Crack resistant for cable

The problem of cracking of Halogen Free Flame retardant (HFFR) cable sheaths first became apparent over 15 years ago in the early HFFR markets, such as the Far East, where cables were subjected to high ambient temperatures.  Cables were sometimes exposed to prolonged direct sun on building sites, causing surface temperatures to reach up to 50-70°C.

The sheath became softened and mechanically weak and even whilst still on the drum, cracks began to appear.  This is thought to be caused by poor high temperature tear and tensile strength coupled with low thermal crack resistance.  Even sheaths that had not cracked on the cable drum, could be easily torn by the slightest abrasive or sharp contact during cable installation. Significant claims were made against cable suppliers requiring cable removal & replacement.

More recently this cracking problem has been found in the Middle Eastern markets, which are relatively new to the use of this type of compound.  The growth in the HFFR markets has attracted new compounders with little experience in such sheath cracking issues and some cheap low cost sheathing materials have demonstrated cracking issues leading to significant commercial claims.

Several test methods have been developed to assess thermal cracking performance.  The initial test method we used was conducted on a small diameter cable which was wound around a mandrel & controlled knife cuts were introduced to the surface. The sample was then placed in an oven at various temperatures to check for crack propagation.

Cable manufacturers have also developed their own internal tests where controlled cuts are placed in the sheath of armoured cables in the direction of armour wire lay and the cable sample is placed in an oven at various temperatures. In poorly designed sheath compounds, cracking propagation is observed as the temperature is raised.

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Crack resistance is a critical property for cable sheaths, especially in **Halogen Free Flame Retardant (HFFR)** cables. This ensures the durability and mechanical integrity of cables under extreme environmental conditions such as high temperatures, mechanical stress, and exposure to harsh environments.

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1. The Problem of Cracking in HFFR Cables

a. Causes of Cracking:
1. **High Ambient Temperatures:**
   - Cables exposed to direct sunlight on building sites can reach surface temperatures of 50–70°C.
   - Prolonged exposure leads to:
     - Sheath softening.
     - Reduced mechanical strength.
     - Increased susceptibility to cracks and tears.

2. **Material Weakness:**
   - Poor high-temperature tear and tensile strength in some HFFR materials.
   - Inadequate thermal crack resistance in compound formulations.

3. **Mechanical Stresses:**
   - Abrasion or sharp contact during installation can tear weakened sheaths.
   - Cracks may even propagate while cables remain on the drum.

b. Impact:
- Compromised insulation and protection.
- Safety risks and operational failures.
- Significant commercial losses due to cable removal and replacement.

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2. Contributing Factors in Cracking

1. **Low-Quality Compounds:**
   - Entry of inexperienced compound manufacturers in the HFFR market.
   - Use of low-cost, substandard materials with poor thermal and mechanical properties.

2. **Inadequate Testing:**
   - Absence of rigorous testing protocols during production to assess real-world performance.

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3. Testing for Thermal Crack Resistance

a. IEC Standards for Testing:
- **IEC 60811-508:** Resistance to cracking at low temperatures.
  - Tests cable flexibility and mechanical integrity under stress at specified low temperatures.

- **IEC 60332-1-2:** Flame retardance testing.
  - Ensures HFFR materials maintain fire safety properties despite cracking.

- **IEC 60216:** Tests for thermal aging.
  - Evaluates long-term material behavior under elevated temperatures.

b. Manufacturer-Developed Tests:
1. **Mandrel Test:**
   - Small-diameter cable wound around a mandrel.
   - Controlled knife cuts introduced to the sheath.
   - The sample is placed in an oven at various temperatures to monitor crack propagation.

2. **Armored Cable Test:**
   - Controlled cuts placed parallel to the armor wire lay.
   - Cable sample exposed to cyclic heating in an oven.
   - Observations made for crack propagation with increasing temperature.

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4. Solutions to Improve Crack Resistance

a. Material Improvements:
1. **Advanced Polymer Blends:**
   - High-performance elastomers and fillers to enhance thermal crack resistance.
   - Improved high-temperature tear and tensile strength.

2. **UV-Stabilized Compounds:**
   - Addition of UV stabilizers to protect the sheath from degradation due to sunlight exposure.

3. **Optimized Flame Retardants:**
   - Balancing fire safety properties with mechanical strength to avoid brittleness.

b. Process Enhancements:
1. **Controlled Extrusion:**
   - Precise temperature control during extrusion for consistent material homogeneity.

2. **Enhanced Cross-Linking:**
   - Cross-linking improves thermal stability and mechanical strength.

c. Compliance with Standards:
- Adherence to **IEC 60811** and **IEC 60216** ensures cables meet global performance benchmarks.

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5. Applications for Crack-Resistant Cables

a. High-Temperature Environments:
- Middle East, Far East, and other hot climates with direct sunlight exposure.

b. Construction Sites:
- Prolonged sun exposure and mechanical stresses during installation.

c. Industrial Applications:
- Armored cables for heavy-duty environments where mechanical integrity is critical.

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6. Conclusion

Crack resistance in HFFR cables is essential for ensuring durability and safety under challenging conditions. By improving the thermal and mechanical properties of sheathing compounds and implementing rigorous testing methods like those outlined in **IEC 60811** and **IEC 60216**, manufacturers can produce reliable, long-lasting cables for high-stress environments.