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Chapter 2 Electrical Components The satisfactory performance of any modern aircraft depends on the continuing reliability of electrical systems and subsystems. One of the most critical factors for obtaining a high degree of reliability involves the quality of workmanship that a technician uses when installing electrical connectors and wiring Improperly or carelessly installed and maintained electrical wiring can be a source of both immediate and potential danger. Conductors are copper and may be either solid or stranded. Although more expensive, stranded wire is stronger, more flexible and resistant to flexing failure than single conductors. Stranded conductors are usually plated in silver or nickel. When choosing the wire for an electrical system, there are several factors that must be considered. For example, the wire selected must be large enough to accommodate the required current without producing excessive heat or causing an excessive voltage drop. In addition, the insulation must prevent electrical leakage and be strong enough to resist damage caused by abrasion. • Most wires used in a aircraft have a core of pure copper. • The majority of wiring in aircraft is made from stranded copper. • Now in some modern aircraft aluminum wire was used as a engine crank circuit. In most cases, the wiring is coated with tin, silver, or nickel to help prevent oxidation. In the mid-1990s it was discovered that polyvinyl chloride insulation emits toxic fumes when it burns. Therefore, a new group of mil spec wires, MIL-W-22759, was introduced which consists of stranded copper wire with Teflon® insulation. Cable The simplest form of cable is two insulated conductors twisted together to form a unit. • Conductors: Nickel plated annealed flexible copper • Insulation: SIF (Silicone rubber) & GFB (Glass fibre braid) • Voltage: 600V RMS (at a maximum frequency 1600 c/s) • Operating temperature: Maximum: 190°C, minimum bending -55°C Requirement for choosing wire • Large enough to supply the required current without producing excessive heat or causing an excessive voltage drop. • The insulation must prevent electrical leakage and be strong enough to resist damage caused by abrasion. TYPES of AIRCRAFT WIRE The majority of wiring in aircraft is made from stranded copper. In most cases, the wiring is coated with tin, silver, or nickel to help prevent oxidation. polyvinyl chloride (PVC), nylon, and glass cloth braid for insulation purposes. The smallest size wire normally used in aircraft is 22-gauge wire, which has a diameter of about .025 inch. American Wire Gage (AWG) Wire Marking Wire Marking Color Meaning Yellow Power Red Power Black Return Blue Signal Green Signal White Signal Shielded Wire With the increase in number of highly sensitive electronic devices found on modern aircraft, it has become very important to ensure proper shielding for many electric circuits. Shielding is the process of applying a metallic covering to wiring and equipment to eliminate interference caused by stray electromagnetic energy. Shielded wire Shielded wire or cable is typically connected to the aircraft’s ground at both ends of the wire, or at connectors in the cable. Make sure you connect the shield at the ground end and cut it off flush at the other end. NOMINAL SYSTEM VOLTAGE ALLOWABLE VOLTAGE DROP CONTINUOUS OPERATION INTERMITTEN OPERATION 14 0.5 1.0 28 1.0 2.0 115 4.0 8.0 200 7.0 14.0 Before wiring a new component in an aircraft, the system voltage, the allowable voltage drop, and the component’s duty cycle must be known. All of these factors are interrelated. CIRCUIT VOLTAGE 115 200 14 28 800 100 200 600 1 20 ELECTRIC WIRE CHART 18 12 14 16 700 630 560 490 420 350 50 100 45 90 40 80 35 70 30 60 25 50 160 280 20 8 6 4 2 1 1 2 0 0 3 0 4 0 AMPERES 75 150 400 360 320 280 240 200 10 3 40 2 120 100 80 72 64 56 48 40 36 32 28 24 20 210 175 140 120 112 98 84 70 63 56 49 42 35 15 12 10 9 8 7 6 5 4 3 2 30 25 20 18 16 14 12 10 9 8 7 6 5 4 7 .5 1 VOLTAGE DROP CURES 1 CONTINOUS RATING-AMPERES CABLES IN CONDUIT AND BUNDLES 2 CONTINUOUS RATING-AMPERES SINGLE CABLE IN FREE AIR 3 INTERMITTENT RATING-AMPERES MAXIMUM OF 2 MINUTES 4 20 18 14 16 12 10 8 5 WIRE SIZE 6 4 2 1 1 2 0 0 3 0 4 0 Wiring Installation • Electrical wiring is installed in aircraft either as open wiring or in conduit. • With open wiring, individual wires, or wire bundles, are routed inside the aircraft structure without protective covering. • On the other hand, when installed in conduit, electrical wiring is put inside either a rigid or flexible tubing that provides a great deal of protection. Several methods of identifying wire bundles include pressure-sensitive tape or sleeve markers tied in place. Regardless of the method of bundle assembly, a plastic comb can be sued to keep individual wires straight and parallel in a bundle. Grounding and bonding One of the more important factors in the design and maintenance of aircraft electrical systems is proper bonding and grounding. Inadequate bonding or grounding can lead to unreliable operation of systems, e.g., EMI, electrostatic discharge damage to sensitive electronics, personnel shock hazard, or damage from lightning strike. This section provides an overview of the principles involved in the design and maintenance of electrical bonding and grounding. Grounding Grounding is the process of electrically connecting conductive objects to either a conductive structure or some other conductive return path for the purpose of safely completing either a normal or fault circuit. The design of the ground return circuit should be given as much attention as the other leads of a circuit. A requirement for proper ground connections is that they maintain an impedance that is essentially constant. Ground return circuits should have a current rating and voltage drop adequate for satisfactory operation of the connected electrical and electronic equipment. EMI problems, that can be caused by a system’s power wire, can be reduced substantially by locating the associated ground return near the origin of the power wiring (e.g. circuit breaker panel) and routing the power wire and its ground return in a twisted pair. Special care should be exercised to ensure replacement on ground return leads. Power ground connections, for generators, transformer rectifiers, batteries, external power receptacles, and other heavy-current, loads must be attached to individual ground ing brackets that are attached to aircraft structure with a proper metal-to-metal bonding attachment. This attachment and the surrounding structure must provide adequate conductivity to accommodate normal and fault currents of the system without creating excessive voltage drop or damage to the structure. If the structure is fabricated of a material such as carbon fiber composite (CFC), which has a higher resistivity than aluminum or copper, it will be necessary to provide an alternative ground path(s) for power return current. Special attention should be considered for composite aircraft. Bonding Equipment Bonding Low-impedance paths to aircraft structure are normally required for electronic equipment to provide radio frequency return circuits and for most electrical equipment to facilitate reduction in EMI. The cases of components which produce electromagnetic energy should be grounded to structure. Metallic Surface Bonding All conducting objects on the exterior of the airframe must be electrically connected to the airframe through mechanical joints, conductive hinges, or bond straps capable of conducting static charges and lightning strikes. Static Bonds All isolated conducting parts inside and outside the aircraft, having an area greater than 3 in2 and a linear dimension over 3 inches, that are subjected to appreciable electrostatic charging due to precipitation, fluid, or air in motion, should have a mechanically secure electrical connection to the aircraft structure of sufficient conductivity to dissipate possible static charges. A resistance of less than 1 ohm when clean and dry will generally ensure such dissipation on larger objects. CONNECTORS Connectors • Connectors are devices attached to the ends of cables and sets of wires to make them easier to connect and disconnect. • Each connector consists of a plug assembly and a receptacle assembly. (a) Circular or Cylindrical (b) Rectangular (c) Coaxial (d) Terminals and Splices Figure 2-22. Four basic types of connector • Contacts are typically male (pin) and female (socket) which are installed in plug connectors which are usually attached to the cable end or in receptacle connectors which are usually attached to a bulkhead or other fixed object. • When it is necessary to use an electrical connector in an area where it may be exposed to moisture, special moisture proof connectors should be used. The ground side of an electrical power conductor is typically connected to a male connector while the power side of the conductor is attached to the female connector. This is done to reduce the chance of an accidental short between the power side of circuit and any conductive surface when the mating connectors are separated. Electrical connectors BUSBAR Busbar systems • “Busbar systems” refers to conductors that take the form of a bar or bars of copper conductor. • Bus bar is simply a copper strip acting as a junction for the generator(s), battery and the various loads. Busbars • Busbars are used in aircraft for power distribution. • The most commonly used materials for busbars are plated copper. • A bus bar is very simply a central point where wires from electrical equipment are grouped together and attached to a metal bar that is then attached to a power source. Without a bus bar we would have to connect every electrical component directly to the power source. This would be very complicated and impractical. In some general aircraft, avionics require special attention. Avionic applications are typically split off on to their own power bus. It is very desirable for the power fed from the main bus to the avionics bus to pass through an avionics master switch. The switch allows the pilot to turn off all avionics before engine start up and shutdown. Applications which should be powered from the avionics bus: • Radio • Intercom • Transponder • Altitude encoder • GPS • EFIS systems • Autopilot Applications which should NOT be powered from the avionics bus: • ETC (and other gyro instruments) • EMS systems • Strobes • Lighting • Fuel pumps • Electrical flap drives SWITCHES Switch • The purpose of a switch is to interrupt the flow of current to the component it controls. • Each switch is rated with regard to the voltage it can withstand and the current it can carry. Switches • Contacts • Actuator A pair of contacts is said to be ‘closed’ when there is no space between them, allowing electricity to flow from one to the other. When the contacts are separated by a space, they are said to be ‘open’, and no electricity can flow. • Contacts touch to make a circuit, and separate to break the circuit. • Contact materials are also chosen on the basis of electrical conductivity, hardness (resistance to abrasive wear), mechanical strength, low cost and low toxicity. switch contacts • Pole :a set of contacts that belong to a single circuit • Throw :one of two or more positions that the switch can adopt • single-pole, single-throw (SPST) • single-pole, double-throw (SPDT) • double-pole, single-throw (DPST) • Double-pole, double-throw (DPDT) TYPE OF SWITCHES • Toggle switch • Rocker switch • Push-button • Rotary switch • Micro switch • …. TOGGLE AND ROCKER SWITCHES TOGGLE AND ROCKER SWITCHES PULL-TO-UNLOCK OPTION Guarded switch ROCKER SWITCHES/MASTER SWITCHES The battery switch which connects battery power to the bus bar (electrical load distribution point or bar). the alternator switch, for energizing the alternator. It connects the alternator field to the bus bar, thus providing the alternator with battery power for field excitation. Both switches must be ON for normal operation of the electrical system. If either switch has to be turned OFF in flight, then you should consider terminating the flight as soon as possible. They can be switched on separately, but only the alternator can be switched off separately – switching the battery OFF will automatically switch the alternator off as well. The master switch (or battery switch/ alternator switch) controls all of the airplane's electrical systems, with one important exception – the ignition system, which receives electrical power directly from the engine-driven magneto. • The master switch needs to be ON for any other electrical system to receive power, or for the battery to be recharged when the engine is running. • It should be turned OFF after stopping the engine, to avoid the battery discharging via services that are connected to it. Push-button switches Push switches are used for momentary actions when a circuit is to be completed or interrupted for a finite time. An example of this type of circuit is the start circuit of many turbine aircraft. Many push switches incorporate illuminated lens caps to indicate that the specific circuit has been selected. ROTARY SWITCHES • When it is necessary to select several conditions for a circuit, a rotary switch may be used. • Rotary switches are manually operated and are often used as selector switches such as when selecting a single voltmeter to measure voltage across different busbars or generators. ROTARY SWITCHES PRECISION (MICRO) SWITCHES • These switches require only a slight movement of the operating plunger to cause the internal spring to snap the contacts open or closed. • When precision switches are used to limit the movement of a mechanism, they are typically referred to as limit switches. Some typical circuits using micro switches include: • Landing gear systems • Door warning systems • Power lever sequencing of system operation (arming of power augmentation systems) • Weight on wheels sensing, which isolates circuits that should not operate on the ground Rheostats • Rheostats are used to alter the amount of current in a circuit by varying the total resistance (e.g. to vary the intensity of panel or flight deck lighting). • They normally also have an OFF position to completely remove the current. Proximity switches • Proximity switches open or close an electrical circuit when they make contact with or come within a certain distance of an object. Types of proximity switches • Infrared • Acoustic • Capacitive • Inductive. RELAYS AND SOLENOIDS When the bar magnet is at rest, the current ceases to flow. If the bar magnet is then removed, the current will again flow. An Electro Motive Force (EMF) is induced in a conductor whenever the magnetic flux is changed. • One of the features of an electrical system is the ability to remotely control components which are located in some far corner of the aircraft. • By using a solenoid, a very small switch can be used to control the current needed to operate an aircraft engine starter or other high-current device. Figure 2-17. Electrical circuits can be controlled remotely by use of relays (a), or by solenoids (b). Both are sometimes referred to as contactors. • Relays and solenoids (contactor) are quite similar, with only a mechanical difference. • Both relays and solenoids are used for electrical controls. Normally a relay has a fixed soft-iron core around which an electromagnetic coil is wound. Movable contacts are closed by the magnetic pull exerted by the core when the coil is energized, and are opened by a spring when the coil is de-energized (normally open relay). A solenoid has a movable core that is pulled in to the center of and electromagnetic coil when the coil is energized. Due to the movable core, solenoids respond quicker and are stronger than relays. • Solenoids are typically used for high current applications and also find important use as mechanical control devices; • For example, to move locking pins into and out of mechanically actuated devices. SOLENOIDS Relays • A relay is an electrical switch that opens and closes under the control of another electrical circuit. • Electromagnet is used to open or close one or many sets of contacts. Solenoids are relays also but the very large types which carry huge amounts of current. Types of Relays • Electro mechanical • Solid-state • Hybrids Relay Armature relays are the oldest , but first-rate. Plenty turns of very fine magnet-wire are wound around an iron core to form an electro-magnet. A Polarized Relay placed the armature between the poles of a permanent magnet to increase sensitivity. The more common magnetically polarized relay uses the effect of a permanent magnet introduced into the magnetic circuit or circuits. Solid-State Relay A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. • A solid state relay (SSR) is a solid state electronic component that provides a similar function to an electromechanical relay but does not have any moving components, increasing long-term reliability. • Solid-state relays are available in AC and DC versions. Advantages of Solid State Relays • low EMI/RFI • long life • no moving parts • no contact bounce • and fast response. The drawback to using a solid state relay it can only accomplish single pole switching. Hybrid Relays • a solid-state relay • an electromagnet relay Applications of relay • To control a high-voltage circuit with a lowvoltage signal • To control a high-current circuit with a lowcurrent signal • To detect and isolate faults on transmission and distribution lines by opening and closing circuit breakers (protection relays), Applications of relay(2) • To isolate the controlling circuit from the controlled circuit when the two are at different potentials, for example when controlling a mains-powered device from a low-voltage switch. • To perform logic functions. Applications of relay(3) • Early computing. Before vacuum tubes and transistors, relays were used as logical elements in digital computers. • Safety-critical logic. Because relays are much more resistant than semiconductors to nuclear radiation, they are widely used in safety-critical logic. • To perform time delay functions. Contactor Contactor • A contactor is an electrically controlled switch (relay) used for switching a power circuit. • contactors are designed to be directly connected to high-current load devices, • includes Power Contacts , Auxiliary Contacts, and Contact Springs. CIRCUIT PROTECTIONS Circuit Protections • Fuses • Circuit breakers FUSES Fuses are thermal devices whose primary function is to protect the distribution cables of a circuit against excess current flow due to short-circuit or overload. • The fuse is placed in series with the load (component) it protects. • The fuse wire is normally made of a zinc alloy, which has the desired low melting point. • All fuses are rated in amperes (amps). • In general fuses are selected on the basis of the lowest rating which will ensure reliable operation of the system • When replacing a blown fuse it is important that the new fuse be of the correct rating. Fuse Heavy Duty fuses fuse with an indicating lamp When replacing fuses, ideally the power to the circuit in question should be switched off. Circuit Breakers • A circuit breaker or thermal trip is designed to isolate a circuit by means of a mechanical trip that opens a switch whenever a surge of current, or overload, occurs. • The advantage of a circuit breaker over a fuse is that a circuit breaker can be reset once the overload situation has been remedied. Types of circuit breaker • Trip-free • Non trip-free trip-free Breakers • Depressing the reset button will not remake the circuit • Cannot reset the circuit breaker, until the overload condition has been cleared Non trip-free Breakers • it is possible to remake the circuit by holding the button in • The circuit breaker cannot be reset until the overload has been cleared • Some are equipped with manual trip buttons so as to manually operated switching device. • In the past, non trip-free circuit breakers were installed in some essential service circuits to permit emergency manual reconnection of supply under overload conditions, despite the fire hazard involved. Circuit breakers make use of bimetallic strips, which bend by an increasing amount as the temperature of the strip increases. At the temperature matched to the rated current flow for that particular circuit the bimetallic strip bends sufficiently to break the circuit. NOTES: • The circuit breaker must hold 100% of the rated current, must trip at 150% and above, within the limits shown in the curve. Trip times specified are at 25℃ ambient with no pre-loading. • To adjust the circuit breaker rating for ambient temperature multiply the breaker rating by the factor. For example, 5 Amp rating at 0℃: 5 ×0.67 = 3.3 Amp. • Therefore select 3 Amp rating. END OF CHAPTER 2