What is Insulation and Jacket Shrinkback?

Shrinkback is the tendency for the insulation of a wire to pull back from a splice or termination connector. If shrinkback occurs, an excessive amount of copper (or aluminum) conductor can be exposed and increase the risk of a short circuit. Shrinkback can also occur on the jacket of a multi-conductor cable. If the jacket pulls back from a splice or termination, it exposes the cable to dirt and/or moisture. Common causes of shrinkback and industry methods used to measure and control it are discussed below.

What Causes Shrinkback?

Shrinkback can occur when built-in mechanical stresses are present in the insulation or jacket material as the result of the manufacturing process. For example, if a copper conductor moves through an insulation extruder head at a speed slightly faster than the molten insulation compound coming out of the extruder head, the molten insulation becomes stretched. When the insulation compound is cooled, the insulation can become "frozen" in this stretched condition. If the wire is later cut into short lengths, the insulation may pull back if friction with the copper conductor is too low to hold it in position. High temperatures and wide temperature excursions tend to accelerate the process. Polymers vary in their susceptibility to shrinkback as a result of their basic chemical structure. For example, the thermal coefficient of expansion of a polymer can affect shrinkback. A higher thermal expansion coefficient means a larger volumetric change during temperature changes. This could lead to larger shrinkbacks. Table 1 contains coefficients of linear thermal expansion for common wire and cable polymer resins.1



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Insulation and Jacket Shrinkback in Cables

Definition:
Insulation and jacket shrinkback refer to the retraction or shrinkage of the insulation or outer jacket of a cable due to thermal, mechanical, or environmental stress. This phenomenon can cause exposed conductors or weaken the cable's structural integrity, leading to performance issues or safety hazards.

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Causes of Shrinkback

• Thermal Expansion and Contraction: During cable extrusion, the insulation and jacket materials expand due to heat. If not properly annealed, they contract when exposed to lower temperatures.
• Improper Cooling During Manufacturing: If cables are cooled too rapidly, internal stresses remain in the polymer, leading to shrinkage over time.
• Material Properties and Memory Effect: Some thermoplastic materials (e.g., PVC, polyethylene) retain internal stresses and tend to revert to their pre-extruded state when exposed to temperature fluctuations.
• Exposure to High Temperatures in Service: Excessive operating temperatures can cause insulation/jacket materials to soften and shrink over time.
• Mechanical Stress and Flexing: Frequent bending, pulling, or twisting can accelerate shrinkback, especially in flexible cables.

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Effects of Shrinkback

• Exposed Conductors: Increased risk of short circuits, arcing, and electrical failure.
• Reduced Dielectric Strength: Insulation shrinkage may compromise electrical performance.
• Compromised Cable Sealing: Entry points for moisture and contaminants may lead to cable degradation.

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Prevention and Mitigation

• Proper Manufacturing Controls: Ensure uniform cooling during extrusion and use appropriate annealing techniques to relieve internal stresses.
• Selection of Proper Materials: Use cross-linked insulation (XLPE) or materials with low shrinkback properties.
• Pre-Shipment Testing: Conduct heat aging tests to check for shrinkage tendencies.
• Installation Best Practices: Avoid excessive bending or pulling during installation and use heat-resistant termination techniques.
• Environmental Considerations: Avoid prolonged exposure to high-temperature environments beyond the cable's rated specifications.

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Industry Standards and Testing

• IEEE 1210: Specifies test methods for measuring insulation shrinkback.
• UL 2556: Covers jacket and insulation shrinkage evaluation.
• ICEA S-95-658: Provides guidelines for controlling shrinkback in power cables.

Understanding and controlling insulation and jacket shrinkback is critical for ensuring long-term cable reliability and performance, particularly in high-voltage and harsh environmental applications.