Electric cables are normally installed on the assumption of a safe working life
Electric cables are normally installed on the assumption of a safe working life of at least 20 years. Changes in the insulating material take place with the passing of time and these changes, which may eventually result in an electrical breakdown, are accelerated at higher temperatures. Thus, if the working life is fixed, the limiting factor is the temperature at which the cable is required to operate.
During operation, the temperature at which the cable will operate depends upon the ambient temperature and the heating effects of the current produced due to the resistance of the cable conductors.
The heat dissipation of buried cables depends on the depth of laying, ground ambient temperature and its thermal resistivity, these being dependent on their geographical location and the season of the year. Nearby cables would also affect the ground temperature. Cables in air reach steady operating temperatures more quickly than similar cables underground and large cables take longer than small ones.
The heat may cause a change in the properties of an insulating material or in extreme cases, deformation may occur. It is important, therefore, to realise that there is “a cable for the job”.
There is a very wide range of cables designed to operate at voltages up to 400 kV. It is not possible to discuss all these in this book, but the reader is referred to a publication, Copper Cables, published by the Copper Development Association.
The majority of cables have copper conductors and in a cable these may vary from a single conductor to stranded construction.
The number of electric wire contained in most common conductors is 3, 7, 19, 37, 61 or 91. Thus, 37/0·083 indicates that the conductor has 37 wires each having a diameter of 0·083 in.
Study of electric cable used for 18 years outdoors in Romania shows that only 2% of original quantity of di-(2-ethylhexyl) phthalate has been lost during service life. Formulation was stabilized with lead stabilizer. Twenty percent of original stabilizer was used and required replacement in recycling process.3
A similar study in Sweden (see formulation in the next section) showed that only 1% of extractable matter was lost during 30-40 years of cable use, material was thermally stable, and mechanical performance measured by elongation changed very little. Experimental studies conducted in laboratory which simulated service life by thermal aging at 80°C and considering activation energy in Arrhenius equation at 95 kJ/mol showed that cables should perform for at least 44 years. The cables collected from field are suitable for recycling with minimal adjustments to formulation. Figure 13.19 shows that stability of insulation has linear relationship with duration of aging. Figure 13.20 shows that changes in elongation are very small.4
Degradation of insulation performance of electric cables is basically evaluated by tests and analyses. Based on the result of equipment qualification tests, subsequent analyses to confirm the integrity after a 60-year service period of cables and the result of insulation resistance measurement and insulation diagnostic tests, it has been concluded that immediate degradation of insulation performance is unlikely to occur for most types of cables.
Degradation of insulation performance is detected by the insulation resistance measurement, insulation diagnostic tests and performance tests of systems and components, which are performed during the inspection.
The Japanese government commenced a national R&D project on cable ageing to have more accurate prediction. Under this project many experiments are being performed to acquire time dependent data of cable ageing. Superposition of the time dependent data proposed by IEC 1244-2 is proposed as a suitable method to predict cable ageing.
The Japanese plant utilities conduct measurement of insulation resistance to monitor degradation of insulation performance and are planning to perform sample investigation to acquire actual degradation data of cable insulations.
An area of rubber cable technology where much research and development work has been concentrated in recent years is that of the behaviour of cables in fires. Although they may overheat when subject to current overloads or mechanical damage, electric cables in themselves do not present a primary fire hazard. However, cables are frequently involved in outbreaks of fire from other causes which can eventually ignite the cables. The result can be the propagation of flames and production of noxious fumes and smoke. This result, added to the fact that cables can be carrying power control circuits which it is essential to protect during a fire to ensure an orderly shutdown of plant and equipment, has led to a large amount of development work by cablemakers. This work has included investigations on a wide range of materials and cable designs, together with the establishment of new test and assessment techniques.