- - AGRICULTURE CORE CURRICULUM - - (CLF2000) Advanced Core Cluster: AGRICULTURAL MECHANICS (CLF2650) Unit Title: ELECTRICITY ___________________________________________________________________________ (CLF2653) Topic: Conductors & Overcurrent Time Year(s) Protection 2 hours 1 / 2 / 3 / 4 ___________________________________________________________________________ Topic Objectives: Upon completion of this lesson, the students will be able to: Learning Outcome #: (N-2) - Select correct fuses & circuit breakers for a given circuit. (N-3) - Select wire sizes for a given circuit. Special Material and Equipment: Samples of wires, cables, cords, fuses, breakers References: Cooper, E. L. (1987). AGRICULTURAL MECHANICS: FUNDAMENTALS AND APPLICATIONS. Albany, NY: Delmar Publishers. Erpelding, L. H. (1971). AGRIBUSINESS ELECTRICAL LESSON PLANS. Danville, IL: Interstate Printers & Publishers. Evaluation: Quiz by instructor. TOPIC PRESENTATION: Conductors AND Overcurrent Protection A. Electrical Conductors 1. Fundamentals a. Conductors are materials that provide a good path for electron flow. b. Conductors used in wiring may be made of copper or aluminum. 1) When the price of copper is high, aluminum may be used. 2) Since aluminum is not as good a conductor as copper, it tends to build up more heat from resistance. c. Conductors may be solid or made of many strands bundled together. 1) Stranded wire, especially for wire sizes No. 8 and larger, improve flexibility and conductivity. 2) Since electricity travels on the outer surface of wires, stranded wire increases conductivity by providing more total surface area. 2. Sizes of Conductors a. Conductors are classified into sizes by American Wire Gauge numbers. b. Common wire sizes run in even numbers from No. 18 to No. 6. The lower the gauge number, the larger the wire size. c. Commonly used wire gauge sizes and their ampere ratings for copper conductors are presented below. 1) No. 14 is rated for 15-ampere circuits. 2) No. 12 is rated for 20-ampere circuits. 3) No. 10 is rated for 30-ampere circuits. d. Aluminum conductors require one wire size larger than copper conductors to provide the same amperage. 3. Types of Conductors a. Electrical Wire 1) It is a single conductor. 2) It may be bare or insulated. 3) It is generally used for permanent installation in conduit or electrical metallic tubing. b. Electrical Cable 1) It is a protective sheath containing two or more insulated wires. 2) It may contain a bare ground wire also. 3) It is generally used for permanent indoor installation. c. Electrical Cord 1) It is a conductor consisting of two or more insulated, stranded wires. 2) It may have a ground wire. 3) It is generally used where flexibility is required. B. Classification of Conductors According to Use 1. Types of Wires a. Type R wire is insulated with rubber and is used for indoor installation. 1) Type RH is used in dry locations with high temperatures. 2) Type RHW is used in wet locations with high temperatures. b. Type T wire is insulated with thermoplastic and is gradually replacing Type R wire. 1) Type T is used in dry locations. 2) Type TW is used in dry or wet locations. 3) Type THHN is used in dry locations with high temperatures. 4) Types THW and THWN are used in wet locations with high temperatures. 2. Types of Cables a. Cable type markings indicate where the cable can be used. 1) Type USE is an underground cable which may be used for service feeders and branch circuits. 2) Type UF is an underground cable which may only be used for feeders and branch circuits. 3) Type NM is a moisture resistant, nonmetallic sheathed cable --often referred to by the trade name, Romex--and is only used indoors. 4) Type NMC is nonmetallic sheathed cable that is flame retardant and resistant to moisture, fungus, and corrosion. 5) Type SE is a service entrance cable used to carry current into a building. It is also used to serve welders and 220 volt appliances. b. Other cable markings indicate the size and number of wires in a cable. 1) 12-2 indicates the cable has two No. 12 wires, one black and one white. 2) 12-2 w/g indicates the cable has two No. 12 wires plus a ground wire, one black, one white, and one green or bare. 3) 14-3 indicates the cable has three No. 14 wires, probably one black, one red, and one white. 4) 14-3 w/g indicates the cable has three No. 14 wires, the same as 14-3 cable with the addition of a green or bare ground wire. 3. Types of Cords a. Parallel cords are commonly used on lamps, radios, clocks, etc. They are available in wire gauge sizes 16 and 18, and have a groove down the center to make separation of the two wires easy. 1) Type SP is insulated with rubber. 2) Type SPT is insulated with thermoplastic. b. Junior hard-service cords are used on machines, power tools, trouble lamps, etc. They are round and available in wire gauge sizes 16 and 18. 1) Type SJ has rubber outer insulation. 2) Type SJT has thermoplastic outer insulation. 3) Type SJO has oil-resistant rubber outer insulation. c. Hard-service cords are used similarly to junior hard-service cords, but have a thicker outer cover for rough use. They are available in wire gauge sizes 10, 12, 14, 16, and 18. 1) Type S has rubber outer insulation. 2) Type ST has thermoplastic outer insulation. 3) Type SO has oil-resistant rubber outer insulation. d. Heater cords have two common types. 1) Type HPN has a thermosetting insulation that can be used in damp places, but is not designed for hard use. It comes in wire gauge sizes 12, 14, 16, and 18. 2) Type HST has either a rubber and asbestos or neoprene insulation that can be used in damp places. It comes in wire gauge sizes 16 and 18. _________________________________________________________ ACTIVITY: 1. Identify the various wires, cables, and cords in the shop by reading their markings. 2. Take apart various electrical cables with a cable stripper. 2. Create a conductor display that can be used as a teaching aid. _________________________________________________________ C. Overcurrent Protection Devices 1. Fundamentals a. Each wire size is rated to safely carry a certain amount of electrical current. b. If wires carry current beyond their rated amperage, they will overheat, damaging insulation and causing a fire. c. Overcurrent protection devices are, therefore, necessary to protect the wires in an electrical system from excessive heat. 2. Fuses a. Fuses are overcurrent protection devices containing a strip of low-melting-temperature metal, called a link, which is designed to carry the amperage stamped on the fuse. b. When a circuit is overloaded, an excessive amount of current passes through the link causing it to melt, and thus opening the circuit before the wires are damaged. c. Two types of fuses are used, plugs and cartridges. 1) Plug fuses are available in ordinary and time-delay types. a) Ordinary plug fuses range in size from 3 to 30 amperes and are used in circuits which do not have to handle the high starting current required by motors. b) Time-delay plug fuses range in size from .4 to 30 amperes and are designed to carry temporary current overloads in electric motor circuits. 2) Cartridge fuses are used to protect circuit with ratings beyond 30 amperes and are also available in the ordinary and time-delay types. a) Ferrule cartridge fuses range in size up to 60 amperes. b) Knife-blade contact, cartridge fuses are made in sizes higher than 60 amperes. 3. Circuit Breakers a. Circuit breakers are current overload protective devices that can be reset and used again after an overload. b. Breakers contain a metal alloy strip through which current must pass. This strip has calibrated amperage indicated on the breaker switch. 1) Excessive heat from a current overload causes the strip to expand and change shape. 2) This action trips the breaker switch opening the circuit. c. To function properly, the breaker amperage rating must never exceed the circuit amperage rating. _________________________________________________________ ACTIVITY: 1. Identify the different types of fuses and circuit breakers. 2. Replace various fuses and reset a circuit breaker. Locate the emergency cutoff switches in the shop, push them, and reset the breaker to that circuit. _________________________________________________________ 6/26/91 OLR/tf #%&C