Chapter 2 Electrical Components

1
Chapter 2 Electrical Components
2

• 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.

3

• 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.

4

• 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.

5

• 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.

6

• In most cases, the wiring is coated with tin,
• silver, or nickel to help prevent oxidation.

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• 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.

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Cable

• The simplest form of cable is two insulated
  conductors
• twisted together to form a unit.

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• 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 190C, minimum
  bending -55C

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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.

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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.

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• The smallest size wire normally used in
• aircraft is 22-gauge wire, which has a
• diameter of about .025 inch.

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American Wire Gage (AWG)
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Wire Marking
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Wire Marking
Color Meaning
Yellow Power
Red Power
Black Return
Blue Signal
Green Signal
White Signal
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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.

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Shielded wire

• Shielded wire or cable is typically connected
• to the aircrafts ground at both ends of the
• wire, or at connectors in the cable.

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• Make sure you connect the shield at
• the ground end and cut it off flush at
• the other end.

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NOMINAL SYSTEM VOLTAGE ALLOWABLE VOLTAGE DROP ALLOWABLE VOLTAGE DROP
NOMINAL SYSTEM VOLTAGE 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 components duty cycle must be known. All of
these factors are interrelated.
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Wiring Installation
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• 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.

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Several methods of identifying wire bundles
include pressure-sensitive tape or sleeve
markers tied in place.
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Regardless of the method of bundle assembly, a
plastic comb can be sued to keep individual
wires straight and parallel in a bundle.
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• Grounding and bonding

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• One of the more important factors in the design
  and maintenance of aircraft electrical systems is
  proper bonding and grounding.

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• 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.

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• This section provides an overview of the
  principles involved in the design and maintenance
  of electrical bonding and grounding.

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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.

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• 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.

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• EMI problems, that can be caused by a systems
• 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.

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• 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.

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• 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.

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• 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.

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• Bonding

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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.

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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.

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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.

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• CONNECTORS

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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.

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(a) Circular or Cylindrical
(b) Rectangular
(c) Coaxial
(d) Terminals and Splices Figure
2-22. Four basic types of connector
43

• 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.

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• 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.

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Electrical connectors
46

• BUSBAR

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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.

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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.

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• Without a bus bar we would have to connect
• every electrical component directly to the
• power source. This would be very
• complicated and impractical.

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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.
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Applications which should be powered from the
avionics bus

• Radio
• Intercom
• Transponder
• Altitude encoder
• GPS
• EFIS systems
• Autopilot

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Applications which should NOT be powered from
the avionics bus

• ETC (and other gyro instruments)
• EMS systems
• Strobes
• Lighting
• Fuel pumps
• Electrical flap drives

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SWITCHES
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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.

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Switches

• Contacts
• Actuator

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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.
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• 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.

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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

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• single-pole, single-throw (SPST)
• single-pole, double-throw (SPDT)
• double-pole, single-throw (DPST)
• Double-pole, double-throw (DPDT)

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TYPE OF SWITCHES

• Toggle switch
• Rocker switch
• Push-button
• Rotary switch
• Micro switch
• .

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• TOGGLE AND ROCKER SWITCHES

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TOGGLE AND ROCKER SWITCHES
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PULL-TO-UNLOCK OPTION
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Guarded switch
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ROCKER SWITCHES/MASTER SWITCHES
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• The battery switch which
• connects battery power
• to the bus bar (electrical
• load distribution point or
• bar).

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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.
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• 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.

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• 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.

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• 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.

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• 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.

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Push-button switches
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• 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.

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• Many push switches
• incorporate illuminated
• lens caps to indicate
• that the specific circuit
• has been selected.

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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.

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ROTARY SWITCHES
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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.

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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

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Rheostats

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• 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.

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Proximity switches

• Proximity switches open or close an
• electrical circuit when they make contact
  with or come within a certain distance of
• an object.

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Types of proximity switches

• Infrared
• Acoustic
• Capacitive
• Inductive.

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RELAYS AND SOLENOIDS
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• When the bar magnet is at rest, the
• current ceases to flow. If the bar magnet
• is then removed, the current will again
• flow.

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• An Electro Motive Force (EMF) is induced in
• a conductor whenever the magnetic flux is
• changed.

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• 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.

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• 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.

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• Relays and solenoids (contactor) are quite
  similar, with only a mechanical difference.
• Both relays and solenoids are used for
  electrical controls.

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• 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).

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• 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.

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• 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.

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SOLENOIDS
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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.

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• Solenoids are relays also but the very large
• types which carry huge amounts of current.

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Types of Relays

• Electro mechanical
• Solid-state
• Hybrids

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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.

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• A Polarized Relay placed the armature
• between the poles of a permanent magnet to
• increase sensitivity.

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• The more common magnetically polarized
• relay uses the effect of a permanent magnet
• introduced into the magnetic circuit or
• circuits.

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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.

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• 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.

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Advantages of Solid State Relays

• low EMI/RFI
• long life
• no moving parts
• no contact bounce
• and fast response.

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The drawback to using a solid state relay

• it can only accomplish single pole switching.

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Hybrid Relays

• a solid-state
• relay
• an electro-
• magnet relay

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Applications of relay

• To control a high-voltage circuit with a low-
• voltage signal
• To control a high-current circuit with a low-
• current signal
• To detect and isolate faults on transmission and
  distribution lines by opening and closing circuit
  breakers (protection relays),

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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.

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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.

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Contactor
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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.

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CIRCUIT PROTECTIONS
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Circuit Protections

• Fuses
• Circuit breakers

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FUSES
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• 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.

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• 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.

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• 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.

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Fuse
Heavy Duty fuses
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fuse with an indicating lamp
127

• When replacing fuses, ideally the power to the
  circuit in question should be switched off.

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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.

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Types of circuit breaker

• Trip-free
• Non trip-free

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trip-free Breakers

• Depressing the reset button will not
• remake the circuit
• Cannot reset the circuit breaker, until the
  overload condition has been cleared

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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.

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• 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.

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• 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.

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• 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.

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END OF CHAPTER 2