INSTALL CIRCUIT PROTECTIVE DEVICES

Transcription

1 SUBCOURSE EN5146 EDITION B US ARMY ENGINEER CENTER AND SCHOOL INSTALL CIRCUIT PROTECTIVE DEVICES

2 INSTALL CIRCUIT PROTECTIVE DEVICES Subcourse Number EN5146 EDITION B United States Army Engineer School Fort Leonard Wood, Missouri Credit Hours Edition Date: November 1998 SUBCOURSE OVERVIEW This subcourse is designed to teach the knowledge necessary to perform tasks related to installing fuses, circuit breakers, 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: CONDITION: STANDARD: You will describe the procedures used to install circuit protective devices. You will be given subcourse booklet EN5146 and an examination response sheet To demonstrate competency of this task, you must achieve a minimum of 70% on the subcourse 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 permission from McGraw-Hill Book Company, New York. i

3 TABLE OF CONTENTS Section Page Subcourse Overview...i Lesson: Install Circuit Protective Devices Part A: Identifying Circuit Protective Devices Part B: Installing Fuses Part C: Installing Circuit Breakers Part D: Installing Ground-Fault Circuit Interrupters Practice Exercise Answer Key and Feedback Appendix A: List of Common Acronyms...A-1 Appendix B: Recommended Reading List... B-1

4 LESSON INSTALL CIRCUIT PROTECTIVE DEVICES Critical Task: 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: CONDITION: STANDARD: REFERENCES: Describe the procedures used to install circuit protective devices. You will be given subcourse booklet EN5146 and an ACCP examination response sheet You will work at your own pace and in your own selected environment with no supervision. Within approximately 2 hours, you should be able to study the lesson resources and complete the practice exercise. 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 from National Fire Protection Association Publications, Batterymarch Park, Quincy, MA 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. 1-1

5 PART A. IDENTIFYING CIRCUIT PROTECTIVE DEVICES Circuit protective devices are installed in the circuit to protect it and other equipment in the circuit from an overcurrent or overload. The words overcurrent and overload are used interchangeably, but thy have different meanings. 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 heavy current 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 protective devices. 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 because a fuse contains a thin metal strip called a fuse element. This strip provides protection against overcurrent and overload. The element has a low melting point. The size of the element determines how much current it can carry before heating to the melting point. 1-2

6 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 circuit and prevents a possible fir or other overcurrent damage. Types of Fuses Fuses are divided into two general categories-plug and cartridge. (Figure 1-2). Each category has many types of fuses 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 circuitbreaker. Standard. The standard fuse has an element designed to melt when the current through the fuse exceeds its rated amperage. Therefore, the standard fuse can only be used one time. Standard fuses are only 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 the protection of a standard fuse for shorted circuits. It also provides protection against heating caused by light 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 and overloads. 1-3

7 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 added advantage 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). For example, a 20-ampere fuse cannot be put into a 15-ampere base. The adapter base is designed to stay in place 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 has the 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). 1-4

8 Figure 1-7. Type-S fuse and fuse base Figure 1-8. Ferrule-contact cartridge fuse Figure 1-6. Circuit-breaker 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 Time-delay cartridge fuses 1-5

9 Replacing Fuses The length and diameter of cartridge fuses increase in steps with the ampere rating. This limits, but does not eliminate, the possibility of replacing a fuse with one of the wrong size. Blown fuses must be replaced with fuses of 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 the element 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 Checking a cartridge fuse Figure Replaceable element cartridge fuse Using Spare Fuses Fuses are a simple, highly reliable, and inexpensive way of providing overcurrent protection. Fuses have no mechanical parts to fail. When fuses blow, there is often no visible evidence. Therefore, it can be timeconsuming to locate, test, and replace blown fuses. Always keep spare fuses of the proper sizes near the panel box. 1-6

10 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 are made for commercial or industrial application. Circuit breakers, like fuses, are rated in amperes. Figure 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 Figure 1-14 Circuit breaker Bimetallic strip. This strip is made of two different metals, such as steel and bronze, fused together. It acts as a latch to hold the contacts together. When more than the rated current flows through the breaker, the heat makes the two metals expand at different rates, causing the strip to bend (Figure 1-15, page 1-8). 1-7

11 Figure Bimetallic strip Spring-loaded contact. When the bimetallic strip bends, the spring-loaded contacts are released and current flow 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 to reach 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 switch to OFF. NEUTRAL. The third is the NEUTRAL position. If the breaker has been tripped by an overcurrent or 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 first and then to the ON position as shown in Figure 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 on and off indicators visible through an opening on the front of the breaker. 1-8

12 Figure 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 contact with 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 a ground 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 wire touches 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 contact with the ground or a grounded conductor. For example, if an electric hair dryer has a break in the insulation near 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 1-9

13 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 live conductor. Under these conditions, the current flow will continue to pass through the user's body and cause serious injury and perhaps even death. Shock protection. GFCI devices have been developed to protect against shock hazard. Short circuits that create leakage paths of only 0.2 ampere will cause the GFCI to trip and cut off the power. The NEC requires that GFCI 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 the neutral (white) wire. When a ground fault happens, current flow can follow two paths--the neutral path and the fault path. As in any parallel resistance circuit, the current now divides, with the heavier current flowing through the lower resistance. In the example previously described, when the user provided a path for current flow from the insulation break to a ground point, current could flow through the device (hair dryer) to the neutral wire and also through the fault path (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 wire and the neutral wire was no longer equal. This imbalance in current flow resulted from a ground fault and was sensed 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. The circuit 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 magnetic field 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 neutral wire, the field of the hot wire increases, and current is induced in the differential secondary. This secondary output 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. 1-10

14 Types There are three basic types of GFCI. All have two common features--a test button and a reset button. The test button simulates a leakage condition and ensures that proper tripping or turnoff occurs. The reset button restores current 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 Components of a GFCI Figure Plug-in GFCI The front of this unit contain the test and reset buttons and either one or two three-prong receptacles. This GFCI 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 quite costly. 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 receptacle GFCI 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 all devices 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 deep or deeper. It is available for two-wire, 120-volt circuits of 15 or 20 amperes. 1-11

15 Figure Receptacle GFCI Combination. For ground-fault protection on any circuit, a combination GFCI unit is made to include both circuit-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-volt and three-wire, 240-volt circuits in 15 to 30 ampere ratings. Figure Service-panel GFCI 1-12

16 LESSON PRACTICE EXERCISE The following items will test your grasp of the material covered in this part of the lesson. There is only one correct answer for each item. When you complete the exercise, check your answer with the answer key that follows. 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 standard fuse? 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? 1-13

17 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 are these shortcomings? 8. Circuit breakers have a bimetallic strip in them. What happens to the metals of the bimetallic strip in an overcurrent situation? 9. Where are GFCI required by the NEC? 1-14

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19 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 it heats 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 against heating 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 fuse base adapter is made for a particular fuse. Therefore, a 20-ampere fuse cannot be put into a 15-ampere base. (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 the metals are different, one expands faster than the other, causing the strip to bend and release the springloaded 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.) 1-16

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