Cable Performance

The definition of cable performance has increased in complexity and precision with the reduction of insulation thickness and weight. Some of the cables now used for airframe wiring have no more than 0.006” of insulation thickness and thus there is little margin for error in manufacture or in an aircraft installation. The operating temperature dictates to a large extent the materials and constructions used, but installation requirements need to be satisfied by defining properties such as resistance to insulation “cut-through” and abrasion. It follows that cables need to be selected with care and the factors detailed below should be considered in relation to any intended duty.

Application

It should be noted that under one generic name there may be a range of insulation thicknesses, which will be appropriate for Airframe or for Interconnect cable and thus correct identification (by part number) is particularly important.

Temperature

The temperature rating of a cable must be defined to permit comparison with the worst case requirements of the application. It follows that the location of a cable, relative to hot air ducts and local hot spots such as power transformers and some filament lighting must be known. Cables have a specified maximum continuous operating temperature, and for many types, this may be achieved by any combination of ambient temperature plus temperature rise due to I²R losses. However, it should be noted that it is undesirable to contribute more than a 40°C rise by electrical heating and that operating temperature and installed life are directly related. The temperature rating of an airframe cable is determined by its construction and will be classified at one of the following temperatures:

• 105°C (obsolescent cable types), 135°C, 150°C, 210°C and 260°C.

Clearly this temperature rating has to be known when evaluating any design application.

Cable Size

Cable is usually identified by a size number, which approximates to the A.W.G. (American Wire Gauge) size of the conductor. However, some cables enjoy a number that refers to the square millimetres of a conductor cross section, which is a system used extensively for commercial cables. The size of cable is the primary determinate of the electrical protection level set by the circuit breaker or fuse and should never be reduced below the level established by proper coordination data. Manufacturers publish rating data for single cables in free air and for bundles of three cables in free air. By study of the short term and continuous ratings for a given cable type and size, the correct protection can be determined (CAA Airworthiness Notice No. 12 and appendix No. 32 should be observed). Current rating data usually relates to a temperature rise of 40^oC above ambient as stated above and due allowance must be made for such electrical heating. Manufacturers’ data will normally include conductor resistance in ohms per km at 20^oC and a temperature correction may be necessary if accurate voltage drop calculations are necessary.

It should be noted that cable ‘size’ relates only to the conductor and thus the overall diameter and surface finish for a given size may vary slightly between cable types. Such differences in overall diameter may have an effect on cable sealing in connectors and pressure bungs, and also the selection of pre-insulated terminal ends where a dielectric crimp provided.

Voltage Rating

All cables have a rated voltage and equipment wires may be specified by voltage. Particular reference should be made to the specified voltage of any cable where higher than normal potentials may be used, examples being discharge lamp circuits and windscreen heating.

Flammability and Toxicity

All cables are required to have a defined level of resistance to burning when exposed to standard flame tests. In addition to the requirements of flammability, there exists within BCARs, JARs and FARs, general requirements relating to the hazards of smoke and toxicity. In recent years, greater emphasis has been placed upon these characteristics and whilst they are not yet defined in many civil cable specifications. It is generally true that new cable types have been more thoroughly investigated, albeit on an empirical or subjective basis.

Wet Arc Tracking

A requirement has now been formulated to assess the ‘resistance to failure’ of cables when subjected to a combination of insulation damage and fluid contamination. The propensity of some insulating materials to ‘track’ has long been studied in high voltage systems but it has now been found necessary, following a failure.

BS G230 now includes a test to determine resistance to Wet Arc Tracking (Test No. 42), and Airworthiness Notice No. 12, Appendix No. 32 will be used to keep Industry advised of the CAA position on this subject.

Tracking can also occur under dry conditions and this is being studied. This failure mode reinforces the need for good cable installation and maintenance practices.

Mechanical Properties

The assessment of cable insulations includes the ability to withstand the pressure of a sharp edge (cut-through) and for the ability to withstand scraping with a defined blade. It is these tests which figure significantly in assessing airframe cable and which are the controlled methods of replacing assessment by scraping with the thumb nail. As noted earlier, differing constructions result in marked changes in handling properties especially with regard to stiffness and ‘springiness’. Installation of looms of thin wall hard dielectric cable has to have regard to the reluctance of such looms to be ‘set’ in position, especially if the supporting structure is flimsy. However, it must not be assumed that this apparent strength is translated into the ability to withstand physical abuse.

Fluid Contamination

Cables are required to display a defined level of resistance to the effects of commonly used aircraft fluids. But, this is not to say that cables can withstand continuous contamination, which should be avoided. A related hazard is that presented by sealing compounds because these may contain agents which are aggressive to cable insulation. It follows that where a new cable type is introduced, the compatibility with such compounds should be checked. Equally, the use of a new fluid on an aircraft should be considered in relation to the ability of cables to withstand contamination. Contamination of cables by toilet or galley waste has to be rigorously prevented or corrected as detailed in Airworthiness Notice No. 12 Appendix No. 32.