead to electrical leakage or breakdo­w­ns. Meanwhile, the dielectric strength te­st determines the ability of cables to withstand high voltage without failing. It ensures that cables can handle the expected voltage levels and do not pose a safety risk. The continuity test verifies the electrical continuity of a cable or conductor by checking for any breaks or open circuits. It is crucial for ensuring proper signal or power transmission. Capacitance testing asse­sses the ability of a cable or conductor to store electrical charge, determining its suitability for specific applications and preventing signal distortion or loss. Further, impe­da­nce and attenuation measurement tests evaluate the resistance, inductance and capacitance of cables and co­nductors, providing information about signal integrity and transmission efficiency. They are particularly important for high frequency applications, such as telecommunications and data transmission.

Cable testing

Underground cables: A popular method for examining underground cables is the tan delta test. It is a diagnostic method of testing cables to determine the quality of the cable insulation. This helps in creating a systemised method to analyse the ageing of cables in the long run. It utilises phase shifts caused by displacement current, which is induced due to impurities in the insulation or the stress on cables. The cable insulation is subjected to a positive feedback process, increasing the risk of a temperature breakdown. Insula­tion properties change with time, and it is necessary that key parameters are tested accurately to ascertain the de­te­rioration level of the insulation. Cables get damaged due to adverse installation conditions during cable laying. Apa­rt fr­om this, another popular meth­od used to diagnose the health of cables, especially XLPE-insulated power cables, is very low frequency testing, which in­vol­ves the application of AC voltage at a low frequency of 0.01-1.00 Hz, depen­ding on the cable’s inbuilt voltage. The strength of the insulating material used in cables is tested using the dielectric withstand test, which verifies that the insulation of a product or component is sufficient to protect it from electrical shocks.

Overhead cables: Overhead power lines are periodically inspected using both on-ground and helicopter-aided visual inspection. Factors including sun glare, cloud cover, close proximity to power lines, and rapidly changing visual circumstances make airborne inspection of power lines a particularly hazardous task. Overhead lines are more accessible than underground cables and can be tested with the help of online tests. For example, punctures in insulation on live wires can be monitored using punctured insulator detectors. These are moved along live wires, and send readings to graphic displays, which detect any leaks in electricity by monitoring variations in fields. Other monitoring methods inclu­de distributed temperature sensor (DTS) and sheath current monitoring. Temp­e­ra­ture detection is very important be­cau­se the condition of insulation and the carrying capacity of a cable can be obtained by monitoring the temperature. A built-in DTS that uses optical fib­re temperature sensor systems can measure the cable temperature on a real-time basis. Besides, it is anti-corrosive, is not susceptible to electromagnetic interference and has high sensitivity. Ca­pacitive coupling sensors and other sensors are also used to detect partial discharge signals. However, a problem with online detection of partial discharge is that cross-bonding of transmission cab­les makes it difficult to identify from whi­ch phases a partial discharge signal has emanated.

Conductor tests

Type tests are most commonly used as routine testing techniques to assess the quality and performance of cables and conductors. These tests encompass a range of methodologies, including partial discharge tests, tan delta measurement, conductor resistance tests, tensile tests, hot deformation tests, hot set tests, shrin­kage tests and thermal stability tests. During a type test, samples are ta­ken at regular intervals along the length of the conductor to undergo various ex­a­mina­tions. A visual examination invol­ves a phy­sical inspection of the conductor to ensure high quality workmanship and an overall satisfactory surface finish. When conducting a visual examination, it is im­perative that the conductor drums be re­wound in the presence of the inspecting officer. The inspector meticulously examines the conductor for any signs of scrat­ches, joints or other potential defe­cts. Only after a thorough visual examination and confirmation of conformity to specified requirements, such as those outlined in IS 398 (Part II)-1996, can the conductor be considered compliant.

The surface condition test is an additional technique used to test conductors. In this test, a sample of the finished conductor, designed for use in an 11/33 kV system, is selected. The sample sh­ould have a minimum length of 5 metres and be equipped with compression-type dead-end clamps that securely hold both ends, allowing the conductor to maintain its natural straight line shape. During the surface condition test, the sample conductor is subjected to a tension equivalent to 50 per cent of its ultimate tensile strength. The purpose of this test is to assess the integrity of the conductor’s surface. It is important that the surface remains cylindrical, without any significant deviations or distortions. The strands of the conductor should not shift or move relative to each other, en­suring they remain in their intended po­sitions and do not disturb the conductor’s longitudinal smoothness.

The measured diameter of the conductor at any given point should not be less than the sum of the minimum specified diameters of the individual aluminium and steel strands. This criterion ensures that the conductor maintains the required ­di­mensions and structural integrity. In so­me cases, utilities may also conduct str­e­ss and strain tests on conductors. Ho­w­ever, these tests are typically performed to gather creep data directly from the ma­nufacturer. For this purpose, a conductor sample with a minimum length of 10 me­tres is used, and it is appropriately com­pressed using dead-end clamps.

The way forward

In the next few years, technological ad­vancements and the emergence of ro­botics and autonomous systems are expected to provide increased support for  the monitoring, inspection and mai­ntenance of offshore wind farms, transmission li­nes, underground cables, etc. The­se de­velopments aim to address the existing health and safety risks faced by hu­man operators while inspecting or monitoring such infrastructure. Both industry and government are driving the development and availability of AI and robotics technologies that can be dep­loy­ed to ga­ther data on the condition of assets and assist technicians in taking necessary actions. Further, machine le­ar­ning techniques provide advanced computational tools to efficiently and accurately process and analyse all the data (corrosion, erosion, fatigue, cracks, etc.) provided by various technologies.

Net, net, these advanced technologies are expected to meet various testing re­quirements and facilitate thorough evaluations of cables and conductors. This can contribute to enhanced safety, reliable system performance, cost savings, and compliance with regulations.