Screening/braiding is primarily designed to protect the cable from outside sources of electromagnetic interference, or to protect other cables from interference from the cable that is screened. In some cases, screening or braiding is designed to give the cable some torsional strength.
Armouring cable is designed to provide the cable with some mechanical protection to maintain its structural integrity, which means the cable can withstand higher stresses, is suitable for direct burial, and can be used in external underground and underwater projects.
Most armouring is made from steel, and can be in either wire (commonly referred to as SWA), or tape form. Other materials are also used to provide armoured protection to cables, including aluminium, but steel is the most common. SWA cable is made by winding solid steel wires of between 0.9mm to over 3mm diameter around the bedded cable in a spiral form.
Each individual wire is laid parallel to each other to give the desired coverage. This technique provides for one of the toughest forms of mechanical protection but it does have its drawbacks, most notably that it substantially reduces the cable’s flexibility and can be more difficult for installers to terminate.
Metallic armour are used when cables have to be installed direct buried, or if mechanical protection is required. Following points must be considered:
·Required tensile load
·Expected pressure on cable during service
·Protection against rodent
·Protection against accidental damage
·Minimum required bending radius
SWA: single layer of galvanized steel wires, with diameters according to relevant standards, coverage min. 90%. This armour assure a very good mechanical protection and tensile strength. An additional counterspiral tape increases solidity, if required.
STA: using two helically wound galvanised steel tapes, wound in the same direction, it can sometimes be used in a corrugated or interlocking fashion. This method can increase both strength and flexibility.
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