Test for Thickness of Insulation of Power Cable

This test covers the technique to confirm the thickness of insulation and sheath of electric power cables. Adequate thickness of insulation and sheath is provided to power cable meet up the voltage stresses and mechanical stresses imposed on the cable during its service life. The measurement of such thickness is necessary to verify whether it is as per specified limit or not. These dimensions ensure safe and reliable performance of the cable.

Apparatus required to test for thickness of insulation of Power Cable

This is purely a measurement process, hence the apparatus for the test are to be chosen very carefully. There should be, a micrometer gauge capable of measuring at least 0.01 mm variation, a vernier caliper which can clearly read at least 0.01 mm count, a measuring microscope with linear magnification of not less than 7 times and the possibility of reading at least to 0.01 mm, and a graduated magnifying glass can read clearly at least 0.01 mm.

First different pieces of specimen are to be prepared for different measuring instruments and methods. The specimen may be of 2 types 1st one is pieces of core cable and 2nd one is slice pieces.

Procedure for testing of Thickness of Insulation of Power Cable

Approximately not less than 300 mm long pieces of core or cable cut from final product is used in the case of round conductor and outer sheath. The specimen must cut from the final product and remove all the covering materials above insulation or sheath without damaging the insulation and sheath. Slice pieces cut from the cable are used for optical measurement. In this case the materials outside and inside of the insulation or sheath which to be measured, can be removed if required. The slice is cut in sufficiently thin pieces along the plane perpendicular to the axis of the cable. The measurements are preferably taken at room temperature. The diameter of core of the cable and the diameter of insulated core and cable with insulation as well as the sheath is measured with the help of micrometer gauge or a vernier caliper. The measurement must be taken perpendicular to the axis of the core or cable.

The measurements should be taken at different 3 points of equal interval along the length of the specimen. These intervals may be about 75 mm in the case of 300 mm long piece of specimen. Each measurement is taken for the diameter of inside and outside of the insulation or sheath. At each point two measurements shall be made for better accuracy. Hence total 6 measurements of the diameter of below as well as above of insulation/sheath are taken. Taking an average of 6 measured outer diameters; we get the average measured outer diameter of the insulation / sheath. Similarly, by taking an average of 6 measured inner diameters, we get the average measured inner diameter of the insulation / sheath. Difference of average outer and inner diameter divided by two is nothing but the average radial thickness of the insulation/sheath.

Where the visual examination of the specimen reveals eccentricity, the optical method shall be used by taking slice section the specimen.
In the case of sliced section the specimen is placed under a measuring microscope along the optical axis. For circular specimen 6 such measurements are taken along the periphery at a regular interval. For noncircular conductor, such measurement is done radially at each point where the thickness of insulation appears minimum. The numbers of slices are taken from a specimen at regular interval along its length in such a manner that the total of such measurement would be not less than 18. For example, in case of circular conductor, at least 3 slices are taken from a specimen and 6 measurements in each slice to be carried out. In case of noncircular conductor, the number of slices taken from a specimen depends upon the number of points of minimum thickness of insulation. As in this case measurement is only made at the minimum thickness points.

Calculation for Thickness of Insulation/Sheath of Cable

For Core/Cable Piece
Where, Dout is the average of six measurements taken for outer diameter of the insulation/sheath
Where, Din is the average of six measurements taken for inner diameter of the insulation/sheath.
For Slice Piece – The average of 18 optical measurements is taken as the minimum thickness of insulation/sheath.

Report
Heading – Test for Thickness of Insulation/Sheath
Cable Type –
Batch No./Lot No. –
Cable No./Drum No. –

Results:
Reference Specification ………………………………

Conclusion – Specimen meets/does not meet the requirements of specification.

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