What is the Hot Set Test?

The hot set test is a critical procedure used in the cable and wire industry to evaluate the thermal and mechanical performance of polymeric materials, particularly insulation and sheath materials. This test is performed to determine how a material reacts under high-temperature conditions and mechanical stress, ensuring that cables can maintain their integrity and functionality in harsh operating environments. By simulating high-temperature conditions, the hot set test helps manufacturers and engineers assess whether a material is suitable for long-term applications involving heat exposure.

Purpose of the Hot Set Test

The primary purpose of the hot set test is to verify the thermal stability and mechanical durability of the polymer materials used in cables. Cables often operate in environments with elevated temperatures, whether in industrial settings, power plants, automotive applications, or aerospace environments. Without adequate thermal resistance, the insulation or sheathing of cables may degrade, leading to potential failure, short circuits, or even fire hazards.

The hot set test ensures that the materials used for cable insulation and sheathing can withstand high temperatures without excessive deformation or loss of mechanical properties. Specifically, this test assesses:

• The elongation under load at elevated temperatures
  
• The permanent deformation after cooling
  

Standards Governing the Hot Set Test

Several international standards outline the procedures and requirements for conducting the hot set test. The most commonly referenced standards include:

• IEC 60811-507 - "Electric and optical fibre cables - Test methods for non-metallic materials - Part 507: Hot set test for cross-linked materials"
  
• ISO 188 - "Rubber, vulcanized or thermoplastic - Accelerated ageing and heat resistance tests"
  
• UL 44 - "Standard for Thermoset-Insulated Wires and Cables"
  
• ASTM D2671 - "Standard Test Methods for Heat-Shrinkable Tubing for Electrical Use"
  

These standards provide detailed instructions on how to perform the test, including the equipment to be used, the test conditions, and the criteria for evaluating the results.

Test Methodology

The hot set test involves subjecting a sample of cable insulation or sheathing material to a specified load at an elevated temperature for a defined period. The key steps in the procedure are as follows:

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• Sample Preparation
  The test begins by cutting a uniform strip of the insulation or sheath material from the cable. The dimensions of the sample, such as length and thickness, must comply with the requirements specified in the applicable standard.
  
  
• Initial Measurement
  The initial length of the sample is measured under no load and at room temperature. This length is recorded as the baseline for calculating elongation.
  
  
• Application of Load
  A specified load, typically expressed in terms of stress (N/mm²), is applied to the sample. The load is determined based on the cross-sectional area of the sample and the requirements of the standard.
  
  
• Exposure to High Temperature
  The sample, with the load applied, is placed in an oven or a controlled heating chamber set to the specified test temperature (usually between 150°C and 250°C, depending on the material and standard). The sample is maintained at this temperature for a predetermined duration, typically 15 minutes.
  
  
• Measurement at Elevated Temperature
  After the specified exposure time, the length of the sample under load is measured while it is still at the elevated temperature. This measurement is used to calculate the elongation under load.
  
  
• Cooling and Final Measurement
  The sample is then removed from the oven and allowed to cool to room temperature with the load still applied. Once cooled, the final length of the sample is measured. This measurement is used to calculate the permanent deformation after cooling.
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  Calculations
  
  Two key parameters are calculated in the hot set test:
  
  
  • Elongation under load (EUL)
    This parameter is calculated as:
    
    EUL (%) = [(L1 - L0) / L0] × 100
    
    Where:
    - L0 = Initial length of the sample (mm)
    - L1 = Length of the sample under load at elevated temperature (mm)
    
    
  • Permanent deformation (PD)
    This parameter is calculated as:
    
    PD (%) = [(L2 - L0) / L0] × 100
    
    Where:
    - L0 = Initial length of the sample (mm)
    - L2 = Length of the sample after cooling (mm)
    
  Acceptance Criteria
  
  The acceptance criteria for the hot set test depend on the specific standard and application. Generally, a material is considered acceptable if:
  
  
  • The elongation under load (EUL) does not exceed a specified maximum value (e.g., 100%)
  • The permanent deformation (PD) does not exceed a specified maximum value (e.g., 25%)
  
  Importance of the Hot Set Test in Cable Manufacturing
  
  The hot set test is crucial for ensuring the reliability and safety of cables used in various applications. It helps manufacturers:
  
  
  • Ensure compliance with international standards and regulations
    
  • Verify material quality during the production process
    
  • Improve product performance by selecting appropriate materials
    
  • Reduce the risk of failures in high-temperature environments
    
  Common Applications Requiring Hot Set Testing
  
  Cables used in the following applications typically require hot set testing:
  
  
  • Power transmission and distribution - High-voltage and medium-voltage cables
    
  • Automotive wiring - Engine compartment cables
    
  • Aerospace wiring - High-performance cables exposed to extreme temperatures
    
  • Industrial machinery - Control and instrumentation cables
    
  Conclusion
  
  The hot set test is an essential tool in the cable and wire industry for ensuring the thermal and mechanical performance of polymeric materials. By simulating real-world high-temperature conditions, this test helps manufacturers produce reliable and durable cables that meet the stringent requirements of modern applications. Adhering to international standards and performing rigorous testing not only ensures product quality but also enhances safety, reduces maintenance costs, and improves overall customer satisfaction.
  
  Understanding and applying the principles of the hot set test can greatly contribute to the development of advanced cable solutions that withstand challenging environments, making it a fundamental aspect of cable manufacturing and quality control.
  
  

Many important properties of polyethylene insulation of cables depend on the degree to which the material is cross-linked. Determination of whether or not the dependent properties are fully realized after cross-linking can be carried out with the help of a hot set test.

In this test the elongation of the insulating material at an elevated temperature under the action of a specified tensile load and its recovery after removal of the load are determined to evaluate the efficiency of the cross-linking. A dumbbell of specified shape and size is suspended from a vertical frame inside a hot air oven with a specified load suspended from its lower end. The elongation of the test specimen after fifteen minutes under the above conditions and its subsequent recovery after removal of the load are then evaluated.

Related standards

IS 7098 (Part 2) - 1985 : Specification for Cross Linked Polyethylene Insulated PVC Sheathed Cables

IS 10810 (Part 30) - 1984 : Methods of Test for Cables

Part 30 : Hot Set Test

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Conditioning - All the test specimens shall be kept at a temperature of 27 f 2°C for a period
of not less than 16 h Immediately before test.

Procedure

8.1 The distance between marker lines shall be measured.

8.2 The test pieces shall be suspended in the oven and weights attached to the bottom grips to
exert a force of the value specified for the material in the relevant cable standard.

8.3 After 15 min in the oven at 'the temperature specified for the material in the relevant cable
standard, the distance between the marker lines shall be measured. If the oven does not have a
window and the oven door needs to be opened to make the measurement, the measurement shall
be made not more than 30 s after opening the door.
fin the case of dispute the test shall be carried out in an oven with a window and the measure-
ment made without opening the door.

8.4 The tensile force shall then be removed from the test piece (by cutting the test piece at the
lower grip), and the test piece left to recover for 5 min at the specified temperature.

3.5 The test pieces shall then be removed from the oven and allowed to cool slowly to ambient
temperature, after which the distance between the marker lines shall be measured again.


Calculation
The percentage hot set elongation = (B-A)/A x 100
Permanent set percent = (C -A)/A x 100

initial Length Length of the Gauge mark A, mm
Between the Mark after 15 Min with load Gauge Mark B, mm
Length of the Gauge Mark at Room Temperature after Test, C mm