By testing for the Tensile strength and Elongation at break, it is observed that the XLPE cables have a tensile strength of 12.5 N/mm2 and elongations of 200 to 300%.
From 20 to 30 N/mm2 for the sheath and 15 to 20 N/mm2 for the core and elongation is 300 to 400% Rubber and Elastomer cables have tensile strength depending upon the material as shown below.
Natural Rubber 5 to 7 N/mm2 Ethylene Propylene 4.2 N/mm2 Polychloroprene, Chlorosulphonate and Nitrile Butadiene 8 to 11 N/mm2Silicon Rubber 5 N/mm2 Butyl Rubber 4.2 N/mm2.
3. Hot Set Test is served as a means for determining whether or not the depending properties are fully realized after cross-linking.
The temperature for testing for XLPE cables is 200± 3?C under load for 15 minutes and mechanical stress of 20 N/cm2, elongation under load is 175% maximum and permanent elongation is not more than 15%.
For PVC cables, the permanent elongation can be in negative value with a maximum limit of 12%.
For Elastomeric cable testing, the temperature is set at 250±3?C and the maximum limit for elongation and the permanent set is 175% and 15%.
3. Cold Bend Test is carried out to ascertain the suitability or withstanding ability of the insulating materials at low temperatures or frost conditions.
The temperature setting is around -40?C. The thickness of the insulation is kept such that the insulation doesn’t get cracked up.
Depending upon the type of cable used, the thickness of the insulation is varied.
Generally, single PVC has an insulation thickness of .711mm and a wire thickness of 1.88 mm, stranded red copper has 0.9398 and 2.3114 respectively, and silicon rubber cable has 0.9652 and 2.3876 respectively.
5) Ageing assesses assessment of change in tensile strength and elongation of the material.
The treatment given to XLPE cable for the accelerated aging of air consists of keeping the cable at a temperature of 135± 3?C.
The tensile strength variation is ±25% maximum and the Elongation variation is ± 25%.
The treatment given to PVC cables constitutes of keeping the cable under temperatures of 80 ±2 ?C,100±2?C,135±2?C for Type A, Type B, and Type C insulation respectively for 7 days.
Tensile strength variation is ± 20%,25% and 25% respectively and elongation variation is ±20%, 25%, 35%.
The Elastomer insulated cables are kept under a temperature of 70?C for IE 1, 135?C for IE 2, IE 3, IE 4, and 200?C for IE 5 insulation cables for a duration of 10,7, and 7,7,10 days respectively.
And Tensile strength variation is observed as ±40%, ± 30%, ±30%, ±50%, ±20% respectively. The variation of elongation at break is ±40%, ± 30%, ±30%, ±50%, and ±20%.
6. Shrinkage test is to determine the shrinkages due to residual strain which is relieved on heating.
This test is important to be carried out so that these shrinkages do not cause any problem in terminations where the heating of cables is observed.
The XLPE insulated cables should allow only 4% maximum shrinkage when the cable is subjected to 130±3?C for a 1-hour duration.
And the PVC cables are kept under 150 ±2?C temperature for 15 minutes and the maximum shrinkage has to be 4%.
7. Loss of mass test for insulation and sheath is done to find out the progressive changes in the cable when they are exposed to elevated temperature and the changes depend on the severity of exposures.
The cables are exposed to a temperature of 80±2?C for 7 days and the maximum loss of mass observed is 2 mg/cm2.
8. Water Absorption test is carried out to indicate the degree of water absorption and thus the quality of the compound used.
The XLPE cables are kept at a temperature of 85±2?C for 14 days and the maximum water absorbed value is 1 mg/cm2.
PVC cables are given the same treatment but the maximum water absorbed is 10 mg/cm2.
The Elastomeric cables are subjected to average stress of 800 V/mm2 and the maximum water absorbed is 10% for the first seven days and 3% for the next seven days.
9. Cold Blend and Cold Impact test, sometimes the cables have to be laid in the area of low-temperature climatic conditions or frost.
Due to constant exposure of cables to low temperature or frost conditions, the insulation or sheath of the cables becomes hard and brittle.
Any impact on the surface of the sheath or cable under these conditions may fracture the insulation, thus making it weak or causing it to fail.
The suitability of the insulation material against low-temperature brittleness is determined with the help of an impact test conducted on the cable maintained at a specified low temperature.
In this test, the cooled cable is placed on an anvil inside a cold chamber and an indenter is dropped on its upper surface to give it a specified impact.
The effect of an impact is then checked to evaluate the quality of the cable insulation.
The Cold Bend test is for cable diameter of 12.5 mm and is subjected to a temperature of -5 ±2?C. Mainly PVC cables are used under these cold conditions.
10. Oil resistance test of cables has now become a critical performance parameter as cables that were able to sustain functional and operational integrity a decade ago would not be adequate to survive in the environment of a present-day manufacturing site.
Certain types of PVC have a lower degree of oil resistance while Elastomeric insulated cables have better oil resistance.
The cable is tested at an oil temperature of 100 ± 2?C for 24 hours. The tensile strength variation is ±40% maximum and the elongation variation is ±40% maximum.
11. Partial Discharge testing of cable is done to check for voids and cavities inside the dielectric of cables.
Voids of a minimum of 1mm may cause a Partial Discharge level of about 1pC but the voids lesser than 1 mm can’t be detected.
12. Hot Deformation test for cables is designed to determine the resistance of PVC insulation and sheaths to pressure at elevated temperatures.
The apparatus consists of a support on which the test specimen is kept and a stainless steel blade of a specified thickness with an arrangement to apply the specified load on the cable under test through the blade.
A set of weights is provided to give the specified load on the test specimen. The cable is kept under the temperature of 80?C for four hours, and the depth of indentation is a maximum of 50 percent.
13. Heat shock test is performed to find out the cracking of thermoplastic insulation and sheath of cables on overheating.
The test specifies the heat treatment given to the thermoplastic insulation and sheath at an accelerated temperature of 150?C and a duration of one hour to ascertain the withstand ability at that condition.
It is only a visual examination. The specimen is wound over a smooth round shaped mandrel for three turns in a close helix and then the mandrel is kept in an oven at a specified temp. and for the duration specified.
At the end of this period, the samples are cooled in the air for one hour. The test specimen is then examined for any cracks visible to the naked eye.
14. Ozone Resistance Test is carried out for Elastomeric cable because concentrations of ozone, that are reached in our natural envisage can cause deep cracks in elastomeric materials.
That normally lead to a failure of a component. The cables are subjected to .025 to .030 percent for 30 hours and the cable passes this test if the cable is free of cracks.
15. Bleeding and Blooming Test is used to check the stability of color of the sheath so that there is no chance of staining of the cores.
For the test, the cable is subjected to 50?C for 72 hours, and then filter paper or indicator compound is observed for any stain.
16. Insulation Resistance Test consists in measuring the Insulation resistance of a device under test, while phase and neutral are short-circuited together.
The measured resistance has to be higher than the indicated limit from the international standards.
A mega ohmmeter (also called insulation resistance tester, tera ohmmeter) is then used to measure the ohmic value of an insulator under a direct voltage of great stability.
For the best result, the insulation resistance of the cable is kept around 200 mega ohms per km at starting.
For PVC cables, the insulation resistance constant is around 36.7 MΩ km at 27?C and .036 MΩ km at maximum rated temperature.
For Elastomer Cables, these values are 700 MΩ km and 3.67 MΩ km. The insulation resistance of XLPE is around 1000 times higher than that of PVC cables.
17. Spark Test in cables is done for evaluation of insulation and sheathing materials for specific uses.
This test is also used for In-line fault testing on single conductors and jackets during the extrusion or rewinding process.
Copyright ? 2010 - 2025 zmscable.com All Rights Reserved.
