CableDataSheet, Cable and Wire Technical Consulting Service

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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