Test for Thickness of Insulation of Power Cable

Contents

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Key learnings:

Definition of Insulation Thickness Test: A test to confirm the insulation and sheath thickness of power cables to ensure they meet specified standards.
Apparatus Needed: Requires precise instruments like a micrometer gauge and vernier caliper for accurate measurements.
Specimen Preparation: Specimens are cut from the cable and prepared for different measurement techniques.
Measurement Process: Involves measuring diameters at multiple points and calculating the average thickness.
Cable Insulation Importance: Ensures the cable can safely handle voltage and mechanical stresses throughout its service life.

This test measures insulation and sheath thickness on electric power cables. IS 10810-6 is the Indian method. A power cable needs enough wall to stand the rated voltage and the pull of laying. The product standard then compares the measured average and the measured minimum with its own limits.

Apparatus required to test for thickness of insulation of Power Cable

The work is a dimensional check, so the tools must read to 0.01 mm. IS 10810-6 lists a micrometer, a vernier caliper, a measuring microscope of at least 7 times linear magnification and a graduated magnifying glass. IEC 60811-201 now treats a measuring microscope or profile projector (10 times in the 2023 amendment) as the reference optical tool.

Prepare a specimen for each of the measuring instruments and methods. There are two specimen forms: a length of core or cable, and a thin slice.

Procedure for testing of Thickness of Insulation of Power Cable

For the micrometer method on round cores and outer sheaths, IS 10810-6 uses pieces at least 300 mm long cut from the finished cable. Strip outer coverings without cutting the insulation or sheath. For optical work, cut a thin slice in a plane perpendicular to the cable axis. Take readings at room temperature. Measure core diameter and insulated-core diameter with a micrometer or vernier, across the axis.

On a 300 mm piece, take readings at three stations about 75 mm apart. At each station measure inner and outer diameter twice, at right angles, so you have six inner and six outer readings. Average those six outer values and those six inner values. Mean radial thickness is then (mean outer diameter minus mean inner diameter) divided by two. That figure is an average wall, not the thinnest wall.

If the wall looks eccentric, use the optical method on a slice.
Place the slice under a measuring microscope with the cut face square to the optical axis. For a circular core, take six radial readings around the periphery. IEC 60811-201 starts the first of those six at the point that looks thinnest. For a non-circular core, measure radially at each point where the wall looks thinnest. IS 10810-6 takes enough slices along the length that the total number of optical readings is not less than 18. A circular core then needs at least three slices with six readings on each. The mean of those readings is average thickness. The smallest reading is minimum thickness.

Tabulation of Observations

For Core/Cable Piece

Observations Nos.	Outer Diameter of Insulation/Sheath (D_out)	Inner Diameter of Insulation/Sheath (D_in)
1.	–	–
2.	–	–
3.	–	–
4.	–	–
5.	–	–
6.	–	–

For Slice Piece –

Observations Nos.	Minimum Thickness of Insulation/Sheath for Slice 1	Minimum Thickness of Insulation/Sheath for Slice 2	Minimum Thickness of Insulation/Sheath for Slice 3
1.	–	–	–
2.	–	–	–
3.	–	–	–
4.	–	–	–
5.	–	–	–
6.	–	–	–

Calculation for Thickness of Insulation/Sheath of Cable

For Core/Cable Piece

Where D_out is the average of six measurements taken for outer diameter of the insulation/sheath
Where D_in is the average of six measurements taken for inner diameter of the insulation/sheath.
For Slice Piece – The mean of the 18 optical readings is the average thickness. The smallest of those 18 readings is the minimum thickness of insulation or sheath.