Flexibility is an important feature to be considered in the selection of cables, as it affects durability, ease of handling and installation, and termination types. Construction elements included in the cable design will have a significant effect on the flexibility of the cable; including conductor class and material, insulation and sheathing material and additional items such as termite protection, screening and armoring.
Construction elements of a flexible cable:
- Conductor:
Flexible cables are usually Class 5 or class 6 conductor stranding, as opposed to Class 1 or 2. The definition of the classes can be found in IEC 60228 or AS/NZS 1125, however, an example would be 2.5mm2 – class 2: 7/0.67, class 5: 30/0.25, class 6: 140/0.15. Class 5 conductors are made up of many finer strands of wire, allowing for greater flexibility and bending capabilities, where more movement or bending is anticipated.
- Insulation and sheath materials:
Insulation material can affect the flexibility of a cable; XLPE, HDPE and PVC materials typically impede the flexibility of a cable, and materials formulated for flexibility are recommended, such as EPR, or Amplex’s specially formulated X-HF-110 and HFS-110-TP combination.
- Armouring:
Armouring will affect the flexibility of cables; especially galvanized steel wire armours. Amplex’s specially prepared tensile aluminium wire armour can improve flexibility where mechanical protection can’t be avoided.
- Screens:
Screens can affect the flexibility of a cable. Copper and brass tape and wire screens will impede flexibility to an extent; this can be overcome by using a tinned copper wire braid instead. Tinned copper wire only has around 80% EMI coverage, so this can be coupled with a aluminium/polyethylene tape.
- Nylon Jackets;
Nylon termite jackets will impede the flexibility of a cable; they can be used on flexible cable, however care is needed in installation as the nylon jacket can crease and crack if over bent.
Flexible cable considerations:
- Durability
Cables that are constantly subject to movement, torsion, flexing or vibration can fail quickly. The movement causes work hardening within the conductor strands and this leads to brittleness and breakage. The most finely stranded conductors are recommended to combat this, allowing the conductor strands to move in relation to each other rather than the within the structure of the strand.
- Handling and Installation
Where space for handling or installation is limited, this is exacerbated by the lack of ability to bend the cable. This will affect not only the ability to install the cable into its final position but also the ability of the tradesmen to work with cable on site.
- Termination
Termination of flexible cables should be considered separately to typical stranded conductors. Flexible cables have a larger conductor diameter, and may need special lug barrel dimensions and crimping methods to accommodate this. Shearbolt lugs cannot be used on flexible conductors.
