Download 4.21 Arrangement Of Bell Circuits

UNIVERSITY OF TECHNOLOGY, JAMAICA
FACULTY OF EDUCATION AND LIBERAL STUDIES
School of Technical and Vocational Education
Electrical Technology (Year 2)
Group Members
Yassan Lindo
ID#: 0602002
Rochede Sibblies
ID#: 0801100
Wesley Esson
ID#:0806674
Nu-Senbi Imhotep
ID#: 0111204
Kevin Coke
ID#: 0800538
Llewellyn Frances
ID# 0804266
Course Name: ……………………………………………Instructional Technology
Date of Submission: ………………………………………….. November 20, 2009
Lecture’s Name: ……….……………………..…….Eraldine Williams-Shakespeare
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Grade:10
Subject: Electrical Technology
Unit Title: Electrical Installation
Protective devices
4.1 Protective devices are devices use to protect electrical apparatus against breakdown
whenever an undesirable event, like short circuit, occurs. These devices are fuses, circuit
breakers, and relays; they only operate whenever a high current is detected which exceed their
rating. Upon operating they break or interrupt the circuit preventing the current from damaging
the protected equipment.
4.11 A short circuit is an abnormal low-resistance connection between two nodes of an electric
circuit that are meant to be at different voltages. This results in an excessive electric current
(over-current) in the network and potentially causes circuit damage, overheating, fire or
explosion.
Examples : In electrical devices, unintentional short circuits are usually caused when a wire's
insulation breaks down, or when another conducting material is introduced, allowing charge to
flow along a different path than the one intended.
In mains circuits, short circuits may occur between two phases, between a phase and neutral or
between a phase and earth (ground). Such short circuits are likely to result in a very high current
and therefore quickly trigger an overcurrent protection device. However, it is possible for short
circuits to arise between neutral and earth conductors, and between two conductors of the same
phase. Such short circuits can be dangerous, particularly as they may not immediately result in a
large current and are therefore less likely to be detected. Possible effects include unexpected
energizing of a circuit presumed to be isolated.
4.12 The methods of short circuit protection discuss why, when and where to apply fuses, circuit
breakers, bimetallic strips, and relays. These are applied to protect various equipment
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depending on their operations. There are different types of fuses and circuit breakers used in
domestic electrical installations.
A fuse interrupts excessive current (blows) so that further damage by overheating or fire is
prevented. Wiring regulations often define a maximum fuse current rating for particular circuits.
Overcurrent protection devices are essential in electrical systems to limit threats to human life
and property damage. Fuses are selected to allow passage of normal current and of excessive
current only for short periods. Fuses once operates will have to be replaced as they become
useless.
A circuit breaker is an automatically-operated electrical switch designed to protect an electrical
circuit from damage caused by overload or short circuit. Its basic function is to detect a fault
condition and, by interrupting continuity, to immediately discontinue electrical flow. Unlike a
fuse, which operates once and then has to be replaced, a circuit breaker can be reset (either
manually or automatically) to resume normal operation.
A bimetallic strip is used to convert a temperature change into mechanical displacement. The
strip consists of two strips of different metals which expand at different rates as they are heated,
usually steel and copper, or in some cases brass instead of copper. The strips are joined together
throughout their length either riveting, brazing or welding. The different expansions force the flat
strip to bend one way if heated, and in the opposite direction if cooled below its normal
temperature. The metal with the higher coefficient of thermal expansion is on the outer side of
the curve when the strip is heated and on the inner side when cooled.
The sideways displacement of the strip is much larger than the small lengthways expansion in
either of the two metals. This effect is used in a range of mechanical and electrical devices. In
some applications the bimetal strip is used in the flat form. In others, it is wrapped into a coil for
compactness. The greater length of the coiled version gives improved sensitivity.
A relay is an electrically operated switch. Many relays use an electromagnet to operate a
switching mechanism, but other operating principles are used. Relays find applications where it
is necessary to control a circuit by a low-power signal, or where several circuits must be
controlled by one signal. The first relays were used in long distance telegraph circuits, repeating
the signal coming in from one circuit and re-transmitting it to another. Relays found extensive
use in telephone exchanges and early computers to perform logical operations. A type of relay
that can handle the high power required to directly drive an electric motor is called a contactor.
The reed switch is an electrical switch operated by an applied magnetic field. It was invented at
Bell Telephone Laboratories in 1936 by W. B. Ellwood. It consists of a pair of contacts on
ferrous metal reeds in a hermetically sealed glass envelope. The contacts may be normally open,
closing when a magnetic field is present, or normally closed and opening when a magnetic field
is applied. The switch may be actuated by a coil, making a reed relay, or by bringing a magnet
near to the switch. Once the magnet is pulled away from the switch, the reed switch will go back
to its original position.
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Signal circuits
4.21 Arrangement Of Bell Circuits
A simple bell circuit is shown in Fig. 1. A battery of two Leclanche cells c, c connected in series
furnishes current to the bell b, located at any part of the house, and the push button p is placed at
any convenient point.
figure 1
4.22 Demonstrate how to connect and test burglar points of a buzzer in a laboratory environment.
Examine the different types of configurations such as normally open (no), normally closed (nc),
and magnetic reed switched.
4.23 A telephone has two main parts: (1) the transmitter and (2) the receiver.
The Transmitter of a telephone serves as a sensitive "electric ear." It lies behind the mouthpiece
of the phone. Like the human ear, the transmitter has an 14 eardrum." The eardrum of the
telephone is a thin, round metal disk called a diaphragm. When a person talks into the telephone,
the sound waves strike the diaphragm and make it vibrate. The diaphragm vibrates at various
speeds, depending on the variations in air pressure caused by the varying tones of the speaker's
voice.
Behind the diaphragm lies a small cup filled with tiny grains of carbon. The diaphragm presses
against these carbon grains. Low voltage electric current travels through the grains. This current
comes from batteries at the telephone company. The pressure on the carbon grains varies as
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sound waves make the diaphragm vibrate. A loud sound causes the sound waves to push hard on
the diaphragm. In turn, the diaphragm presses the grains tightly together. This action makes it
easier for the electric current to travel through, and a large amount of electricity flows through
the grains. When the sound is soft, the sound waves push lightly on the diaphragm. In turn, the
diaphragm puts only a light pressure on the carbon grains. The grains are pressed together
loosely. This makes it harder for the electric current to pass through them, and less current flows
through the grains.
Thus, the pattern of the sound waves determines the pressure on the diaphragm. This pressure, in
turn, regulates the pressure on the carbon grains. The crowded or loose grains cause the electric
current to become stronger or weaker. The current copies the pattern of the sound waves and
travels over a telephone wire to the receiver of another telephone.
The Receiver serves as an "electric mouth." Like a human voice, it has "vocal cords." The vocal
cords of the receiver are a diaphragm. Two magnets located at the edge of the diaphragm cause it
to vibrate. One of the magnets is a permanent magnet that constantly holds the diaphragm close
to it. The other magnet is an electromagnet. It consists of a piece of iron with a coil of wire
wound around it. When an electric current passes through the coil, the iron core becomes
magnetized. The diaphragm is pulled toward the iron core and away from the permanent magnet.
The pull of the electromagnet varies between strong and weak, depending on the variations in the
current. Thus, the electromagnet controls the vibrations of the diaphragm in the receiver.
The electric current passing through the electromagnet becomes stronger or weaker according to
the loud or soft sounds. This action causes the diaphragm to vibrate according to the speaker's
speech pattern. As the diaphragm moves in and out, it pulls and pushes the air in front of it. The
pressure on the air sets up sound waves that are the same as the ones sent into the transmitter.
The sound waves strike the ear of the listener and he hears the words of the speaker.
Lighting fixtures and calculations
Types of lamps
Arc lamp
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An arc lamp or arc light is the general term for a class of lamps that produce light by an electric
arc (also called a voltaic arc). The lamp consists of two electrodes, typically made of tungsten,
which are separated by a gas. The type of lamp is often named by the gas contained in the bulb;
including neon, argon, xenon, krypton, sodium, metal halide, and mercury.
The electric arc in an arc lamp consists of gas which is initially ionized by a high voltage and
therefore becomes electrically conductive. To start an arc lamp, a very high voltage is pulsed
across the lamp to "ignite" or "strike" the arc across the gas. This requires an electrical circuit
with an igniter and a ballast.
Incandescent light bulb
The incandescent light bulb, incandescent lamp or incandescent light globe is a source of
electric light that works by incandescence (a general term for heat-driven light emissions which
includes the simple case of black body radiation). An electric current passes through a thin
filament, heating it until it produces light. The enclosing glass bulb prevents the oxygen in air
from reaching the hot filament, which otherwise would be destroyed rapidly by oxidation.
Incandescent bulbs are also sometimes called electric lamps, a term also applied to the original
arc lamps.
Gas-discharge lamp
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Gas-discharge lamps are a family of artificial light sources that generate light by sending an
electrical discharge through an ionized gas, i.e. a plasma. The character of the gas discharge
critically depends on the frequency or modulation of the current: see the entry on a frequency
classification of plasmas. Typically, such lamps use a noble gas (argon, neon, krypton and
xenon) or a mixture of these gases. Most lamps are filled with additional materials, like mercury,
sodium, and/or metal halides. In operation the gas is ionized, and free electrons, accelerated by
the electrical field in the tube, collide with gas and metal atoms. Some electrons circling around
the gas and metal atoms are excited by these collisions, bringing them to a higher energy state.
When the electron falls back to its original state, it emits a photon, resulting in visible light or
ultraviolet radiation. Ultraviolet radiation is converted to visible light by a fluorescent coating on
the inside of the lamp's glass surface for some lamp types. The fluorescent lamp is perhaps the
best known gas-discharge lamp.
Fluorescent lamp
A fluorescent lamp or fluorescent tube is a gas-discharge lamp that uses electricity to excite
mercury vapor. The excited mercury atoms produce short-wave ultraviolet light that then causes
a phosphor to fluoresce, producing visible light. A fluorescent lamp converts electrical power
into useful light more efficiently than an incandescent lamp. Lower energy cost typically offsets
the higher initial cost of the lamp. The lamp is more costly because it requires a ballast to
regulate the flow of current through the lamp.
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Wiring installation
Testing procedures
Polarity
Testing for polarity with circuit dead.
An ohmmeter is used for testing the polarity of the circuit switches.
 The main switch and fuses are out.
 Lamps and other apparatus are out or disconnected, and the single-pole switches are on.
 One lead of the polarity tester is connected to phase or outer conductor at the outgoing
side of the main fuses. The other lead is touched to the terminals of the single-pole
switches in turn.
Insulation resistance
These tests are performed between the conductors themselves and the conductors and earth.
There are two kinds of leakage path:

Direct leakage through the insulation

Surface leakage
Testing the installation to earth:
 Lamps and other apparatus disconnected.
 The wires of both pole of the supply to the main switch are twisted together and
connected to the line terminal of the ohmmeter.
 The earth terminal of the ohmmeter is connected to the consumer’s earthing terminal.
 The three terminals of 2-way switches should temporarily be connected together.
 Heating appliance should be tested to earth separately.
Ring continuity
An ohmmeter is used to carry out the ring continuity test.
 Remove the load from the circuit.
 Connect the ohmmeter across the circuit positive and negative terminals.
 Close switch last.
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Earthing continuity
An ohmmeter is used to carry out this test.
 Connect one lead of the ohmmeter to the earthing conductor.
 Use the other lead to touch the grounded apparatus and test for continuity.
Sheathed and armoured cables
Cable construction
Conductors - Copper and aluminum are the materials most used for conductors
Aluminium conductors of cross-sectional area less than 16 mm2 have proved difficult to
terminate due to its tendency to 'cold flow'. Therefore below 16 mm2, aluminum cables are not
generally used.
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Three core cable
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