INSTALL SERVICE- ENTRANCE SYSTEMS

Transcription

1 SUBCOURSE EN5141 EDITION B US ARMY ENGINEER SCHOOL INSTALL SERVICE- ENTRANCE SYSTEMS

2 INSTALL SERVICE- ENTRANCE SYSTEMS Subcourse Number EN5141 EDITION B United States Army Engineer School Fort Leonard Wood, Missouri Credit Hours Edition Date: June 1999 SUBCOURSE OVERVIEW This subcourse is part of Soldier Training Publication (STP) 5-51R12-SM-TG, Install Service Entrance Systems. It is designed to teach the knowledge and skills necessary to perform tasks related to installing service-entrance (SE) systems. The subcourse is presented in two lessons, each corresponding to a terminal learning objective as indicated below. There are no prerequisites for this subcourse. This subcourse reflects the doctrine which was current at the time the subcourse 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. TERMINAL LEARNING OBJECTIVE ACTION: CONDITION: You will learn the procedures used to install SE systems. You will be given the material in this subcourse and an Army Correspondence Course Program (ACCP) examination response sheet. STANDARD: To demonstrate competency of this task, you must achieve a minimum of 70 percent on this subcourse. i EN5141

3 TABLE OF CONTENTS Section Page Subcourse Overview...i Administrative Instructions...iii Grading and Certification Instructions...iii Lesson 1: Overhead Entrance Systems Practice Exercise Answer Key and Feedback Lesson 2: Underground Service Practice Exercise Answer Key and Feedback Examination...E -1 Appendix A: List of Common Acronyms... A-1 Appendix B: Recommended Reading List...B-1 Student Inquiry Sheets ii EN5141

4 LESSON 1 OVERHEAD ENTRANCE SYSTEMS Critical Task: OVERVIEW LESSON DESCRIPTION: In this lesson you will learn to describe an overhead entrance system. TERMINAL LEARNING OBJECTIVE: ACTION: CONDITION: STANDARDS: REFERENCES: You will learn the procedures used to install an overhead entrance system. You will be given subcourse booklet EN5141 and an ACCP examination response sheet. You will work at your own pace and in your own selected environment with no supervision. You will correctly answer practice exercise questions at the end of this lesson. The material contained in this lesson was derived from STP 5-51R12-SM-TG and FM INTRODUCTION Overhead entrance systems are the conductors and the equipment used for the control and protection of electrical circuits, equipment, and personnel. The SE is the end of the interior electrical system. This lesson will teach you how to describe an overhead entrance system. The conductors from the power pole to the building are called service drops as shown in Figure 1-1, page 1-2. The minimum height of a service drop above the ground and on the building is determined by its location. 1-1 EN5141

5 Figure 1-1. Clearances for a service drop Above foot-traffic areas, 10 feet of vertical clearance is necessary for service drops supported and wrapped on a ground messenger having 150 volts to ground between the current-carrying conductors (Figure 1-2). Remember, the 10-foot minimum clearance is measured from the lowest point of the SE cable. Figure 1-2. Service drop above a foot-traffic area 1-2 EN5141

6 Above residential property, driveways, and areas not subject to truck traffic, 12 feet of vertical clearance is maintained for conductors having 300 volts to ground (Figure 1-3). Figure 1-3. Service drop above residential property not subject to truck traffic (300 volts to ground) Above residential property, driveways, and commercial property not subject to truck traffic, 15 feet of vertical clearance is maintained for conductors having 600 volts to ground (Figure 1-4). Automobiles and small vehicles can clear an SE cable with a minimum clearance of 15 feet to ground. Figure 1-4. Service drop above residential property not subject to truck traffic (600 volts to ground) 1-3 EN5141

7 Above public streets and parking areas subject to truck traffic, 18 feet of vertical clearance is required in Figure 1-5. A 20-foot-high load did not clear the service drop. Figure 1-5. A high load damaging a service drop Above a peaked roof, having a 1:3 pitch or more, an 18-inch vertical clearance is necessary when the voltage between conductors is less than 300 volts and the SE is a mast within 48 inches of the roofs edge (Figure 1-6). Figure 1-6. Clearance between the roof and the service drop (300 volts) 1-4 EN5141

8 Conductors that carry more than 300 volts require an 8-foot vertical clearance (Figure 1-7). Figure 1-7. Clearance between the roof and the service drop (300 volts) Conductors that are installed at the side of a window or below it must have a 3-foot clearance (Figure 1-8). Figure 1-8. Conductor clearance from windows 1-5 EN5141

9 For fire escapes, porches, or platforms, conductors must have 3 feet of horizontal clearance from the object and 10 feet of vertical clearance from the ground (Figures 1-9 and 1-10). Figure 1-9. Fire escape/platform clearance Figure Clearance above ground level 1-6 EN5141

10 SE Requirements: Attachment of Service Drops Use back supports in walls that are not strong enough to support a service drop in adverse conditions (wind, snow, or ice) (Figure 1-11). A 2-inch by 4-foot back support gives added strength to the wall. Figure Back support installed in wall A single-cable system supports hot conductors and is hooked to the feed-through dead end (Figure 1-12). The cable that supports the service-drop conductors is clamped by the feed-through clamp and coupled to the ground conductors. The cable support is part of the ground system. Figure Grounded messenger cable providing support 1-7 EN5141

11 The bare neutral conductor is used as the messenger cable (Figure 1-13). By wrapping the hot conductors around the messenger cable, all the weight of the service drop is on the uninsulated neutral/messenger cable. Figure Hot conductors wrapped around a neutral/messenger cable. In a typical service-drop cable, the neutral conductor is used to attach the cable to the dead end (Figure 1-14). The feed-through clamp is built so that the pull of the conductors makes the clamp tighter on the neutral/messenger cable. Figure Feed-through clamp attached to the service spool 1-8 EN5141

12 Service drops are also dead-ended to a building by using secondary racks (Figure 1-15). When heavy service drops are installed on buildings, use secondary racks. Figure Service drop attached to a secondary rack When individual conductors are installed, many of the installations are dead-ended by using service spools made of porcelain (Figure 1-16). One service spool is screwed or bolted to the wall. Figure Service drop attached to service spools 1-9 EN5141

13 Service masts can dead-end individual conductors and an insulator bracket is used for this purpose (Figure 1-17). An insulator bracket can also be used when service conductors are wrapped around a messenger cable but feed-through clamp will be required. Figure Use of insulator brackets 1-10 EN5141

14 Most service masts use a single, dead-end bracket and a service cable (Figure 1-18). This makes a neat installation. Although time-consuming to install, a service mast is the best method for service installation. Figure Insulator bracket with feed-through clamp SE Requirements: Installation Procedures SE cable (Figure 1-19) has a flame-retardant, moisture-resistant covering. The neutral conductor (copper or aluminum) is twisted into a pigtail so that it can be clamped into the grounding system. Figure SE cable 1-11 EN5141

15 SE cable must be supported within 12 inches of the service head and all boxes, and then supported every 4 1/2 feet thereafter. When a service head is unavailable an expedient service head can be made. The SE cable is bent into a gooseneck and secured with tape (Figure 1-20). Service heads are located above the service drop to prevent water from entering the cable run. Figure Expedient head When installed as a service mast, an SE using rigid conduit provides protection for the conductors and a strong contact point for the service drop. A service-mast system is used when greater overhead roof clearance is needed (Figure 1-21). Figure Service mast using rigid conduct 1-12 EN5141

16 To provide adequate strength of the service mast, supports must be attached to the structure at critical points. A typical arrangement for supports is shown in Figure Figure Mast supports 1-13 EN5141

17 The conductors are pulled through the rigid conduit to provide total protection of the SE from the service head into the power panel (Figure 1-23). A well-built service mast will hold up under all types of weather. Figure Service mast Power-panel (Figure 1-24) and service-drop amperages must match. A smaller amperage power panel cannot be used as a main disconnect for a larger amperage service drop. A 210-ampere cable must be used to feed a 200-ampere power panel. Figure Power panel 1-14 EN5141

18 Conductors carrying current within a building are disconnected at the main power panel (Figure 1-25). When the main switch or circuit breaker is in the OFF position, all branch circuits will be disconnected. Figure Main switch inside a power panel A building can have no more than six switches or circuit breakers mounted in a single enclosure that disconnects all power from the building (Figure 1-26). In an emergency situation, all power from this service to the building can be disconnected by using no more than six operations of the hand. When a main switch (breaker) is provided inside the same panel, 42 branch circuit breakers can be installed. Figure Switches in a power panel 1-15 EN5141

19 Two- or three-pole breakers that can be joined together with a tie handle or a master handle can be counted as a single switch if all conductors are disconnected with a single pull or push of a set of switches. By adding tie handles, the power panel in Figure 1-27 follows the six-switch-maximum NEC rule. Label each breaker to indicate the load serviced. Figure Breakers joined with a tie handle The NEC contains important exceptions to the maximum-switch rule (Figure 1-28). The rule can be exceeded for a fire pump or an emergency service such as a hospital, but the switch should be located away from the other disconnects and the power panel should be labeled. Figure Labeled power panel 1-16 EN5141

20 The code also states that up to 42 overcurrent devices can be put into one panel; however, the power must be disconnected from all circuits with six hand operations. For the 42-device role, a two-pole circuit breaker is counted as two circuit breakers (Figure 1-29). One main disconnect removes the power from all circuits in 'this panel. Not counting the main, there are ten circuit breakers in this panel and four of them are tied together. Figure overcurrent devices in a power panel No circuit should be loaded greater than 80 percent of the breakers amperage capacity. In Figure 1-30, the breaker is rated at 20 amperes. Therefore, the ampere load to be applied cannot be greater than 16 amperes. Each heater uses 8 amperes. This circuit has the maximum allowable load. Figure Maximum ampere load 1-17 EN5141

21 Power panels must be accessible. This is accomplished by mounting the panel so that the top of the panel is no higher than 6 1/2 feet and the middle no lower than 4 12 feet (see Figure 1-31). Figure Panel mounting location 1-18 EN5141

22 THIS PAGE IS INTENTIONALLY LEFT BLANK EN5141

23 LESSON 1 PRACTICE EXERCISE The following items will test your grasp of the material covered in this 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. A service drop, limited to 300 volts to ground, is run over a sidewalk and goes to a house. What is the minimum distance, in feet, from the finished grade up to the lowest point of the SE drop? A. 10 B. 12 C. 14 D When a service drop is run over a public street that is not subject to truck traffic, what is the minimum distance, in feet, between the conductors and the finished surface? A. 9 B. 12 C. 15 D When the voltage exceeds 300 volts to ground, what is the minimum distance, in feet, from the roof to the service conductors? A. 2 B. 4 C. 6 D When installing a service drop beside a window, what is the minimum distance, in feet, from the side of the window to the point of attachment of the service drop? A. 2 B. 3 C. 6 D EN5141

24 5. How many feet can a service drop be mounted from the side of a porch? A. 2 B. 3 C. 6 D In which of the following situations can the maximum-switch rule be exceeded? A. Outside lighting B. Elevator C. Fire pump D. Telephone power 7. When installing a service drop using a feed-through dead end, the grounding conductor must be insulated from the cable support. A. True B. False 8. A service mast is not a good SE installation. A. True B. False 9. When installing a service, the SE cable can be rated at a lower amperage than the power panel. A. True B. False 1-21 EN5141

25 LESSON 1 PRACTICE EXERCISE ANSWER KEY AND FEEDBACK Item Correct Answer and Feedback 1. B. 12 Above residential property (page 1-3) 2. C. 15 An automobile can clear (page 1-3) 3. D. 8 Conductors that carry (page 1-5) 4. B. 3 conductors must have (page 1-6) 5. B. 3 For fire escapes, porches, or (page 1-6) 6. C. Fire Pumps The rule can be exceeded (page 1-16) 7. B. False A single cable system (page 1-16) 8. B. False Most service masts use (page 1-11) 9. B. False A 210-ampere cable must (page 1-14) 1-22 EN5141

26 LESSON 2 UNDERGROUND SERVICES Critical Task: OVERVIEW LESSON DESCRIPTION: In this lesson you will learn to describe an underground electrical service. TERMINAL LEARNING OBJECTIVE: ACTION: CONDITION: STANDARD: REFERENCES: You will learn the procedures used to install an underground electrical service. You will be given subcourse booklet EN5102 and an ACCP examination response sheet. You will work at your own pace and in your own selected environment with no supervision. You will correctly answer practice exercise questions at the end of this lesson. The material contained in this lesson was derived from STP 5-51R12-SM-TG and FM INTRODUCTION Underground electrical services are not subject to as much mechanical damage from equipment, personnel, and weather as overhead systems. Cable can be buried in the ground or pulled through conduit systems. To avoid a drop in voltage, underground systems usually run no more than 360 feet after the last transformer in the system. The underground service uses sizes of No. 8 copper or No. 6 aluminum conductors. This lesson will teach you how to describe an underground electrical service. The ground-cover requirements for underground conductors or conduit vary according to the site. In nontraffic areas, SE cable should be buried at least 18 inches and covered with a 2-inch-thick concrete pad (Figure 2-1). If rigid or intermediate conduit is installed, the depth can be reduced to 6 inches. 2-1 EN5141

27 Figure 2-1. Buried SE cable (nontraffic area) When SE cable is subject to vehicle traffic, at least 24 inches of ground cover is required (Figure 2-2). The 24-inch-depth requirement will allow the service to withstand the weight of vehicles passing over it. The depth requirement is the same when conduit is used. Figure 2-2. Buried SE cable (traffic area) 2-2 EN5141

28 SE cable buried in solid-rock areas can be installed down to the surface of the rock if the cable is covered with at least 2 inches of concrete that extends to the rock surface (Figure 2-3). The same requirement exists for conduit. Figure 2-3. Buried cable in solid rock area Conductors coming out of the ground should be protected by enclosures, raceways, or conduit from below grade to 8 feet above the ground (Figure 2-4). Figure 2-4. Conductor protection When backfilling a trench where there are cables or conductors, do not backfill with- Large rocks. Paving materials. Cinders. Sharp substances. Corrosive materials. 2-3 EN5141

29 Placement of SEs. Underground services can be installed using either underground service entrance (USE) cable, direct-burial conductors, or conduit (Figure 2-5). Conductors should not be spliced unless they are placed in a properly installed splice box. Figure 2-5. Types of installation When direct-burial conductors or USE cables are above grade they must be protected in the plastic or metallic conduit at both ends (Figure 2-6). Figure 2-6. Protection at ends of SE cable 2-4 EN5141

30 The installation of a USE to the power panel varies according to the site and the equipment. (Figure 2-7) shows a typical setup that is subject to change to fit the installation. NOTE: surface. A 1/4-inch air space is required when mounting a power panel to a damp wall or Figure 2-7. Typical USE power-panel setup 2-5 EN5141

31 LESSON 2 PRACTICE EXERCISE The following items will test your grasp of the material covered in this 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. When using SE cable in nontraffic areas, what is the minimum depth, in inches, for a service having a 2-inch-thick concrete pad over its length? A. 2 B. 6 C. 18 D How deep, in inches, should a service subject to vehicle traffic be buried? A. 2 B. 6 C. 12 D When cable is installed in solid-rock areas, how many inches thick is the concrete pad covering the cable? A. 2 B. 6 C. 12 D How many feet above finished grade should USE cable be protected? A. 6 B. 8 C. 10 D Which of the following materials can be used to backfill a cable trench? A. Sand B. Large rocks C. Cinders D. Paving materials 2-6 EN5141

32 6. A direct-burial conductor can be spliced anywhere along its length if plastic, self-sealing tape is used to replace the insulation. A. True B. False 7. Rigid conduit cannot be used for underground services. A. True B. False 2-7 EN5141

33 LESSON 2 PRACTICE EXERCISE ANSWER KEY AND FEEDBACK Item Correct Answer and Feedback 1. C. 18 In nontraffic areas SE (page 2-2) 2. D. 24 When SE cable (page 2-2) 3. A. 2 SE cable buried in solid rock (page 2-3) 4. B. 8 Conductors coming out of the ground (page 2-3) 5. A. Sand When backfilling (page 2-3) 6. B. False Protection of SE (page 2-4) 7. B. False Placement of SE (page 2-4) 2-8 EN5141