H-3 LEARNING GUIDE H-3 INSTALL PROTECTIVE DEVICES

Transcription

1 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Level 2 Line H: Install Electrical Equipment H-3 LEARNING GUIDE H-3 INSTALL PROTECTIVE DEVICES

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3 Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

4 Acknowledgements and Copyright Copyright 2011, 2014 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program. Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Beverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Dennis Evans, Photography A special thank you to Ken Holland at Camosun College for assisting Open School BC with additional photographs. Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia Copyright Permissions Photo of magnetic-only breaker used with permission of Schneider Electric Canada Inc., HIC breaker with current-limiting fuses used by permission of Eaton Corporation To order copies of any of the Electrician Apprenticeship Program Learning Guides, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: Toll Free: Fax: crownpub@gov.bc.ca Website: Version 1 Revised, April 2014 New August 2011

5 LEVEL 2, LEARNING GUIDE H-3: INSTALL PROTECTIVE DEVICES Learning Objectives Learning Task 1: Describe the application of circuit protection devices Self-Test Learning Task 2: Describe the characteristics and ratings of circuit protection devices Self-Test Learning Task 3: Describe the features of low-voltage fuses Self-Test Learning Task 4: Describe the features of low-voltage circuit breakers Self-Test Answer Key CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 5

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7 Learning Objectives H-3 Learning Objectives The learner will be able to identify protective devices for single-phase installations. The learner will be able to determine protective device requirements in single-phase installations. The learner will be able to describe procedures to test protective devices in single-phase installations. Activities Read and study the topics of Learning Guide H-3: Install Protective Devices. Complete Self-Tests 1 to 4. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources You are encouraged to obtain the following textbook for supplementary information: Canadian Electrical Code, Part 1 (latest edition); Published by the Canadian Standards Association. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 7

8 Learning Objectives H-3 BC Trades Modules We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

9 Learning Task 1: Describe the application of circuit protection devices Role of protective devices The purpose of a protective device is to open the circuit before any damage can be done to the conductor and equipment. There are several kinds of protective devices, and within each type there are many variations, classes and ratings. The circuit breaker and the fuse are the most commonly used types. A few, such as the motor relay and the rectifier fuse, provide specialized protection for individual pieces of equipment. In each case, however, all the devices serve the same purpose: they protect conductors and insulation from excess amounts of heat arising from overload and overcurrent. The Canadian Electric Code (CEC) specifies maximum ampacity values for protective devices, but you may use lower values. Definitions, causes and means of protection Overload Overload is a moderate increase in current beyond the normal rated current value. When a circuit is overloaded, equipment draws more current than it is designed or rated for. The heat produced by an overload quite often causes insulation to deteriorate and fail. If overheating is permitted to persist for any length of time, it can cause other problems as well, such as: Annealing, whereby the conductor undergoes a softening process that might cause it to flow out from under a connection. This can produce a loose connection and a high resistance at the connection and create even more heat. Rapid oxidation, which again produces increased resistance at terminals and connections. Burning insulation, which can give off toxic fumes. Cracking insulation, which might allow conductors to touch each other and produce a short circuit. Typical values of overload currents may range as high as six times the normal currents. Typical causes for an overload are: Too much equipment connected to a circuit, referred to as the octopus effect (Figure 1) Use of improper size equipment for example, small equipment for a big job (Figure 1) A worn-out or seized motor bearing, or excessive mechanical load. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 9

10 Learning Task 1 H-3 Figure 1 Typical causes of excessive mechanical load In an overload situation, the protective device must open the circuit before damage occurs. The circuit opens when a fuse link melts or a bimetallic strip causes an overload to trip (Figure 2). Figure 2 Protection against overload Overcurrent An overcurrent may range from six times to many hundreds of times the normal rated current, and, unlike an overload, it happens over a very short time. An overcurrent can result when: A motor starts. When a motor starts, it initially draws a current much larger than its normal full-load current. But even though this surge lasts a very short time, the protective device must be capable of withstanding the inrush current without opening. A short circuit occurs. Within a few thousandths of a second, a short-circuit current can become hundreds of times larger than the normal operating current. Two causes of a short circuit are: Failure of the conductor s insulation, which causes the conductor to touch a metal casing and create a line-to-ground short. A foreign object makes accidental contact, causing a line-to-line short (Figure 3). 10 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

11 Learning Task 1 H-3 Figure 3 Accidental connection by a foreign object Short circuit A short circuit is a larger-than-normal current that flows outside the normal current path. It is a parallel path of low resistance. A large amount of damage can occur very quickly in short circuits. The damage may be thermal, magnetic or the result of arcing. The magnetic damage can result in large explosions almost instantly. For this reason, short-circuit protective devices must respond as quickly as possible. Ground faults A ground fault is another form of short circuit. It is a short circuit to ground. Sometimes currents from such faults may be very small, resulting in a circuit current only slightly greater than normal (high resistance ground). Overload protection may be too slow to respond to this type of fault, and damage may occur before the fault is cleared. There are two main types of ground fault protective devices: one protects against fire damage; the other protects human life. Devices used to protect against ground faults are known as ground-fault circuit interrupters (GFCIs) or simply as ground-fault interrupters (GFIs). Arc Fault Circuit Interrupters (AFCIs) Starting with the 2002 edition of the Canadian Electrical Code, the CEC requires arc fault circuit interrupters (AFCIs) in all circuits that feed outlets in bedrooms of dwelling units. Conventional circuit breakers only respond to overloads and short circuits, so they do not protect against arcing conditions that produce erratic current. One of the leading causes of household fires in the home is arc faults. An AFCI is selective so that normal arcs (such as those which can occur when a switch is opened, or a plug is pulled from a receptacle) do not cause it to trip. The AFCI circuitry continuously monitors the current through the AFCI to discriminate between normal and unwanted arcing CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 11

12 Learning Task 1 H-3 conditions. Once the signal from an unwanted arcing condition (such as a lamp cord that has a broken conductor in the cord from overuse) is detected, the control circuitry in the AFCI trips the internal contacts, thus de-energizing the circuit and reducing the potential for a fire to occur. Although AFCIs resemble GFCIs in that they both have a test button, it is important to distinguish between the two. GFCIs are designed to protect against electrical shock while AFCIs are primarily designed to protect against arcing and/or fire. Both types of interrupters protect against overload and short circuit conditions as well. Effects of a short-circuit current A short-circuit current flows in a circuit during a fault. Usually this current is very large. Often, within a few thousandths of a second, it becomes hundreds of times larger than the normal operating current. It is limited only by the impedance of the circuit involved. Ohm s law states that a current in a circuit is directly proportional to the voltage applied and inversely proportional to the resistance or impedance. The familiar formulas are: I = E/R R = E/I E = IR Substituting impedance for resistance, these become: I = E/Z Z = E/I E = IZ Consider a system with a single-phase, 150 kva transformer and 2% impedance at 600 V, as shown in Figure 4. Figure 4 One-line diagram 12 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

13 Learning Task 1 H-3 When the transformer is fully loaded under normal conditions, it will deliver its rated output of 625 A at 240 V, single phase. But what determines the current that normally flows? First, note that it is the load in the secondary circuit that determines the current. Second, remember that the load is made up of resistance and reactance, which in combination is called impedance. It is the value of this impedance that determines the current. If a line-to-line fault occurs on the system between the transformer and the load (Figure 5), the load is now effectively bypassed. The total impedance is now reduced to that of the transformer alone. Figure 5 With a fault in the system, the load is bypassed and the impedance reduced The fault current will, of course, increase. The amount by which it increases depends on the total impedance of the transformer, which in this example is 2%. The anticipated fault current is equal to: Rated current of 625 A (100 2) = A (at transformer terminals). This is 50 times larger than the normal full-load current of the transformer. Again refer to Ohm s law. It is known that thermal energy is expressed as its current square multiplied by the resistance, as follows: P = I 2 R The longer this fault current is allowed to flow, the more the heat increases and the temperature rises. The protective device must be able to stop this enormous short-circuit current before any damage is done to the equipment or the circuit. The effects of the short-circuit current are as follows: Mechanical forces: High-fault currents, even if only a few thousandths of a second, exert terrific magnetic stress on bus bars and equipment. Buses can be bent out of shape, possibly to such an extent that new parallel fault paths develop. Thermal energy: The intense thermal energy that develops is known as I 2 R t. This large current, if allowed to flow for a longer time, can destroy electrical equipment. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 13

14 Learning Task 1 H-3 Such thermal energy is minimized by using a current-limiting device that clears a highfault current within the first half cycle. Figure 6 shows the current-limiting action of a fuse. The fuse opens within the first half cycle before the prospective fault current reaches its maximum value. I p is the amount of current let through before the fuse is open. Figure 6 Fuse let-through current wave Other facts about fault current Fault current is not normally present in a circuit, but is the maximum current that can flow should a fault occur. In effect, it is the transformer that produces this current during a fault. Fault current from the transformer terminals is approximately equal to 100 times the fullload transformer current divided by the percent impedance (%Z) of the transformer. How fast a circuit protection device must operate depends on how much current the equipment can withstand. How, then, is electrical equipment protected from damage by fault current? The most difficult solution is to design electrical systems to prevent faults. But this idea is unrealistic. Another solution might be to make every component wire, switches, breakers and so forth mechanically and electrically strong enough to withstand the mechanical and thermal devastation that potential fault currents at any point in the system can produce. Following through with this idea, however, could be costly. The usual method of protecting a circuit and equipment is to use a high-speed, specially constructed fuse. This fuse is called a high rupturing capacity (HRC) fuse. The principles of its operation are discussed in Learning Task 3. Now do Self-Test 1 and check your answers. 14 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

15 Learning Task 1 H-3 Self-Test 1 1. Explain the difference between overload and overcurrent. 2. An overcurrent may range up to many hundred of times the normal rated current. a. True b. False 3. What damage to a circuit can an overload cause? 4. What is the overall purpose of a protective device? 5. List three typical causes of overload. 6. When is a temporary overcurrent permitted? 7. What is a short-circuit current? CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 15

16 Learning Task 1 H-3 8. List two injurious effects of a short-circuit current. 9. When a fault occurs, what determines the amount of current flowing in the circuit? 10. What device in the circuit protects the circuit and equipment when a fault occurs? 11. What kind of fuse is designated by the letters HRC? 12. What formula will you use to calculate an amount of thermal energy? Go to the Answer Key at the end of the Learning Guide to check your answers. 16 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

17 Learning Task 2: Describe the characteristics and ratings of circuit protection devices Before you select a protective device for an installation, study all the factors influencing the situation. Critical factors are voltage rating, current rating, interrupting rating and time-current characteristics. Equally important is the kind of protection required. There are some applications only circuit breakers can satisfy, other applications only fuses can satisfy, and still others where a circuit breaker and fuse combination performs best. Where you can use either a breaker or a fuse, you should consider such other factors as cost, size, and personal experience and prejudices. Voltage rating Voltage rating relates to the ability of the protective device to: Quickly extinguish the arc after the fuse element melts or the circuit breaker opens Prevent the open-circuit voltage in the system from restriking across the open fuse element or across the circuit-breaker contacts The voltage rating of a fuse or a circuit breaker must be equal to or greater than the voltage of the circuit or system. For example, a 600 V fuse can be used on a 240 V system, but a 300 V fuse cannot be used on a 600 V system. Standard fuse voltage ratings are 600 V, 300 V, 250 V and 125 V. Current rating Every protective device has an ampere rating that has been determined under specified testcircuit conditions. The ampere rating is the current the device can carry continuously without opening, deteriorating or exceeding prescribed temperature limits. Standard fuse ratings are 15 A, 20 A, 30 A, 40 A, 60 A, 100 A, 200 A, 300 A, 400 A, 600 A, 800 A, 1200 A and 1600 A. Before you select a fuse or breaker for an installation, consider both the type of load and the Code requirements for that installation. Normally, the ampere rating of the fuse or breaker should not exceed the current-carrying capacity of the circuit. But there are applications where the ampere rating of the fuse or breaker is permitted to be larger than the current-carrying capacity of the circuit. A typical example is a motor circuit. Interrupting rating Interrupting rating is the maximum short-circuit current an overcurrent protective device can safely interrupt without damaging itself. The Code states that overcurrent protective devices shall ensure safe operation and have interrupting capacity sufficient for the voltage employed and for the anticipated fault current that must be interrupted. HRC fuses commonly have an interrupting rating of A, which is sufficient for many applications. Many circuit breakers and older-style fuses have an interrupting rating of A. When you select a protective device, consider its interrupting rating. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 17

18 Learning Task 2 H-3 Time-current characteristics For any fuse, time-current characteristics in graph form are available from manufacturers. Generally, the more severe the fault (the higher the fault current), the faster the fuse responds. Time-current characteristics compare fault magnitude and speed of response. Fuse manufacturers publish their test information so that engineers can coordinate the fuses within a system. To minimize inconvenience, it is better if the fuse closest to the fault in the circuit opens first. To make sure this happens, engineers must use the manufacturer s published data to select fuses with the proper response speed. This process is often referred to as selective coordination or simply fuse coordination. Figure 1 shows the difference in the melting curve of a manufacturer s 100 A, dual-element (FRN R 100), time-delay fuse, and that of a 100 A fuse without a time delay (KTN R 100). At 500 A, the dual-element fuse melts in about 10 seconds; the fuse without a time delay melts in 0.2 seconds. A brief inrush current is encountered when most motors are started. Motor-starting currents of four to six times full-load current occur for up to several seconds. A fuse with a built-in time delay allows a motor to start without the fuse opening unnecessarily. The Canadian Electrical Code requires that time delay and non-time delay fuses be readily distinguishable from each other: The time-delay fuses must be marked with the letter D for delay. The non-time-delay fuses must be marked with the letter P for low melting point. 18 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

19 Learning Task 2 H-3 Figure 1 Time-current characteristic curve Courtesy of Bussmann Division, Cooper Industries Inverse time-current characteristics Refer back to Figure 1. The curve of the 100 A fuse without a time delay (KTN-R 100) shows its time-current characteristics. The graph shows that the fuse opens at 10 seconds when an excessive overload current of 200 A is flowing through it. If the overload current is larger, such as 500 A, this fuse opens much earlier, at 0.2 seconds. This curve shows that the higher the current, the quicker the fuse opens. This is commonly referred to as an inverse time-current characteristic. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 19

20 Learning Task 2 H-3 Other considerations Besides those instances where the current exceeds the fuse s current rating, a fuse may occasionally open: When the ambient temperature is high, or when there is excessive vibration. When the connection is loose or dirty. (Electricity cannot flow smoothly through a loose or dirty connection, and result of this poor contact is excessive heat.) Replacing a fuse When you replace a fuse, bear in mind the points discussed in this Learning Task. Always observe the correct: Voltage rating Current rating Interrupting-capacity rating Time-current characteristic. Now do Self-Test 2 and check your answers. 20 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

21 Learning Task 2 H-3 Self-Test 2 1. Explain the significance of the following when you select a fuse: Voltage rating Interrupting-capacity rating 2. What should you consider when you replace a fuse? 3. Besides a current that exceeds the rating, what two other conditions may cause a fuse to open? 4. List the standard voltage ratings of low-voltage fuses. 5. What is the interrupting rating expected of an HRC fuse? 6. Will a 100 A fuse open instantly at 101 A? Explain why it will or will not. Go to the Answer Key at the end of the Learning Guide to check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 21

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23 Learning Task 3: Describe the features of low-voltage fuses A fuse is a simple device that protects the conductor of a circuit. A fuse is an insulated tube containing a strip of heat-sensitive metal that has a lower melting point than copper or aluminum. The metal link heats up before the circuit conductors do. Then, when excessive current is passed through the conductors, the fuse automatically opens a circuit. In a short circuit, the fuse element melts in just a fraction of a second. Fuses are available in various sizes, with different ratings and features. There are two general categories: Plug fuses Cartridge fuses Plug fuses The plug fuse is sometimes called the screw-base or Edison-base fuse. Figure 1 shows two basic types: Type W for standard non-time delay, Type D for time delay. Figure 1 Type W (left) and Type D (right) plug fuses The maximum rating of any plug fuse is 30 A. The ratings generally available are 15 A, 20 A, 25 A and 30 A. There are, however, lower ratings for special applications ratings such as 1 A, 2 A, 3 A, 5 A, 6 A, 8 A and 10 A. A plug fuse is not to be used in a circuit with more than 125 V between conductors, except where the circuit is supplied from a system with a grounded neutral and there is no conductor operating at more than 150 V to ground. The Code states that plug fuses with different ratings must not be interchangeable. There are two non-interchangeable plug fuse types available: Type C and Type S. Type C fuses Before a type C plug fuse can be inserted into the panelboard, a rejection ring must be placed in the fuse holder (Figure 2). The thickness and size of the hole in the holder prevent fuses with different ratings from making contact. The rejection ring is identified by colour: blue for 15 A or less, pink for 20 A or less, and green for 30 A or less. The type C fuse is now being replaced by the type S non-interchangeable. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 23

24 Learning Task 3 H-3 Figure 2 Type C fuse and rejection ring Type S fuses Before you can insert a type S plug fuse (Figure 3), a rejection adapter must be installed in the standard fuse holder. Different-sized adapters are used with fuses with different ratings. To ensure non-interchangeability, the adapter is spring-locked once installed it cannot be removed. Figure 3 Type S fuse with adapter Cartridge fuses Cartridge fuses are manufactured in ferrule-contact, knife and bolt-on styles. All three styles are used in standard fuses and high-rupturing-capacity fuses. Standard fuses (Code fuses) Class H The term code fuse was coined a long time ago when this was the only type of fuse required by the Electrical Code. The construction of the ferrule-contact fuse and knife-blade fuse used today is similar; the only difference is that the knife type is larger than the ferrule contact and has blades on both ends. Figure 4 shows these two standard fuses. Ferrule contact (available up to 60 A) Knife blade (Available up to 600 A) Figure 4 Standard cartridge fuses: ferrule contact and knife blade 24 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

25 Learning Task 3 H-3 The fuse is designed to be the weakest link in the circuit. Its job is to break the circuit before damage is done to the conductors. Many of the large cartridge fuses are filled with an arcquenching powder (Figure 5). This is a fire-extinguishing powder that prevents an arc from continuing or bursting through the tubing of the fuse. As a short circuit often causes the fuse link to burst into flame, the arc-quenching powder is important. Figure 5 Fuse with arc-quenching powder Renewable fuse links If a fuse element is not renewable, it must be thrown away when it is open or melts. It is, in other words, a one-time fuse. In contrast, the renewable fuse body (Figure 6) can be used over and over again, and thus it provides low maintenance costs over a long time. When you replace a fuse element, it is important to use one with the proper current rating. The fuse must not be spiked that is, you must not install a larger link, or more than one link, in the fuse than is necessary. (All renewable fuses are non-time-delay.) CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 25

26 Learning Task 3 H-3 Figure 6 Renewable fuses Overload situation If, as previously mentioned, a circuit overload exceeds a fuse s current rating for a sufficient length of time, the fuse element melts. The arc drawn by the resulting gap or break in the element burns back the metal of the element and lengthens the gap. When the gap is large enough to break the arc, the circuit is open. The circuit opens even sooner if there is arc-quenching powder around the fuse element to help interrupt the arc. Figure 7 shows the sequence. 26 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

27 Learning Task 3 H-3, Figure 7 How a link reacts to an overload Courtesy of Gould Shawmut Short-circuit situation When a short circuit or ground fault occurs, several sections of the link melt instantly (Figure 8). The large flow of current causes portions of the link to vaporize. Again, the arc-quenching powder helps to extinguish the arc while it is still safely contained in the fuse tubing. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 27

28 Learning Task 3 H-3 Figure 8 How a link reacts to a short circuit Courtesy of Gould Shawmut Time-delay features As noted in Learning Task 2, a fuse that delays opening in an overload situation is called a timedelay fuse. The time required for the fuse to open depends on its time-current characteristic curve. The time-delay fuse (Figure 9) consists of two fuse links joined by a thermal cutout unit connected in series. The thermal cutout unit, or overload element, has a copper centre (a heat sink) and a spring-loaded connector. The connector, which connects the fuse elements, is held in place by solder with a low melting point. Figure 9 Cutaway view of time-delay mechanism The time-delay fuse is sometimes referred to as a dual-element fuse. CSA standards require that this fuse be marked with the letter D, and the Code requires it on cycling loads such as motor circuits. Type P fuses Every fuse, regardless of type, must continuously carry its normal rated current. It must also be able to interrupt the fault current (short circuit). Almost all standard fuses have an interrupting capacity rating of A. But there is a new one-time fuse manufactured to exceed CSA specifications. This type P fuse has a low melting point and an interrupting capacity rating of A. High-rupturing capacity (HRC) fuses High-rupturing capacity (HRC) fuses interrupt large short-circuit currents without disintegrating. At the same time they protect cables and equipment. 28 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

29 Learning Task 3 H-3 As well, HRC fuses do not deteriorate. They have moisture-proof fibreglass or ceramic barrels, filled with high-quality silica sand for arc-quenching. Under heavy current and short-circuit conditions, the silica sand quickly turns into a glasslike material, which blocks the formation of an arc between the fuse ends. Finally, HRC fuses are non-interchangeable that is, they are designed in such a way that other fuses will not fit into their holders. This is a safeguard to stop you from substituting a fuse with a lower interrupting capacity rating. HRC fuse designations There are two basic categories of HRC fuses: HRCI (previously called Form I) and HRCII (previously called Form II). The HRCI fuse provides protection from both overloads and short circuits in cables and equipment. It has a high interrupting capacity (usually A) and is made in a variety of types and classes. The HRCII fuse gives short-circuit protection only. If it is used for over-current protection on a motor, an overload device must also be installed. Common HRCI and HRCII fuses and their current ratings are described below. As you study them, bear in mind that these descriptions may differ somewhat from those of manufacturers who may have another way of designating types and ratings. Class R fuses (Figure 10) Available in time-delay and fast-acting types. Normal current ratings range from 0 to 600 A in both the 250 V and 600 V category, with A interrupting capacity. Provides a rejection feature and is non-interchangeable. Previous CSA designation of HRCI. Figure 10 Class R fuses CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 29

30 Learning Task 3 H-3 Class J fuses (Figure 11) Available in time-delay and fast-acting types, from 0 to 600 A in 600 V ratings, with A interrupting capacity. Designed with lower resistance than standard fuses, which makes it a cooler operating fuse. Designed with an end-to-end length shorter than that of other fuses to make it noninterchangeable. Previous CSA designation of HRCI-J. Figure 11 Class J fuses Class T fuses There are two kinds of Class T fuses, formerly designated HRCI-T and HRC-T. HRCI-T (Figure 12): Current ratings from 0 to 600 A, available in both 300 V and 600 V ratings with A interrupting capacity. Very fast acting. Extra-small size provides non-interchangeability. Figure 12 Class HRCI-T fuses 30 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

31 Learning Task 3 H-3 HRC-T (Figure 13): Current rating from 601 to 1200 A for 300 V and from 601 to 800 A for 600 V, with A interrupting capacity. Very fast acting. Class L fuses (Figure 14) Figure 13 Class HRC-T fuse The Class L fuse is an extension of the Class J fuse. Provides both overload and overcurrent protection and is available in time-delay and fastacting types, from 601 to 6000 A in 600 V rating. Used in large-current installations such as those for service and feeder protection. Has A interrupting capacity. Non-interchangeable because of its different size and mounting holes. Previous CSA designation of HRC-L. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 31

32 Learning Task 3 H-3 Figure 14 Class L fuse Three new classes of Form I fuses are now available: Class CA, Class CB and Class CC. Although their ratings are similar to each other, their time and physical characteristics are different. Class CA (Figure 15) Current ratings from 0 to 30 A in 600 V, with A interrupting capacity. Maximum peak let-through current of 8000 A. Non-interchangeable. Previous CSA designation of HRCI-CA. Figure 15 Class CA fuse 32 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

33 Learning Task 3 H-3 Class CB (Figure 16) Current ratings from 0 to 60 A in 600 V, with A interrupting capacity. Maximum peak let-through current of A for 30 A or less, A for A. Non-interchangeable. Previous CSA designation of HRCI-CB. Class CC (Figure 17) Figure 16 Class CB fuse Current ratings from 0 to 30 A in 600 V, with A interrupting capacity. Maximum peak let-through current of A. Non-interchangeable. Previous CSA designation of HRCI-CC. Figure 17 Class HRCI-CC fuse Class C (Previously designated HRCII-C) This HRC fuse (Figure 18) gives short-circuit protection only (no time delay). There are two classes: Class C and HRCII-Misc. Class C: This fuse has a high interrupting capacity of A. Its current rating is from 0 to 1200 A in 600 V. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 33

34 Learning Task 3 H-3 HRCII-Misc: Figure 18 Class C fuse Current ratings from 0 to 1200 A in 600 V, with A interrupting capacity. Non-dimensional (must not be interchangeable with standard fuses). CSA designation of Misc will remain in use for fuses that do not have a Class designation. Other fuses In addition to the fuses discussed above, there are many different fuses for special applications, but their features are similar to those just described. These other types include the following: Semiconductor fuse: A, interrupting rating. Designed for semiconducting devices such as SCR, diodes, thyristors, triacs and transistors. Similar to rectifier fuses. Midget fuse: A, interrupting rating, small physical size. Electronic fuse: For electronic and communication equipment. Special fuses: Various shapes and ratings. Used in automobiles. In-line, panel, flat blade types. Locomotive fuse, lift-truck fuse, capacitor fuse and so forth. Current-limiting ability A fuse must have adequate interrupting capacity to safely interrupt the maximum fault current. This means that the fuse must open without damaging the equipment. A fuse with currentlimiting ability opens before the fault current reaches its maximum value. Summary When selecting a fuse, consider its current and voltage ratings, and its interrupting capacity. Standard fuses must not be used in circuits with a current over 600 A and a voltage over 600 V. All fuses must have ratings appropriate for interrupting anticipated short-circuit currents. Now do Self-Test 3 and check your answers. 34 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

35 Learning Task 3 H-3 Self-Test 3 1. What maximum current and voltage ratings are available for plug fuses? 2. Explain why a fuse must not be interchanged with a fuse with a different rating. 3. What letter is used to indicate a time-delay fuse? 4. What is the maximum interrupting-capacity rating for standard fuses? 5. What fuse term is commonly used instead of time delay? 6. What are the maximum current and voltage ratings for standard cartridge fuses? 7. Explain the difference between Class CA and Class C fuses. 8. Under what condition must an HRC fuse be used for circuit protection? Go to the Answer Key at the end of the Learning Guide to check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 35

36 36 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

37 Learning Task 4: Describe the features of low-voltage circuit breakers The Canadian Electrical Code defines a circuit breaker as an electro-mechanical device designed to open a current-carrying circuit under both overload and short circuit conditions without injury to the device. It refers only to the automatic type designed to trip on a predetermined overcurrent. The Canadian Electrical Code requires that all circuit breakers be trip-free. This means the internal tripping mechanism must trip on an overload or short circuit even if the operating lever is held in the ON position. It is free to trip. Circuit breakers are available in single-pole, double-pole and triple-pole models. There are four main types of moulded-case circuit breakers: Thermal Thermal-magnetic Magnetic Solid state Function of the circuit breaker The circuit breaker (Figure 1) protects connected apparatus against overloads and short circuits in low-voltage distribution systems under 750 V. It is often used to control lighting and motor circuits. Figure 1 Circuit breaker CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 37

38 Learning Task 4 H-3 One of the greatest advantages of the circuit breaker is that it is resettable. If the breaker trips, it can be reset by hand without replacing any parts. In a short circuit, arcing is produced within the breaker s moulded case. This arcing must be extinguished at once to prevent restriking. Some breakers have a built-in arc chute. The chute forces the arc into a parallel arc chamber, where it is broken into smaller segments that are quickly extinguished (Figure 2)., Figure 2 Interruption of a current Construction of the moulded-case circuit breaker In a moulded-case circuit breaker, all the circuit-breaker components are mounted inside a frame of insulating material. Each type of moulded case is identified on the frame by the manufacturer s letters. This identification refers to certain characteristics of the breaker, such as maximum allowable voltage and current, interrupting capacity, and physical dimensions. Manufacturers each have their own identification system, as their breakers have unique characteristics. Although manufacturers make different types of circuit breakers, all have the same major components (Figure 3): Frame, or case Operating mechanism Trip elements Contacts 38 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

39 Learning Task 4 H-3 Terminal connection But not all types have an arc extinguisher. Moulded case (frame) Operating mechanism Arc extinguishers Contacts Trip elements Terminal connectors Figure 3 Moulded-case circuit breaker Operation The handle turns the circuit breaker ON and OFF. It is a quick-make, quick-break type, meaning that the speed with which the contacts snap open or closed is independent of how fast the handle is moved. The breaker is also trip-free, which means it cannot be prevented from tripping by holding the handle in the ON position. When the breaker trips, the handle moves midway between ON and OFF. To reset, the handle must be moved from the centre position to OFF, then to ON. Trip elements The trip element trips the operating mechanism under prolonged overload or excessive current conditions and causes the circuit to open. This protection is triggered either by a thermal trip or by a magnetic trip. Many circuit breakers have both trip elements. Thermal element This heat-sensitive element is a bimetal strip composed of two dissimilar metals. Each metal has a different rate of thermal expansion, so heat from excessive current or overload will cause one of the metals to bend and force the other over with it. This deflection triggers the opening of the circuit (Figure 4). CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 39

40 Learning Task 4 H-3 Figure 4 Bimetal strip heats and bends to open overload contact Magnetic element This short-circuit and ground-fault protection element is an electromagnetic device wired in series with the load. When a short circuit occurs, the fault current passing through the circuit causes the electromagnet to attract an armature mounted on the trip bar. This action opens the contacts (Figure 5). Figure 5 Magnetic trip element Other types of circuit breakers Magnetic-only breaker The Canadian Standards Association calls this breaker an instantaneous trip circuit interrupter. It looks like the standard thermal-magnetic breaker except that it has no thermal trip element. This breaker normally has an adjustable trip element on the front (Figure 6). These trip elements are interchangeable within the limits of frame size. The magnetic-only circuit breaker provides short-circuit protection but no protection from overload. It is used where overload protection is provided in some other way. 40 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

41 Learning Task 4 H-3 Figure 6 Instantaneous trip or magnetic-only breaker Photo of magnetic-only breaker used with permission of Schneider Electric Canada Inc., High-interrupting-capacity breaker In many low-voltage distribution systems, the available short-circuit current often exceeds the interrupting capacity of standard breakers. Thus a special high-interrupting-capacity (HIC) breaker is required. The HIC breaker is similar to, and the same size as, the standard thermalmagnetic breaker, except that it provides a higher interrupting capacity. Some manufacturers offer an interrupting capacity of A or higher. As well, a current-limiting device may be integrally installed to provide the breaker with very high interrupting capacity. Solid state breaker In the solid state breaker, a current transformer and semiconductor circuitry replace the conventional thermal-magnetic trip units. The current transformer not only monitors the line current but also reduces it to a lower level for the solid state circuitry. A prolonged overload or a short circuit causes the circuitry to initiate an output to a low-power, shunt-trip solenoid to trip the breaker. Ground-fault interrupter (GFI) The ground-fault circuit breaker is a thermal-magnetic breaker that incorporates a solid state, ground-fault sensing circuit to detect ground currents of 5 ma or greater. The ground-fault section consists of a window-type current transformer monitor (CT), an amplifier and a shunt trip coil. Two wires from the 120 V load pass through the monitor. Normally the currents flowing through the load wires are equal, so no current flows in the secondary winding of the CT monitor. But if a ground fault occurs, or a current leaks to ground, some of the current does not return through the monitor. The supply current then exceeds the current returning through the monitor. In this situation, the unbalanced current is amplified and fed to the shunt trip, which opens the breaker (Figure 8). CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 41

42 Learning Task 4 H-3 Figure 7 Ground-fault circuit breaker and wiring diagram Now do Self-Test 4 and check your answers. 42 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

43 Learning Task 4 H-3 Self-Test 4 1. List the five major components of a circuit breaker. 2. What term is used to describe a breaker that trips even though its handle is held in the ON position? 3. What is the purpose of a circuit breaker? 4. How does a circuit breaker protect equipment from an overload? 5. What is the name of the breaker that provides overcurrent protection only? 6. What type of breaker must be used in installations where a short-circuit current could exceed the interrupting capacity of a standard circuit breaker? CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 43

44 Learning Task 4 H-3 7. What is the minimum tripping current of a ground-fault circuit interrupter? 8. What is the operating principle of a ground-fault circuit interrupter? Go to the Answer Key at the end of the Learning Guide to check your answers. 44 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2

45 Answer Key H-3 Answer Key Self-Test 1 1. Overload: a moderate increase to an above-normal current over a period of time. Overcurrent: a current much larger than its normal current over a very short period of time. 2. a. True 3. heat damage resulting in insulation breakdown, loose connections, high resistance connections or fire. 4. Protective devices protect a circuit from being damaged by overload or overcurrent. 5. Too much equipment plugged into a circuit (called the octopus effect) Improper size equipment Seized or worn-out motor bearing or excessive mechanical load 6. a temporary overcurrent that is necessary for the normal starting of a motor 7. A short-circuit current is the current that flows in a circuit during a fault condition. 8. Mechanical stress: bus bars may be bent out of shape Thermal energy: the heat generated may melt the insulation of wire 9. the impedance of the circuit alone 10. an overcurrent protection device that opens the circuit safely 11. HRC means a fuse with high rupturing capacity. 12. The amount of thermal energy depends on the magnitude of the fault current and the impedance. This is expressed as: J(Energy) = I 2 R t Self-Test 2 1. Proper voltage rating is required in order to prevent the arc from restriking. The fuses must have interrupting capacity higher than the anticipated short-circuit current. 2. Voltage rating Current rating Interrupting capacity Time-current characteristics 3. High ambient temperature Excessive vibration CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2 45

46 Answer Key H V, 250 V, 300 V, 600 V A 6. No. Temporary overloads should be tolerated. The opening time of the fuse depends on the type of fuse and the magnitude of overcurrent. Self-Test A, 150 V to ground 2. to prevent over-fusing by replacing a lower rated fuse with a higher rated one 3. Type D A 5. dual-element A, 600 V 7. Class CA: provides overload and overcurrent protection Class C: provides overcurrent protection only 8. when the fault current exceeds A Self Test 4 1. frame, operating mechanism, terminals, trip elements, contacts 2. trip-free 3. to open the circuit under overload or overcurrent conditions 4. Overload is handled by a thermal action that causes a bimetal strip to bend and activate a trip mechanism. 5. instantaneous trip circuit interrupter, or magnetic-only circuit breaker 6. high-interrupting-capacity (HIC) circuit breaker 7. 5 ma 8. It senses the unbalanced current between the supply and the returning load wires. This unbalanced current is amplified and channelled to a shunt, which opens the breaker. 46 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 2