Lifespan of heating cables

Almost everything, be it mobile phones, cars, trains, planes, buildings, tunnels, bridges have a design life. We are often alarmed by the longevity of our mobile phones, but we never give it a second thought when we drive over a bridge built over 50 years ago or fly in a 30-year-old commercial aircraft. How much is needed to provide efficient and reliable service in the expected environment of materials, equipment and machinery.

Electrical cables are no different and have a useful design life under certain conditions, but unlike many other components, electrical cables remain the main arteries or nerve connections that provide any other item to work. Because “laid” electrical cables are often difficult and expensive to replace, electrical cables must therefore ideally provide a reliable service life equal to or better than the equipment connected and often combined with the design or design life.

Many people think that electrical cables are passive electrical components. When activated, electrical cables must transmit voltage and current over a range of frequencies. Because of the fundamental limitations of conductors and insulation, secondary effects such as resistance, reactance, capacitance, etc., all create unwanted conflicts that must be calculated and designed out. And if not eliminated, at least minimised as far as practicable. Interconnecting equipment can also cause adverse effects on electrical cables, but there is another very important point, this is the inherent or induced effect of conductor resistance from current, and this creates cable heat. *** Translated with www.DeepL.com/Translator (free version) ***

Heat in the presence of air is the main enemy of all polymer coatings, but such as light, certain acids, alkalis, salts and gases such as ozone accelerate degradation. Heat or thermal degradation is a molecular deterioration caused by molecules with long chain breaking (cleaving) and reacting with each other to change the properties of the polymer. These changes typically include reduced flexibility, embrittlement (cracking) brightening, colour changes and reduced relative elongation. In addition to the physical changes of performance aging, performance properties may also be affected and these may include insulation resistance, fire resistance, oil/water resistance, etc.

This standard is widely used worldwide and specifies the calculation and test procedures to be used to derive thermal ageing characteristics from experimental data obtained in accordance with IEC 60216-1 and IEC 60216-2 guidelines, using fixed ageing and ageing temperatures based on a large number of inclusions.
Essentially: The temperature rating gives the insulating material: “This temperature that deteriorates / reduces the
material elongation at break (EB) to 50% absolute elongation during 20000 hours of exposure”(20000 hours = 2.3 years).
The standard cable insulation material operating temperature is defined according to IEC60216:
– PVC = 70°C
– Cross-linked polyethylene = 90°C
– EPR / OPP / CNT rubber = 90 °C
– Silicone rubber = 180°C
– PTFE Teflon = 260°C

Understanding why PVC is rated at 70°C and XLPE is rated at 90°C is better we will be able to understand according to the current standards:
• UK ЕАС;
• IEC 60364-5-52;
• AS / NZS3008-1.
What is perhaps not indicated by these standards is that the reduction in elongation to 50% in absolute value is for 20,000 hours of exposure time at this temperature – that’s only 2.3 years. In fact, these standards do not describe engineers to use cables at (PVC) 70°C or (XLPE) 90°C continuously, or describe the short duration of the cable. These standards assume that usage will be on a burst load basis, where it is not anticipated that cables will be fully loaded 100% of the time. This pragmatic approach is the only way to use polymer insulated cable economically.
A general “rule of thumb” for ageing polymer insulated cable is that a 10°C decrease in the average operating temperature of the cable over its entire life span will double the insulation life time by 50% EB (relative elongation at break), viz:

PVC sheathing:

Running continuously at 70°C will degrade to 50% EB in 20,000 hours (2.3 years)

Operation continuously at 60°C will degrade to 50% EB in 40000 hours (4.6 years)

Continuous operation at 50°C will degrade to 50% EB in 80000 hours (9.2 years)

Continuous operation at 40°C will degrade to 50% EB in 160,000 hours (18.4 years)

Sheath with cross-linked polyethylene:

Running continuously at 90°C will degrade to 50% EB in 20,000 hours (2.3 years)

Operation continuously at 80°C will degrade to 50% EB in 40000 hours (4.6 years)

Operation continuously at 70°C will degrade to 50% EB in 80000 hours (9.2 years)

In continuous operation at 60° C, will degrade to 50% EB in 160,000 hours (18.4 years)

Conversely increasing the continuous exposure temperature by 10°C will half the time to 50% EB.

When considering the above, keep in mind that any additional chemical, ozone, light overload or short circuits will shorten the expected life of the cable.
For illustrative purposes, how quickly polymer cable insulation will degrade with time and temperature, during continuous operation in air at rated temperatures is summarised in a table:

In practice, the application of IEC60216 to determine the temperature of polymer insulation assessments against thermal ageing and the
elongation at break measurements followed by calculations to determine the cable current rating is pragmatic, but only because circuits do not often utilise the accuracy of cable performance. The full load current of cables in circuits may not be frequent and the “average” operating temperature of cables over their lifetime may well be much less than the maximum conductor temperature estimates quoted in the standards, so as to extend the life of the cable for a reasonable period.

In defence of the above standards, to calculate the current rating of polymer coated cable under any more conservative use requires significantly larger conductor sizes with significant economic impact.
Environmental concerns may also need to be considered (although for power circuits, the associated reduction in Watt losses may well offset the additional cost over the lifetime of the cable installation).
It is very important that design engineers understand the aging characteristics of polymer coatings when selecting cables for use in equipment where long life and/or high continuous or near continuous loads are required, especially at high ambient temperatures, in sunlight, or where higher than normal expected ozone levels are present. Examples may include: conventional or nuclear power plants, generators, high temperature industrial facilities, transformers, continuous ventilation fans, continuous pumps, etc. In these cases, “continuous use” are rating factors and should be applied when using cables with the appropriate higher temperature.

There is a single cable technology that has been available and widely used for over 80 years that simply has no effect on aging. Cable with copper outer jacket, with inorganic magnesium oxide insulation and copper conductors do not age, regardless of temperature and heat generated. They will withstand repeated overloads and short circuits without any deterioration. They are unaffected by sunlight, UV, ozone and resist many chemicals.

For this reason, mineral cables are often used in critical installations, for high or continuous loading and for basic safety circuits. Mineral insulated cables are often used for long-life projects, i.e. 50 or more years. The cable is also approved for use in all hazardous locations.
Being inorganic mineral cables are completely flame retardant because they have no combustible element when fire spreads, as it simply cannot spread by flame. For the same reason, cables cannot generate halogen, caustic or any other toxic gases when exposed to high temperature or fire, including CO and CO2. Mineral insulated cables are also mechanically stronger than any other cable construction and under all operating and emergency conditions. They do not soften when exposed to high temperatures, are pressure tested, and are resistant to external impacts and cuts.