Posted 17 July 2019
Part 4 - Braiding, Screening and Armouring
In this series we have looked at cable manufacturing, starting with the original conductor, the insulation material and the process of laying up the cores.
In this instalment of ‘What’s in a Cable’ we look at the next stage – braiding, armouring and screening.
After the cores have been laid up they can either go straight to be sheathed/jacketed or braided or armoured. The purpose of screening or braiding is to act as a barrier, reducing the amount of electromagnetic interference from or to other signals nearby. Types of screening can include woven wire braiding or aluminium coated polyester tape. Armouring, as its name implies, provides mechanical protection to the cable.
Braiding
Braiding can be used for either mechanical protection, electrical screening against electromagnetic interference (EMI) or to give the cable torsional strength. Braiding can be applied straight over the core bundle or over a bedding or inner sheath. If no inner sheath is used clear polyester tape or non-woven polyester fleece is normally applied. This holds the cores together and reduces wear if the cable flexes. This also helps keep the overall diameter of the cable to a minimum. It is normal to extrude a sheath or bedding over the core bundle if the cable is to be armoured or steel braided.
The equipment used to braid cables dates back to the early textile industry and in some factories it is not unusual to see 50 year old machines still working! Nowadays however, most factories have invested in modern computer controlled braiders.
The braiding process requires two sets of bobbins, pre-wound with braid wires, to rotate in opposite directions to plait or weave the wires to form a braid. The density or coverage of the braid, often expressed as a percentage, varies depending on the number of wires or strands in each plait, the pitch of the braid and the thickness of each strand.
Copper braids such as CY cables are generally used for screening against electromagnetic interference and steel braided cables such as SY cables are used for mechanical protection. Other braid materials used for specialist applications include glass or ceramic fibres for high temperature and Rayon for torsional strength. When glass or ceramic fibres are used they are normally varnished to make them easier to handle.
An important point to watch out for is where suppliers talk about braid coverage but omit to mention the diameter of the wires used to braid it. This can easily reduce the metal content of the braid by as much as 50%. In a screened cable such as a CY or coax cables, this may substantially reduce the performance of the cable and its ability to deal with EMI.
With steel braided cables the braid should only be used for mechanical protection. Here the diameter of the braid wires clearly dictates the level of mechanical damage the cable can withstand.
Tape and Aluminium Foil
An alternative and cheaper form of screening is to wrap the cores in an aluminium or polyester laminated tape. This can provide 100% coverage but is not as conductive as a copper braid screen. If aluminium / polyester foils are used they must be in direct contact with a bare wire to act as an earthing point.
The ultimate level of screening is an aluminium / polyester foil in contact with a high coverage copper wire braid as found on some Belden style data cables and the FS Cables Niltox® LF-329 and Risershield screened power or control cables.
Armouring
Steel wire armour (SWA) or to give it its correct designation - single wire armour, is constructed by winding galvanised solid steel wires of between 0.9mm to over 3mm diameter around the bedded cable in a spiral form.
The individual wires are laid parallel to each other to give almost 100% coverage. This is one of the toughest forms of mechanical protection but the drawback is that it substantially reduces flexibility and can be slow to terminate.
Steel tape armour is also popular in some countries particularly on some fibre optic cables. This form of armour uses two helically wound galvanised steel tapes, wound in the same direction – sometimes it is corrugated or interlocking which helps with both strength and flexibility but also increases the price.
The advantage of steel tape armour is its resistance to vermin and termite attack and its relatively quick manufacture and low cost. Once screened or armoured the cable is then sheathed or jacketed. We will look at this process in the next instalment of ‘What’s in a Cable’.