Analysis of thermal shrinkage test of wire and cable insulation

1. Inspection purpose

Thermoplastic insulating materials for wires and cables are affected by thermal expansion and contraction. When the ambient temperature changes sharply, their radial and axial dimensions change, and the change in radial thickness is small and negligible. The change of its axial dimension is relatively obvious, especially the installation wires generally use short segments, so the insulation expansion and contraction phenomenon at both ends is relatively obvious. Considering the safety of the use of the wires, the thermal shrinkage performance should be assessed.

2. Scope of application

It is suitable for insulation heat shrinkage test of wire and cable.

3. Inspection basis

GB/T 2951.13-2008/IEC 60811-1-3:2001 and related product standards.

4. Test equipment and appliances

Air oven, steel ruler

5. Sampling and sample preparation

          1. Cut a sample about 1.5L mm long from each insulated core to be tested at least 0.5m away from the cable end. L should be the length specified in the relevant cable product standards. If there is no specification in the cable product standard, L shall be 200mm, and the sample shall be 300mm.

          2. Remove the adhered extruded semi-conductive shielding layer (if any), and remove all sheaths from the insulated wire core sample in time. Mark the test length of L±5mm in the middle. Measure the distance between marks, accurate to 0.5mm. Then remove the insulation at a distance of 2 to 5 mm from the mark at both ends of each sample.

6. Test steps

          1. Heat the oven to the temperature specified in the standard.

         2. Hang the prepared sample horizontally on the test rack with the help of conductors at both ends, or place it horizontally on the surface of the talcum powder tank so that it can expand and contract freely, and then put it into a heated oven for heating for 15 minutes. When using a talcum powder tank, it should be heated to the specified temperature first.

         3. Take out the sample holder or talcum powder tank with the sample, cool it to room temperature in the air, and re-measure the distance L1 between the marks of each sample, accurate to 0.5mm.

         4. Calculate the shrinkage rate: the shrinkage rate is the percentage of the difference between the distance between the marks before heating and the distance between the marks after heating and cooling, and the distance between the marks before heating.

Formula: η=(L-L1)/L×100%

In the formula: η - the shrinkage rate of each sample, %;

L - the distance between the marking lines before heating, mm;

L1 - distance between marking lines after heating, mm.

7. Evaluation of test results:

The insulation shrinkage rate should not be greater than the standard to be qualified.