Cable testing to ascertain the measurements of tensile strength and elongation is used to determine the mechanical properties of insulating and sheathing compounds.
The Standard BS EN 60811-501 determines the cable test methods applied to cross-linked and thermoset insulation and sheathing materials.
The tensile strength of a material is the amount of force needed to pull that material to the point of break.
The elongation is a measure of the length that the material can be stretched to before breaking.
In order to make a proper comparison between the properties of different materials, standard cable sample preparation methods must be used. Cable samples for tensile and elongation testing are prepared by removing the conductors and any tapes with the size and shape of the cable determining if the test piece is a tubular sample or cut into a smooth dumbbell shape. The sample material is then allowed to condition for three hours before the test at a temperature of 23°C +/-5°C.
The cable sample for testing is marked in the centre, with two marks 20mm apart for larger cables and 10mm for smaller cable types. Cable cross-sectional areas are calculated for the sample pieces so that the force required to pull the sample to point of break can be expressed in terms of force x cross-sectional area (N/mm2). The distance between the marks on the cable are to determine the degree of stretch or elongation to the point of break expressed as a percentage (%).
The test for cable tensile strength and cable elongation is performed on materials in the condition as manufactured, but subsequent testing after accelerated ageing is also required for these material types. Results after ageing give an appreciation of how ageing affects the mechanical properties of the materials.
The materials used for the insulation and sheathing of electric cables have predefined requirements for minimum elongation and tensile strength, and any variation after ageing, all detailed in the relevant material standards.