US Army Engineer course - Electricity VII (Install Circuit Protective Devices)

7/30/2019 US Army Engineer course - Electricity VII (Install Circuit Protective Devices)

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SUBCOURSE EDITION

EN5146 B

US ARMY ENGINEER CENTER AND SCHOOL

INSTALL CIRCUIT PROTECTIVE DEVICES




Subcourse Number EN5146

EDITION B

United States Army Engineer School

Fort Leonard Wood, Missouri 65473

2 Credit Hours

Edition Date: November 1998

SUBCOURSE OVERVIEW

This subcourse is designed to teach the knowledge necessary to perform tasks related to installing fuses, circuitbreakers, and ground-fault circuit interrupters (GFCIs).

There are no prerequisites for this subcourse.

This subcourse reflects the doctrine which was current at the time it was prepared. In your own work situation,always refer to the latest official publications.

Unless otherwise stated, the masculine gender of singular pronouns is used to refer to both men and women.

*This publication contains copyrighted material.

TERMINAL LEARNING OBJECTIVE

ACTION: You will describe the procedures used to install circuit protective devices.

CONDITION: You will be given subcourse booklet EN5146 and an examination response sheet

STANDARD: To demonstrate competency of this task, you must achieve a minimum of 70% on thesubcourse examination.

*Art in Figures 1-1 through 1-5, 1-8 through 1-10, 1-12 through 1-16, and 1-19 through 1-22 is adapted from

Electrical Wiring Fundamentals by Joseph H. Foley. This copyrighted material is reproduced with permissionfrom McGraw-Hill Book Company, New York.

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TABLE OF CONTENTS

Section Page

Subcourse Overview.....................................................................................................................................................i

Lesson: Install Circuit Protective Devices..............................................................................................................1-1

Part A: Identifying Circuit Protective Devices.........................................................................................1-2

Part B: Installing Fuses ............................................................................................................................1-2

Part C: Installing Circuit Breakers ...........................................................................................................1-7

Part D: Installing Ground-Fault Circuit Interrupters ..............................................................................1-9

Practice Exercise.......................................................................................................................................1-13

Answer Key and Feedback ......................................................................................................................1-15

Appendix A: List of Common Acronyms ..........................................................................................................A-1

Appendix B: Recommended Reading List............................................................................................................B-1



LESSON


Critical Task: 051-246-1107

OVERVIEW

LESSON DESCRIPTION:

At the end of this lesson, you will be able to describe the procedures used to install circuit protective devices.

TERMINAL LEARNING OBJECTIVE:

ACTION: Describe the procedures used to install circuit protective devices.

CONDITION: You will be given subcourse booklet EN5146 and an ACCP examination response sheet Youwill work at your own pace and in your own selected environment with no supervision.

STANDARD: Within approximately 2 hours, you should be able to study the lesson resources and completethe practice exercise.

REFERENCES: The material contained in this lesson was derived from the following publications:STP 5-51R12-SM-TG, FM 5-424, and the National Electrical Code (NEC) available fromNational Fire Protection Association Publications, Batterymarch Park, Quincy, MA 02269.

INTRODUCTION

This lesson, part of the MOS 51R Skill Levels 1 and 2 course, is designed to teach the knowledge necessary to

perform tasks related to installing fuses, circuit breakers, and ground-fault circuit interrupters.

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PART A. IDENTIFYING CIRCUIT PROTECTIVE DEVICES

Circuit protective devices are installed in the circuit to protect it and other equipment in the circuit from anovercurrent or overload. The words overcurrent and overload are used interchangeably, but thy have differentmeanings.

• Overcurrent describes a condition in which a circuit is carrying more than its rated load of amperes.

• Overload describes the starting load of motors and motor-driven appliances. Motors require heavycurrent flow when starting and much less current at normal running speed.

In order to prevent overcurrent and overload, electrical circuits have been designed with built-in protectivedevices. Typical circuit protective devices are fuses, circuit breakers, and ground-fault circuit interrupters (GFCIs)(Figure 1-1).

Figure 1-1. Types of circuit protective devices

PART B: INSTALLING FUSES

Fuses become part of the circuit when installed in series with the hot wire of the circuit. This is possible becausea fuse contains a thin metal strip called a fuse element. This strip provides protection against overcurrent andoverload. The element has a low melting point. The size of the element determines how much current it cancarry before heating to the melting point.

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This capacity is the ampere rating of the fuse. The rated current can flow through the element indefinitely.When a greater amount of current passes through the element, it becomes hot and melts. This opens the circuitand prevents a possible fir or other overcurrent damage.

Types of Fuses

Fuses are divided into two general categories-plug and cartridge. Each category has many types of fuses(Figure 1-2).

Figure 1-2. Types fuses

Plug fuses. Types of plug fuses that will be discussed in this lesson are standard, time-delay, type-S, and circuit-breaker.

• Standard. The standard fuse has an element designed to melt when the current through the fuseexceeds its rated amperage. Therefore, the standard fuse can only be used one time. Standard fuses areonly rated up to 30 amperes (Figure 1-3, page 1-4).

• Time-delay. The time-delay fuse is a dual-element fuse (Figure 1-4, page 1-4). It offers theprotection of a standard fuse for shorted circuits. It also provides protection against heating caused bylight overloads. This secondary protection prevents nuisance tripping, which is sometimes caused when

starting motor-driven appliances such as refrigerators and air conditioners. The cutaway of Figure 1-5,page 1-4, shows both elements of a time-delay fuse. Note how the fuse responds to shorts andoverloads.

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Figure 1-3. Standard fuse Figure 1-4. Time-delay fuse

Figure 1-5. Cutaway of a time-delay fuse

• Type-S. The type-S fuse has the operating characteristics of a time-delay fuse, and also the addedadvantage of being nontamperable. The type-S fuse is considered nontamperable because the fuse will

not fit into the base adapter unless it is the correct ampere rating for the circuit (Figure 1-6). Forexample, a 20-ampere fuse cannot be put into a 15-ampere base. The adapter base is designed to stay inplace once it is inserted into the panel.

• Circuit-breaker. The circuit-breaker fuse is a devise that can be screwed into a fuse panel, but it hasthe operating characteristics of a circuit breaker. This device can be rest after an overload (Figure 1-7).

Cartridge fuses. Cartridge fuses are the only type available for circuits rated over 30 amperes.

• Ferrule-contact. Cartridge fuses for 30 to 60 amperes have ferrule contacts (Figure 1-8).

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Figure 1-6. Circuit-breaker fuse

Figure 1-7. Type-S fuse and fuse base

Figure 1-8. Ferrule-contact cartridge fuse

• Knife-blade-contact. Cartridge fuses for 60 amperes and above have knife-blade contacts(Figure 1-9).

• Time-delay. Cartridge fuses with a time-delay feature are available at all ampere ratings(Figure 1-10).

Figure 1-9. Knife-blade-contact cartridge fuse

Figure 1-10. Time-delay cartridge fuses

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Replacing Fuses

The length and diameter of cartridge fuses increase in steps with the ampere rating. This limits, but does noteliminate, the possibility of replacing a fuse with one of the wrong size. Blown fuses must be replaced with fusesof the proper size and ampere value.

Testing Fuses

The cartridge fuses used in residential wiring provide no visible evidence of being blown, as plug fuses usually do.The only way to tell if a cartridge fuse is blown is to perform a continuity test on it (Figure 1-11).

Replacing the Element

Some blown cartridge fuses can be reused by installing a new fuse element in the fuse cylinder. To replace theelement unscrew the end cape, remove the blown element, and insert the new element. It is important to tighten

the end cape firmly when the new element is in place (Figure 1-12).

Figure 1-11. Checking a cartridge fuse

Figure 1-12. Replaceable element cartridge fuse

Using Spare Fuses

Fuses are a simple, highly reliable, and inexpensive way of providing overcurrent protection. Fuses have nomechanical parts to fail. When fuses blow, there is often no visible evidence. Therefore, it can be time-

consuming to locate, test, and replace blown fuses. Always keep spare fuses of the proper sizes near the panelbox.

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PART C: INSTALLING CIRCUIT BREAKERS

Circuit breakers combine the functions of manual disconnect and overcurrent protection in a single device.Circuit breakers are available in ratings of 15 to 200 ampere for residential use (Figure 1-13). Larger sizes aremade for commercial or industrial application. Circuit breakers, like fuses, are rated in amperes.

Figure 1-13. Typical 20-ampere circuit breaker

Internal Mechanisms

The internal mechanisms of residential-type circuit breakers consist of a bimetallic strip and spring-loaded

contacts as shown in Figure 1-14.

Figure 1-14 Circuit breaker

Bimetallic strip. This strip is made of two different metals, such as steel and bronze, fused together. It acts asa latch to hold the contacts together. When more than the rated current flows through the breaker, the heatmakes the two metals expand at different rates, causing the strip to bend (Figure 1-15, page 1-8).

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Figure 1-15. Bimetallic strip

Spring-loaded contact. When the bimetallic strip bends, the spring-loaded contacts are released and currentflow is interrupted. The contacts can also be opened by moving the switch to the OFF position.

Time-delay feature. The bimetallic strip requires time to heat up and trip the breaker. This provides a time-delay feature. Most breakers will carry one and one-half time their rated load for about one minute and as much

as three times their load for about five seconds. This provides enough delay to allow a motor-driven appliance toreach normal operating speed without tripping the breaker.

Positions

Most circuit-breaker switches have three positions.

• ON. The first is the ON position used during normal operating conditions.

• OFF. The second is the OFF position. To turn the power off in the circuit, simply move the switchto OFF.

• NEUTRAL. The third is the NEUTRAL position. If the breaker has been tripped by an overcurrentor overload, it will automatically switch itself to NEUTRAL and turn off the power.

Reset. In order to turn the power on again or to reset the breaker, move the switch to the OFF position firstand then to the ON position as shown in Figure 1-16.

Position indicators. The NEC requires that circuit breakers clearly show whether they are on or off. The ON

and OFF positions of switch-type circuit breakers can be seen on or near the switch. Push-button types have onand off indicators visible through an opening on the front of the breaker.

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Figure 1-16. Reset action

PART D: INSTALLING GROUND- FAULT

CIRCUIT INTERRUPTERS

Ground-fault circuit protection devices are required by the NEC in places where the user may come in contactwith moisture, such as bathrooms, basements, garages, and outside area. Ground-fault protection is not

overcurrent protection.

Leakage Paths

Ground faults can cause leakage paths of very small amounts of current and still be very dangerous. Leakage

paths occur when current flow is altered by a short circuit. The leakage current will flow from the short to aground point. A simple loose wire can cause a leakage path; for example, a loose hot wire in a light switch.Small amounts of electric current may be passing through the cover plate of the light switch if the hot wiretouches it even very slightly.

Fatal Shocks

As little current a 0.1 ampere can cause a fatal shock This can only happen, however, if the victim is in contactwith the ground or a grounded conductor. For example, if an electric hair dryer has a break in the insulationnear the plug on the power cord, current can flow from that break to any ground point. In bathrooms, many

exposed points, such as water faucets, metal sink, and decorative metal trim, are all possible ground points.

Shock factors. The degree of shock that a victim receives depends on two factors:

• The amount of current

• The length of time that the victim receive the current.

As stated previously, small amounts of current (0.1 to 0.2 ampere) can cause fatal shocks, but this small amount

of current will not cause a circuit breaker to trip. If a user receives

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a shock of 0.1 to 0.2 ampere, it can cause the muscles to freeze, so the user will be unable to release the liveconductor. Under these conditions, the current flow will continue to pass through the user's body and causeserious injury and perhaps even death.

Shock protection. GFCI devices have been developed to protect against shock hazard. Short circuits that createleakage paths of only 0.2 ampere will cause the GFCI to trip and cut off the power. The NEC requires thatGFCI protection be installed on all 120-volt, 15-and 20-ampere receptacle circuits outdoors and in areas such as

bathrooms and garages where the user may come in contact with moisture.

Neutral and Fault Paths

Under normal conditions, the current flow in any two-wire circuit is exactly the same in the hot wire and theneutral (white) wire. When a ground fault happens, current flow can follow two paths--the neutral path and thefault path. As in any parallel resistance circuit, the current now divides, with the heavier current flowing throughthe lower resistance.

In the example previously described, when the user provided a path for current flow from the insulation break toa ground point, current could flow through the device (hair dryer) to the neutral wire and also through the faultpath (the user) to the ground point

This second path (fault path) required an increase in the hot-wire current flow. The current flow in the hot wireand the neutral wire was no longer equal. This imbalance in current flow resulted from a ground fault and wassensed by the GFCI. The circuit was interrupted before serious injury occurred.

Parts

The GFCI contains a differential transformer, a sensing-and-testing module, and a magnetic switch (Figure 1-17).

Differential transformer. The differential transformer consist of a circular, iron-core secondary winding. Thecircuit power conductors act as the primary power source of the differential transformer. These conductors pass

through the center of the circular core. As current flows through the hot and neutral conductors, a magneticfield is created around each conductor. The strengths of the opposing magnetic fields are equal and remain in

balance. The fields cancel, and no current flows in the secondary.

Sensing-and-testing module. When the current in the hot wire becomes greater than the current in the neutralwire, the field of the hot wire increases, and current is induced in the differential secondary. This secondaryoutput is sensed and amplified by the sensing-and-testing module.

Magnetic switch. The module output activates the magnetic switch that cuts off the power to the load. The

GFCI also contains a test button to check the operation of the module and the switch.

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Types

There are three basic types of GFCI. All have two common features--a test button and a reset button. The testbutton simulates a leakage condition and ensures that proper tripping or turnoff occurs. The reset button restorescurrent flow after a test shutoff or an actual ground-fault shutoff.

Plug-in. The plug-in GFCI provides the simplest form of ground-fault protection. It consists of a small,

rectangular unit with plug prongs on the back side (Figure 1-18).

Figure 1-17. Components of a GFCIFigure 1-18. Plug-in GFCI

The front of this unit contain the test and reset buttons and either one or two three-prong receptacles. ThisGFCI is available for both two-wire, 120-volt and three-wire, 240-volt circuits in current ratings up to 30 amperes.It requires no special installation. It simply plugs into a receptacle. It has the advantage of simplicity and

portability. If many receptacles require ground-fault protection, using this unit at every receptacle would be quitecostly. In such a case, other methods of providing ground-fault protection should be considered.

Receptacle. To provide ground-fault protection for several receptacles on the same circuit, install a receptacleGFCI unit in the electrical box instead of a standard receptacle (Figure 1-19, page 1-12).

This provides ground-fault protection not only for the devices plugged into the GFCI receptacle but for alldevices plugged into receptacles between the GFCI receptacle and the end of the branch circuit. This GFCI

receptacle is also known as a feed-through unit. A receptacle GFCI can fit any electrical box 1 1/2 inches deepor deeper. It is available for two-wire, 120-volt circuits of 15 or 20 amperes.

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Figure 1-19. Receptacle GFCI

Combination. For ground-fault protection on any circuit, a combination GFCI unit is made to include bothcircuit-breaker and ground-fault protection in a single unit. This unit is installed into a service panel(Figure 1-20).

This GFCI has a white pigtail lead that must be connected to the neutral bus bar. The neutral and hot wires of the circuit must be connected to the GFCI unit. A combination GFCI is manufactured for two-wire, 120-voltand three-wire, 240-volt circuits in 15 to 30 ampere ratings.

Figure 1-20. Service-panel GFCI

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LESSON

PRACTICE EXERCISE

The following items will test your grasp of the material covered in this part of the lesson. There is only onecorrect answer for each item. When you complete the exercise, check your answer with the answer key thatfollows. If you answer any item incorrectly, study again that part of the lesson which contains the portion

involved.

1. What part of the fuse determines its ampere rating?

2. Plug fuses have the lowest ampere rating of all fuses. What is the maximum ampere rating of the standardfuse?

3. What additional feature does the time-delay fuse provide that the standard fuse does not?

4. The type-S fuse has a nontamperable advantage. Explain what this advantage is.

5. What special characteristic does the circuit-breaker fuse have that the other plug-type fuses do not have?

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6. Cartridge fuses have higher ampere ratings than plug fuses. What type cartridge fuse would you use in a 70-ampere circuit?

7. Fuses are a reliable means of providing overcurrent protection. They have two shortcomings. What arethese shortcomings?

8. Circuit breakers have a bimetallic strip in them. What happens to the metals of the bimetallic strip in anovercurrent situation?

9. Where are GFCI required by the NEC?

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THIS PAGE IS

INTENTIONALLY

LEFT BLANK

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LESSON

PRACTICE EXERCISE

ANSWER KEY AND FEEDBACK

Item Correct Answer and Feedback

1. The rating is determined by the amount of current that can be carried through the fuse element before itheats to the melting point. The fuse element is the correct response. (See page 1-3.)

2. Plug fuses are only rated up to 30 amperes. (See page 1-3.)

3. The time-delay fuse provides the protection of a standard fuse. It also offers added protection againstheating due to light overloads such as the starting loads of motors. (See page 1-3.)

4. The type-S fuse is nontamperable because it cannot be put into a base unless the sizes match. Each fusebase adapter is made for a particular fuse. Therefore, a 20-ampere fuse cannot be put into a 15-amperebase. (See page 1-4.)

5. The circuit-breaker fuse has a push-type reset button that allows the fuse to be reset. (See page 1-4.)

6. For circuits rated over 60 amperes, a knife-blade cartridge fuse is used. (See page 1-5.)

7. Cartridge fuses show no visible evidence of being blown. Fuses are time-consuming to test and replace,and they are not always readily available. (See page 1-6.)

8. When more than the rated current flows through the bimetallic strip, it heats the metals. Because themetals are different, one expands faster than the other, causing the strip to bend and release the spring-loaded contact. This interrupts the flow of current. (See page 1-7.)

9. Ground-fault circuit protection is required in places where the user may come in contact with moisture,such as in bathrooms, garages, or outside areas. (See page 1-9.)

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US Army Engineer course - Electricity VII (Install Circuit Protective Devices)