unit 2 electrical devices

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UNITS IN THIS COURSE

UNIT 1 ELECTRICAL TERMS

UNIT 2 ELECTRICAL DEVICES

UNIT 3 ELECTRICAL SAFETY

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TABLE OF CONTENTS

Para Page

2.0 OBJECTIVES 3

2.1 ELECTRICAL DEVICES 4

2.2 BATTERIES 4

2.2.1 Wet Cell Storage Batteries 5

2.2.2 Dry Cells 8

2.3 SWITCHES 9

2.3.1 Push-button Switch 11

2.3.2 Toggle Switch 12

2.3.3 Rotary Switch 13

2.3.4 Multipurpose Rotary Switch 13

2.3.5 Spring - Loaded Rotary Switch 14

2.3.6 Rotary Selector Switch 15

2.4 PROTECTIVE DEVICES 15

2.4.1 Fuses 16

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2.0 OBJECTIVES

After completing this course the student will be able to:

Describe a wet cell battery.

Recognize the hazards of working around storage batteries.

Describe a dry cell battery.

Recognize the uses of different types of switches.

Describe how a fuse works.

Differentiate between a fuse and a circuit breaker.

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2.1 ELECTRICAL DEVICES

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This part of the course will show you some of the common electrical devices you willsee and use everyday in a plant.

2.2 BATTERIES

Batteries supply direct current (DC) power. Batteries contain cells. A cell is a devicethat uses chemical energy to produce electrical energy.

There are two basic types of batteries :

Wet Cell Storage Batteries

Dry Cell Batteries

A cell creates a chemical reaction to produce electricity.

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2.2.1 Wet Cell Storage Batteries

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In a wet cell a chemical reaction which produces electricity takes place in a liquid.Figure 2-1 shows a typical wet cell.

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Figure 2-1 A Single Electrical Cell

The acid electrolyte (sulfuric acid and water) reacts with the zinc bar. Positivelycharged zinc atoms, or ions, go into the electrolyte solution. Free negative

electrons remain in the zinc bar, giving it a negative charge. Positive ions in theelectrolyte (the liquid) collect around the copper bar, giving it a positive charge.

The chemical reaction creates (makes) an electrical potential across the electrodes.Although a voltage potential exists, there is still no electricity.

Recall that electricity is the flow of electrons.

Without a circuit that connects the positive and negative terminals, there is no pathfor the electrons to flow through.

In Figure 2-2, a wire connects the two terminals. Now there is a path, or circuit, for

electrons to follow. The free electrons flow from the negatively charged zinc terminalthrough the wire to the positively charged copper terminal. This electron flow lightsup the lamp in the circuit.

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Figure 2--2 Chemical Action Electricity

A battery is two or more electrical cells connected in series. In series means that thepositive terminal in one cell is connected to the negative terminal in the next cell,and so on.

Figure 2-3 Lead, Acid Battery

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Figure 2-3 shows a storage battery. It is similar to the storage battery in a car. It

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supplies DC voltage when it is needed.

When storage batteries are used a lot, they lose their ability to make electricity.When this happens, they must be recharged.

Recharging battery allows the battery to supply electricity again. Many plants have a

bank of storage batteries. These a . re kept continuously charged by a batterycharger. The battery charger is powered from the AC power supply. If there is amajor power failure, the storage batteries will supply emergency DC power for theplant. (See Figure 2-3).

STORAGE BATTERIES CAN BE DANGEROUS

Large storage batteries can produce a very high current. If you drop metal toolsonto storage batteries it can cause a short circuit. This could cause a large spark(flash) and possibly an explosion.

Another danger with storage batteries is that they contain acid. Do not get close tothem without protective clothing. The acid can burn.

Figure 2-4 Battery Bank for Emergency DC Power

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2.2.2 Dry Cells

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Dry cells produce potential DC energy by the same sort of chemical action used instorage batteries. Dry cells are called dry because they do not use liquid electrolyte.All of the parts in a dry cell are sealed inside a metal container.

Figure 2-5 Common DC Dry Cells

Figure 2-5 shows you two types of dry cells. The materials and the electrolyte(chemical) are different from wet batteries. However, the cells produce potentialenergy the same way as the lead-acid storage battery.

In the past, dry cells were not recharged when they lost their power. They wereusually thrown away and replaced. Now it is possible to get rechargeable dry cellbatteries. Rechargeable means they can be charged again.

Dry cells are often used in test equipment, and, of course, in flashlights, mobiletelephones, wristwatches, pagers, cameras etc.

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2.3 SWITCHES

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There are many different types of switches.

Switches have one thing in common. They all do only one thing. They open orclose an electrical circuit. Recall that an open circuit has a break in it and currentcan not flow through it. A closed circuit is complete so current can flow through it.

No matter how they work or how they are tripped (activated), the end result is thesame. An electrical circuit is either opened or closed by a switch.

Switches are control devices used to operate electrical equipment. Switches openand close the electrical circuits which give power to the equipment.

Figure 2-6 A Switch Circuit

There are many different kinds of switches. There are switches that are activated bychanges in process variables. For example, a rise or fall in temperature or pressure.There are-switches that are tripped by electronic circuits. Switches can be simple orcomplex. (See Figure 2-7)

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Figure 2-7 Types of Switches

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2.3.1 Push-button Switch

A very simple kind of switch is the push button. (See Figure 2-8)

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Figure 2-8 Push-button Switch

There are two types of pushbutton switches.

NO - normally open (push it to close)

NC - normally closed (push it to open)

All push-button switches work in the same way. A spring in the switch holds thebutton open or closed.

A fail open switch will cut off all electrical current in a circuit if there is a fault orfailure in the system.

Most emergency shutdown systems use the FAIL OPEN system. Switches in thesystem are normally closed. They will open if there is a fault.

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2.3.2 Toggle Switch

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The switch in Figure 2-9 is a toggle switch. Some toggle switches do not have aspring like the push-button switches. The toggle switch will stay in the position it isset, open or closed.

Other toggle switches have springs in ' them. Their normal position is neutral,

neither on nor off. The switch is moved to one position or the other, but when it isreleased, the spring returns it to the neutral position.

Figure 2-9 Toggle Switch

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2.3.3 Rotary Switch

The word Rotary in rotary switch simply means that you turn the switch in a circulardirection (rotate it) to operate it. Figure 2-10 shows a simple ON/OFF, two position,rotary switch.

Figure 2-10 Simple Rotary Switch

2.3.4 Multipurpose Rotary Switch

Multipurpose means that the switch serves more than one purpose. Figure 2-11 isactually a circuit breaker switch. A circuit breaker is a device that protects electricalequipment from too much current. When the switch is off, the circuit is off. The typeof switch is often used on electrical control panels.

Turning the switch directly to the ON position will not activate the circuit. The switchmust first be turned to the RESET position, then to ON. In the ON position theswitch closes a circuit breaker.

Turning the switch to the OFF position opens the circuit breaker and deactivates thecircuit. We will look at circuit breakers later in the unit.

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Figure 2-11 Multipurpose Rotary Switch

2.3.5 Spring - Loaded Rotary Switch

Figure 2-12 shows an example of a spring-loaded switch. The switch is normally inthe position shown. It is in a neutral position, which means it is not doing anything.

The operator can turn the switch to the position he wants. If the circuit is off (broken)he turns the switch to START. When he releases the switch, a spring in the switchreturns it to the neutral position. To break the circuit, the operator turns the switch toSTOP. When he releases the switch, the spring in the switch returns it to the neutralposition again.

Figure 2-12 Spring-Loaded Rotary Switch

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2.3.6 Rotary Selector Switch

Figure 2-13 is a rotary selector switch found on a gas turbine engine control panel.The switch is a complex rotary switch as shown in figure 2-7.

This type of switch allows an operator to select the function he wants to perform.

We have said earlier that a switch does one thing. It opens or closes a circuit. Theswitch in figure 2-13 is still only opening or closing a circuit. The switch selectswhich circuit to open or close.

Figure 2-13 Rotary Selector Switch

2.4 PROTECTIVE DEVICES

The electrical equipment used is expensive. Some of it is very expensive. Too muchelectric current can damage or destroy these electrical devices.

Two cheap devices that protect the expensive devices are

Fuses

Circuit Breakers

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2.4.1 Fuses

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A fuse is an electrical device, placed in a circuit, that protects the circuit from toomuch current.

High current in the circuit will cause a wire inside the fuse to melt. This will create anopen circuit which the current cannot flow through.

There are many different types and sizes of fuses. Each circuit needs a specifictype of fuse.

Figure 2-14 shows some of the types of fuses you will see.

Figures 2-14 Examples of Types of Fuses

Figures 2-15A and 2-15B show you the inside of a typical fuse. The fuse wire isdesigned to melt when too much current flows through it. Figure 2-15B shows theresult of too much current in a circuit. The fuse wire has melted and the circuit isbroken.

Figure 2-15 Inside a Fuse

unit 2 electrical devices

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