Cable Construction

Conductors

For equipment interconnection and airframe cables, the conductors are normally of the stranded type and are usually made from plated copper. However, size 24 and smaller sizes of conductor will be of copper alloy having a higher tensile strength. Fire resistant cables may also be of copper alloy or copper conductors throughout all applicable sizes.

The total conductor consists of plated strands which are circular in section and which are laid up into one of a number of stranded forms. Aluminium conductors are also available for cables of size 8 and larger but such cables have not been without any problems. Any modification, which involves conversion from copper to aluminium should be classed as ‘major’ and thoroughly investigated, especially in regard to termination techniques. Obviously, ‘aluminium cables’ will need to be significantly larger in cross section than copper for a given electrical load, because of the higher electrical resistance of aluminium.

Conductor Plating

Plating is employed on copper, copper alloy and aluminium conductors to improve resistance to corrosion and to assist termination techniques. Very often it is the plating which will determine the temperature rating of a given cable and the figures given below are those widely recognised within the UK.

• Tin plated copper maximum continuous temperature - 135^oC.
• Silver plated copper maximum continuous temperature - 200^oC.
• Nickel plated copper maximum continuous temperature - 260^oC.
• Nickel Clad plated copper maximum continuous temperature - 260^oC.

Nickel clad copper is used instead of nickel plate on fire resistant cable to provide a thicker nickel element. The temperature figures quoted above may have to be varied downwards because of limitations imposed by the cable insulation. Higher figures (notably 150^oC for tin plating) are sometimes quoted in the USA but performance at such temperatures, especially in regard to stable crimp resistance and solder ability is the subject of debate, if not dispute. It should be noted that the plating used on crimped terminal ends must be compatible with the conductor plating of the cable, and information should be sought from termination manufacturers.

Dielectrical Materials/Cable Types

In this Leaflet, it is not practicable to review the performance of all of the many types of cable constructions available except in general terms. Extensive studies have been made, especially in the USA, in an attempt to determine an optimum cable type. The conclusion drawn is that there is not an overall best cable and that all the materials studied have advantages and disadvantages. This is little help to a user who is seeking to resolve the conflicting guidance and advice offered by organisations which have a keen commercial interest in the decisions of an intending purchaser. This Information Leaflet is intended to alert staff to the difficulty of making a sound judgement in what has traditionally been considered to be a simple subject.

Insulation material is applied to conductors by one of two basic methods, extrusion and wrapping. In general terms, extrudable materials are ‘heat meltable’ and are not employed for higher temperature applications. It follows that towards the upper limit of their operating temperature, their mechanical strength can be significantly less than that measured at room temperature. Airframe categories of cable usually have a double extrusion which are not always of the same material. A double extrusion is also claimed to impart ‘crack stopping’ qualities. Radiation cross linking of processed material is employed on high performance cables and this eliminates melting, increases strength and allows for thinner wall thickness. Cables employing such construction perform well on the British Standard test for wet arc tracking.

The most commonly used wrapped insulation material is Kapton (Du-Pont trademark), which is the registered trade name to an aromatic polyimide produced by Dupont. Many cable manufacturers world-wide use Kapton, either singly or in combination with other materials to give a so called hybrid construction. Single or double tapes are spirally wound over the conductor to a defined overlap to give the required tape thicknesses at any one point. Kapton is naturally copper coloured and it is usual to apply a top coat to provide a coloured surface which will accept print and also give added protection to the cable. It follows that it is totally incorrect to talk of Kapton cables without further definition. Some constructions, notably cables made in the USA to MIL-W-81381/11, have been the subject of adverse comment and it is possible that the use of this particular type will be discontinued in some environments. This would not reflect general rejection of cables containing Kapton because most constructions provide good overall performance including excellent mechanical strength, especially the newer higher hybrid types.

The process of wrapping insulation provides good control of insulation wall thickness and there are now cable types which employ only 4 layers of ‘Kapton’, giving a total wall thickness of approximately 0.006 inches, and these are being employed throughout the airframe of some recently certified aircraft types. The CAA has not granted an Accessory Approval as ‘Airframe’ types to such cables, these having been accepted on a ‘Component’ basis.