The safe and economic operation of an aircraft is becoming ever more dependent on electrical and electronic systems. These systems are all intercon-nected with wires and cables, and take many forms. Electrical wires and cables have to be treated as an integral part of the aircraft requiring careful installa-tion; this is followed by direct ongoing inspection and maintenance requirements for continued airworthi-ness. Wire and cable installations cannot be consid-ered (or treated) as ‘ fi t and forget ’ . System reliability will be seriously affected by wiring that has not been correctly installed or maintained. Legislation is being proposed to introduce a new term: electrical wire interconnection system (EWIS); this will acknowl-edge the fact that wiring is just one of many compo-nents installed on the aircraft. EWIS relates to any wire, wiring device, or combination of these, includ-ing termination devices, installed in the aircraft for transmitting electrical energy between two or more termination points.

We need to distribute the sources of electrical power safely and effi ciently and control its use on the aircraft. Once installed, the wires and cables must be protected from overload conditions that could lead to overheating, causing the release of toxic fumes, possi-bly leading to fi re. This chapter describes the physical construction of wires and cables together with how they are protected from overload conditions before power is distributed to the various loads on the air-craft. (Practical installation considerations for wiring are addressed in Chapter 20.)

7.1 Overview

In the fi rst instance, we need to make a distinction between wires and cables . Wires are formed from a single solid conductor or stranded conductors, con-tained within insulation and protective sheath materi-als. Cables can be defi ned as:

● two or more separate wires within the same insulation and protective sheath

● two or more wires twisted together ● any number of wires covered by a metallic braid,

or sheath ● a single insulated conductor covered by a metallic

outer conductor (co-axial cable).

The terms wires and cables are often interchanged. In this book, reference will be made to ‘ wiring ’ in the all-embracing generic sense; cables will be referred to in specifi c terms as and when required.

Key maintenance point

Wire and cable installations cannot be considered (or treated) as ‘fi t and forget ’. System reliability will be seriously affected by wiring that has not been correctly installed or maintained.

7.1.1 Types of wire and cable

There is a vast range of wires and cable types used in an aircraft; these can be categorized with their spe-cifi c uses and applications:

● airframe wires and cables ● equipment wires and cables ● ignition system cables ● thermocouple cables ● data bus cables ● radio-frequency (RF) cables.

This chapter addresses airframe wire and cable applications in detail. (Wiring inside of equipment is not specifi cally addressed.) The application of ignition and thermocouple cables are described in Chapter 10 (engine systems). Wires or cables designed for small signals, e.g. data bus cables carrying digital data, are screened to prevent their signals from being affected by electromagnet interference. Wires and cables that carry high power and/or high frequencies are also shielded to prevent them being the cause of

Wiring and circuit protection 7

Chapter

Aircraft electrical and electronic systems138

electromagnetic interference. Screening and shield-ing to protect against electromagnetic interference (EMI) and electromagnetic compatibility (EMC) are described in this chapter; the subject of EMI/EMC protection is discussed in more detail in Chapter 19. Radio-frequency (RF) cables (or feeders) are special-ized types of screened cables. (These are described in more detail in a related book in the series, Aircraft Communications and Navigation Systems .)

7.1.2 Operating environment

Wiring installed in aircraft has to operate in a harsher environment than that found in cars, buildings or industrial applications. In addition to conducting cur-rent (often at high voltages), aircraft wiring will be exposed to a variety of environmental and in-service conditions including contaminants, for example:

● hydraulic fl uid ● fuel and/or oil ● temperature extremes ● abrasion ● vibration

General-purpose wiring used in the airframe com-prises the majority of the installed material. There are also special considerations for wiring, e.g. when routed through fi re zones. The materials used for the conductor and insulation must take these factors into account. Each application has to be planned from the initial design, taking into account the inspection and maintenance required for continued airworthiness of the wiring.

Test your understanding 7.1

Why are cables shielded?

7.2 Construction and materials

Aircraft wiring needs to be physically fl exible to allow it to be installed, and then to withstand the vibration of the aircraft that will cause the wires to fl ex. Multi-stranding of the conductor increases the fl exibility of the wire or cable, making it easier to install and with-stand vibration of the aircraft. The insulating mate-rial has to be able to withstand the applied voltage; the sheath material needs to be able to withstand the

specifi ed contaminants. Conductors need to be able to carry the required current without overheating or burn-ing; they must also have low insulation resistance to minimize voltage drops. From Ohm’s law , we know that (at a given temperature) the voltage across a resist-ance is proportional to the current. For a given cur-rent ( I ) and resistance ( R ), the voltage drop is quoted in terms of IR losses . For these reasons, most aircraft conductors are constructed from copper or aluminium contained within man-made insulating material(s).

Aluminium conductors are sometimes used in air-craft; however, the majority of installations are cop-per. The choice of conductor material is a trade-off; the fi rst consideration is the material’s resistance over a given length (given the term resistivity , symbol ρ , measured in ohm-metres, abbreviated Ω m). Annealed copper at 20°C has a resistivity of 1.725 � 10 � 8 Ω m; copper is more ductile than aluminium, and can be easily soldered. Aluminium has a resistivity value of 2.8 � 10 � 8 Ω m, it is 60% lighter than copper but it is more expensive.

A major consideration for using aluminium is that it is self-oxidizing ; this reduces manufactur-ing costs (no plating required) but extra precautions are necessary for terminating the conductors due to the increased termination resistance. The quantity and gauge of these strands depends on the current-carrying capacity (or rating) and degree of fl exibil-ity required. Individual strands of copper need to be coated to prevent oxidation. The choice of coating for the strands depends on the operating temperature of the wire. In general terms, three types of coating are used: tin, silver or nickel, giving temperature ratings of 135°C, 200°C and 260°C respectively.

To summarize, copper conductors are used exten-sively on aircraft due to the material’s:

● low resistivity ● high ductility ● high tensile strength ● ease of soldering

Test your understanding 7.2

Explain why cables and wires are multi-stranded.

Conductors must be insulated to prevent short-circuits between adjacent circuits and the airframe. Power supplies of 12 or 28 V do not pose a threat of electri-cal shock, but the wires must be insulated to prevent

139Wiring and circuit protection

arcing, loss of system integrity and equipment failure. The combined effects of insulation damage and fl uid contamination gives rise to wet arc tracking . This phenomenon can occur when insulating surfaces are contaminated with any material containing free ions; the surface then behaves as an electrically conductive medium (an electrolyte ). Leakage currents are suffi -ciently high to vaporize the contamination; this drives away the electrolyte and results in the formation of localized dry areas. These areas now offer a higher resistance to the current fl ow. In turn, high voltages will develop across these areas and result in small surface discharges. Initially, these discharges will emit fl ashes of light at the insulation surface, and produce localized temperatures in the order of 1000ºC. These high tem-peratures cause degradation of the insulation material. The ability of aircraft wiring to resist wet arc tracking is highly dependent on the wire insulation material. The conductivity level of the electrolyte will infl uence the failure mode resulting from this wet arc tracking. Higher power supply voltages are potentially lethal and the insulation provides a level of protection against this. The insulation material and its thickness depend mainly on the operating temperature and system voltage; examples of insulating and sheath materials include:

● ethylene tetrafl uoroethylene (ETFE): this is a fl uorocarbon-based polymer (fl uoropolymer) in the form of plastic material

● polytetrafl uoroethylene (PTFE), a synthetic fl uoropolymer

● fl uorinated ethylene-propylene (FEP): this retains the properties of PTFE, but is easier to form

● polyvinylidene fl uoride (PVF 2 or PVFD) has good abrasion and chemical resistance, and (like most fl uoropolymers) is inherently fl ame-retardant.

Test your understanding 7.3

Explain the term wet arc tracking.

7.3 Specifi cations

These have become more complex over the years to address the higher performance needed from wires and cables. This has been driven largely from in-service experience with electrical fi res and the drive to reduce weight as more and more avionics equipment is introduced onto the aircraft. Reduced weight for a

given length and diameter of wire is achieved through reducing the wall thickness of the insulation. Typical specifi cations used for wires and cables are contained in the US military specifi cation MIL-W-M22759E . This specifi cation covers fl uoropolymer-insulated sin-gle conductor electrical wires manufactured with cop-per or copper alloy conductors coated with either tin, silver or nickel. The fl uoropolymer insulation of these wires can either be PTFE, PVF 2 , FEP or ETFE. Wires manufactured to this specifi cation are given a part number using the following format: M22759/x-xx-x.

● M22759/x -xx-x determines the specifi c wire type (Insulation, sheath and wire coating) from a table in the specifi cation

● M22759/x- xx -x determines the wire size from a table in the specifi cation

● M22759/x-xx- x determines the insulation colour from another specifi cation (MIL-STD-681).

Examples of wires are shown in Fig. 7.1 . The single walled construction in Fig. 7.1(a) has a composite insulator and protective sheath to reduce cost and weight. The twin-walled construction, Fig. 7.1(b) ,illustrates the conductor, insulator and separate pro-tective sheath. Outer sheaths on either type of wire can crack over long periods of time; in the single-wall-type wire, moisture ingress can migrate into the wire through capillary action. This can lead to track-ing inside the insulation, leading to overheating; twin walled wiring is more resistant to this effect.

Key maintenance point

The combined effects of insulation damage and fl uid contamination gives rise to wet arc tracking.

7.3.1 Wire size

Wire sizes used on aircraft are defi ned in accordance with the American Wire Gauge or Gage (AWG). For a given AWG, the wire will have a specifi ed diam-eter and hence a known conductance (the reciprocal of resistance). Cable size relates to the conductor’s diameter; the overall wire or cable diameter is there-fore larger due to the insulation. The largest wire size is 0000 AWG; the smallest is 40 AWG. The range of AWG wire/cable sizes is detailed in the Appendices. Selection of wire size depends on the specifi ed cur-rent to be conducted. Larger diameter wires add

Aircraft electrical and electronic systems140

weight, but offer less voltage drop for a given length and lower heating effect due to I2R losses.

Wire identifi cation is printed on the outer surface of the sheath at intervals between 6 and 60 inches. This includes the wire part number and manufac-turers ’ commercial and government entity ( CAGE)designation. The printing is either green or white (depending on the actual colour of the wire).

Key point

Conductors must have low insulation resistance to minimize IR losses.

7.3.2 Performance requirements

Wires manufactured to MIL-W-M22759E have to demonstrate compliance with dimensions and con-struction, together with criteria to meet the following requirements:

● ease of removing the insulation ● ease of soldering ● dielectric testing ● fl exibility ● elongation and tensile strength ● wicking ● high- and low-temperature testing ● fl ammability ● life cycle testing ● fl uid immersion ● humidity ● smoke emission.

7.4 Shielding/screening

Shielded or screened wiring either prevents radia-tion from circuits switching high currents or protects

susceptible circuits (see Fig. 7.2 ). The inner conduc-tor carries the system current; the screen provides a low resistance path for coupling of electromagnetic fi elds. These fi elds are coupled into the shield and are dissipated to ground. (Further details on the the-ory of electromagnetic fi elds and shielding are given in Chapter 19.) Typical applications where shield-ing is used includes wiring installed near generators, ignition systems or contacts that are switching high currents. Shielded wires can be formed with single, twin, triple or quadruple cores. The classic example of screened wiring occurs with the Arinc 429 data bus which uses twisted screened pairs of cable to transmit digital data. This is being transmitted at either 12.5–14.5 (low speed) or 100 (high speed) k bits per second with a differential voltage of 10 V between the pair of wires. Examples of screened wire and cable specifi cations are found in M27500-22TG1T-14.

Key point

Wires and cables that carry high power, and/or high frequencies are shielded to prevent electromag-netic interference.

Figure 7.1 Typical aircraft wires and cables: (a) single-walled, (b) twin-walled

Conductor

Insulation/sheath

(a)

Conductor

Insulation

Outer sheath

(b)

Outer insulationInner insulation

Screen Conductors

Figure 7.2 Screened cable

Aircraft electrical and electronic systems142

they have reduced axial movement. Figure 7.5 gives an illustration of typical coaxial cable connectors and how they are fi tted to the cable.

Test your understanding 7.5

What is the difference between shielded and coax-ial cables?

7.5 Circuit protection

The current-carrying capacity of a wire or cable is determined by its length and cross sectional area; heat dissipation is determined by I2R losses. When the cir-cuit or system is designed, the wire size is selected to safely carry this current. Wires and cable are sub-jected to abrasion during the normal service life of the aircraft; this can lead to the conductor being exposed. This exposure could lead to a low resistance path between the conductor and the airframe and/or an adjacent conductor. Faulty equipment, low resist-ance paths or overloading from additional circuits will cause the current to increase and this might exceed the current-carrying limit of the conductor. Heat will build up in the wire leading to fumes, smoke and ultimately fi re. It is vital that we protect against this whilst allowing for transients; the methods used in aircraft are selected from the following devices:

● fuse ● circuit-breaker ● limiting resistor.

7.5.1 Fuses

Fuses are links of wire that are connected in series with the circuit. Their current-carrying capacity is predetermined and they will heat up and melt when this is exceeded, thereby interrupting and isolat-ing the circuit. Materials used for the fusible linkinclude lead, tin-bismuth alloy, copper or silver alloys. Referring to Fig. 7.6(a) , the fuse wire is contained with a glass or ceramic casing (or cartridge) to pre-vent any particles of hot metal escaping which could cause secondary damage. End-caps provide a connec-tion for the fuse wire and make contact with the cir-cuit wiring. Fuse holders consist of terminals and a panel clamp-nut.

Some fuse holders have an indication of the fuse condition, i.e. if the fuse has blown. The indicat-ing cap is black with an integrated coloured light. When the fuse has blown, the cap illuminates; differ-ent colours indicate different power supply voltages. Heavy-duty fuses (typically protecting circuits with up to 50 A current) are constructed with a ceramic body and terminals, see Fig. 7.6(c) . Fuses are either clipped into position on a terminal board, see Fig. 7.7 , or screwed into a panel, see Fig. 7.8 .

Fuses are relatively low cost items, but they can only be used once. In some applications, the fuse

(a)

Figure 7.5 Coaxial cable connectors: (a) typical end fi ttings, (b) terminating the coax cable

8mm

5mm

1

2

3

4

5

(b)

Aircraft electrical and electronic systems146

7.5.3 Limiting resistors

These are used to limit current surges, primarily in DC circuits, where the initial current surge is large. When these circuits are switched they cre-ate large current fl ows that can be harmful to other components and reduce the power supply voltage for a period of time (determined by the time constants of the circuit). Limiting resistors are connected in series with such circuits and then automatically shorted out once the circuit current has stabilized. Typical applications of limiting resistors are found in engine starting circuits and voltage regulators. Limiting resistors are also used in fi re extinguishing systems (see also Chapter 16). Fire extinguishers are activated by applying direct current (DC) through current-limiting resistors to the associated squib; this ruptures a disc that allows extinguishing agent to be expelled under pressure. The limiting resistors pre-vent inadvertent operation of the squib. In electron-ics, limiting resistors are used to protect devices such as diodes.

7.6 Multiple choice questions

1. Wires and cables that carry high power and/or high frequencies are shielded to prevent:

(a) the wire or cable being the cause of electromagnetic interference

(b) the wire or cable being affected by electromagnetic interference

(c) wet arc tracking.

2. Two or more separate wires within the same insulation and protective sheath is referred to as a:

(a) screened wire (b) coaxial cable (c) cable.

3. Fuses have a rating that determines the: (a) maximum current it can carry without

opening (b) minimum current it can carry without

opening (c) time taken for the fuse to rupture.

4. Conductors must have low insulation resistance to minimize:

(a) current surges (b) electromagnetic interference (c) IR losses.

5. The combined effects of insulation damage and fl uid contamination gives rise to:

(a) electromagnetic interference (b) wet arc tracking (c) current surges

6. For a given AWG, the wire will have a specifi ed: (a) diameter and hence a known conductance (b) length (c) screening.

7. Coaxial cables are normally used for: (a) digital signals (b) motors and generators (c) radio-frequency (RF) signals.

8. Trip-free circuit-breaker contacts: (a) can always be closed whilst a fault exists (b) cannot be closed whilst a fault exists (c) are only used during maintenance.

9. Limiting resistors are connected: (a) momentarily in series (b) momentarily in parallel (c) permanently in series.

10. The white collar just below a circuit-breaker button provides visual indication of the:

(a) circuit-breaker trip current (b) system being protected (c) circuit-breaker being closed or tripped.

Automatic reset

Automatic trippush to reset

Pull to trippush to reset (trip free)

Switch type

Figure 7.11 Circuit-breaker symbols/type