Low Voltage Electric Cables Paul Chaplin Proud to be an Australian Family Business Owner Switches Plus Components 1
Electric Cables are not just Electric Cables 2
Electric Cable Construction 3
Electrical Conductors A conductor is an object or type of material that allows the flow of an electrical current in one or more directions. Materials include Copper, Aluminium, Gold, Silver. • Electrolytic-Tough-Pitch (ETP) is the most common copper used for electrical applications. ETP is required to be 99.9% pure. To go to 99.99% pure copper is more expensive and provides at best a 1% increase in conductivity. • When metal is cold worked or formed, it becomes work hardened, or strain hardened. Copper conductors go through a considerable amount of work hardening as the copper rod is drawn down through ever decreasing die sizes until the required conductor dimension is achieved. Copper in this state is known as hard drawn copper. • Hard drawn copper is difficult to work with. The stranding and bunching of the finer wires in this state would be very difficult. By heat treating the copper at the correct temperatures the ductility can be restored to make the copper soft and flexible again. This heat treating process is known as annealing and the resulting metal is known as soft annealed copper. The degree of annealing is controlled by temperature and time, copper wire is used with different degrees of annealing depending on the application. • Hard drawn copper has a significantly higher tensile strength than soft annealed copper and is used as overhead wire whereas the soft annealed copper is flexible and has somewhat improved conductivity over hard drawn copper conductor. 4
Operating Temperatures of Electric Cable The operating temperature of an electrical cable normally refers to the minimum and maximum temperature that the cable can safely operate at for a sustained period of time. This operating temperature is determined by the insulation and/or sheathing material around the cable. • Each material type will have an upper and lower range of temperatures within which it continues to be suitable for use. This varies widely depending on the material type as well as whether or not the cable is required to be flexible at these temperatures. Generally, materials soften at higher temperatures and become rigid at lower temperatures making the material less suitable for applications involving flexing at either low or high temperatures. • A typical PVC insulation material has a temperature range of -15°C to 70°C for applications. Silicone rubber typically has a temperature range of -60°C to 180°C for fixed applications 5
Life Expectancy of Electric Cables There are many different environmental and operational conditions which are likely to influence the longevity of electrical cables in service. • The insulation and sheathing materials of cables will degrade over time when exposed to heat, UV light, ozone, various chemicals, excessive flexing, or mechanical action, not to mention in certain situations cables may be exposed to attack by termites, birds and rodents. • When a current passes through the cable conductor it generates heat - the higher the current the more heat will be generated. This will have a significant impact if the conductor is undersized or continuously at or near the cable’s maximum permissible resistance or rated load , degrading the insulation and sheathing materials over time until they become dangerous and require replacement. • Although it is primarily the condition of the insulation and sheathing materials rather than the actual conductors that determine the longevity of the cables, water ingress and poor fixings can also cause corrosion and damage. 6
Electrical Insulating materials - Properties of Thermal Endurance. better known as “u seful lifespan ” IEC 60216 -1 page 11 Introduction • The listing of the thermal capabilities of electrical insulating materials, based on service experience, was found to be impractical, owing to the rapid development of polymer and insulation technologies and the long time necessary to acquire appropriate service experience. Accelerated ageing and test procedures were therefore required to obtain the necessary information. The IEC 60216 series has been developed to formalize these procedures and the interpretation of their results. • Physical-chemical models postulated for the ageing processes led to the almost universal assumption of the Arrhenius equations to describe the rate of ageing. Out of this arose the concept of the temperature index (TI) as a single-point characteristic based upon accelerated ageing data. This is the numerical value of the temperature in °C at which the time taken for deterioration of a selected property to reach an accepted end-point is that specified (usually 20,000 h). 7
Electrical Insulating materials - Properties of Thermal Endurance. Common cable insulating materials operating temperature defined by the IEC60216 test method Cable insulation • PVC = 75 ° C degradation caused by • XLPE = 90 ° C thermal aging • EPR, CPE, CSP, Rubbers = 90 ° C • Silicon Rubber = 180 ° C 8
Electrical Insulating materials - Properties of Thermal Endurance. Understanding why PVC is rated at 70°C and why XLPE is rated at 90°C we now better understand why AS/NZS3008-1-1:2017 calculate current ratings for PVC based on a 70°C conductor temperature and for XLPE/EPR based on a 90°C conductor temperature: Perhaps what is not highlighted by this standard is that the elongation reduction to 50% absolute is calculated on 20,000 hours exposure time at this temperature (which is only 2.3 years). In fact this standard does not really expect engineers to use the cables at (PVC) 70°C or (XLPE) 90°C continuously or the cable lifespan will be exceptionally short. They assume usage will be on a basis of discontinuous loading where it is not anticipated the cables will be fully loaded 100% of the time. This pragmatic approach is the only way polymeric cable insulations can be economically viable. A common ‘rule of thumb’ for cable polymer insulation aging is that a reduction of 10 °C in the average cable operating temperature across its life span will double the insulation life time to the 50%EB (Elongation at Break) point: i.e: AS/NZS 3008.1/1:2017 Cl 3.5.6 states : The ratings given are for continuous loading. The question that needs to be asked is “ How long is continuous ?” 9
Electrical Insulating materials - Properties of Thermal Endurance. Thermoplastic V 75-PVC / TPE-7 operated continuously at: 75°C will degrade to 50%EB in 20,000 hours (2.3 yrs) 65°C will degrade to 50%EB in 40,000 hours (4.6 yrs) 55°C will degrade to 50%EB in 80,000 hours (9.2 yrs) 45°C will degrade to 50%EB in 160,000 hours (18.4 yrs) XLPE 90, R-EP 90, CPE/CSP-90 operated continuously at: 90°C will degrade to 50%EB in 20,000 hours (2.3 yrs) 80°C will degrade to 50%EB in 40,000 hours (4.6 yrs) 70°C will degrade to 50%EB in 80,000 hours (9.2 yrs) 60°C will degrade to 50%EB in 160,000 hours (18.4 yrs) NOTE: a 50% EB represents a reduction in the materials original property of ≈ 80% 10
Bending Radius of Electric Cables The cable bending radius is a measurement of the smallest radius a cable can be bent around without damaging the cable. • Factors which influence the minimum bending radius include the cable size, the cable construction, the conductor type and the sheathing and insulation types used. • The bending radius is normally expressed as a factor of the overall dimension of the cable for example, 6D or 6x the outer diameter of the cable. • The cable manufacturer will determine a minimum bending radius so as to protect the integrity and performance of the cable. Where the cable bending radius has been exceeded during installation - the cable can show kinking or other sheath damage as an indication of other possible problems such as hot spots, which combined with over-stressed insulation and sheath may result in premature ageing and potential cable failure. 11
Impedance in Electric Cables • Impedance is measured in Ohms and represents the total resistance that the cable presents to the electrical current passing through it. Impedance is associated with AC circuits. • At low frequencies the impedance is largely a function of the conductor size (resistance), but at high frequencies, conductor size, insulation material and insulation thickness all affect the cable's impedance. Matching impedance is very important, for example, if the system is designed to be 100 Ohms, then the cable should match that impedance, otherwise error-producing reflections are created at the impedance mismatch, seen as lower return loss in bidirectional signal cables. 12
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