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Unformatted text preview: voltage to find the current rating. For example, a 1,000-watt heater plugged into a 120-volt circuit will need almost 10 amps of current. Let's look at another example: A 1-horsepower electric motor uses electrical energy at the rate of almost 750 watts, so it will need a minimum of about 7 amps of current on a 120-volt circuit. But, electric motors need additional current as they startup or if they stall, requiring up to 200% of the nameplate current rating. Therefore, the motor would need 14 amps. Add to find the total current needed to operate all the appliances supplied by the cord. Choose a wire gauge that can handle the total current. American Wire Gauge (AWG) power--the amount of energy used in a second, measured in watts 1 horsepower = 746 watts. Wire size #10 #12 #14 #16 AWG AWG AWG AWG Handles up to 30 25 18 13 amps amps amps amps Remember: The larger the gauge number, the smaller the wire! Do not use extension cords that are too long for the size of wire. The length of the extension cord also needs to be considered when selecting the wire gauge. Voltage drops over the length of a cord. If a cord is too long, the voltage drop can be enough to damage equipment. Many electric motors only operate safely in a narrow range of voltages and will not work properly at voltages different than the voltage listed on the nameplate. Even though light bulbs operate (somewhat dimmer) at lowered voltages, do not assume electric motors will work correctly at less-than-required voltages. Also, when electric motors start or operate under load, they require more current. The larger the gauge of the wire, the longer a cord can be without causing a voltage drop that could damage tools and equipment. Page 44 Section 7 The grounding path for extension cords must be kept intact to keep you safe. A typical extension cord grounding system has four components: a third wire in the cord, called a ground wire; a three-prong plug with a grounding prong on one end of the cord; a three-wire, grounding-type receptacle at the other end of the cord; and a properly grounded outlet. Make sure the path to ground is continuous. Control hazards of exposed live electrical parts: isolate energized components
Electrical hazards exist when wires or other electrical parts are exposed. These hazards need to be controlled to create a safe work environment. Isolation of energized electrical parts makes them inaccessible unless tools and special effort are used. Isolation can be accomplished by placing the energized parts at least 8 feet high and out of reach, or by guarding. Guarding is a type of isolation that uses various structures--like cabinets, boxes, screens, barriers, covers, and partitions--to close-off live electrical parts. Outlets must be grounded properly. guarding--a covering or barrier that separates you from live electrical parts This exposed electrical equipment is guarded by an 8-foot fence. Use covers to prevent accidental contact with electrical circuits. Section 7 Page 45 S AF ET Y M O D EL S TAG E 3--C O N T rOLLING HAZArDS: SAFE WOrK ENVIrON M EN T 20-year-old male laborer was carrying a 20-foot piece of iron from a welding shop to an outside storage rack. As he was turning a corner near a bank of electrical transformers, the top end of the piece of iron struck an uninsulated supply wire at the top of a transformer. Although the transformers were surrounded by a 6-foot fence, they were about 3 feet taller than the fence enclosure. Each transformer carried 4,160 volts. A When the iron hit the supply wire, the laborer was electrocuted. A forklift operator heard the iron drop to the ground at about 8:46 a.m. and found the victim 5 minutes later. He was pronounced dead on arrival at a local hospital. According to OSHA, the enclosure around the transformers was too low. The fence should have been at least 8 feet tall. The company in this case did not offer any formal safety training to its workers. All employers should develop safety and health training programs so their employees know how to recognize and avoid life-threatening hazards. Take the following precautions to prevent injuries from contact with live parts: Immediately report exposed live parts to a supervisor or teacher. As a student, you should never attempt to correct the condition yourself without supervision. Provide guards or barriers if live parts cannot be enclosed completely. Use covers, screens, or partitions for guarding that require tools to remove them. Replace covers that have been removed from panels, motors, or fuse boxes. Even when live parts are elevated to the required height (8 feet), care should be taken when using objects (like metal rods or pipes) that can contact these parts. Close unused conduit openings in boxes so that foreign objects (pencils, metal chips, conductive debris, etc.) cannot get inside and damage the circuit. This cover cannot be removed without special tools. Control hazards of exposure to live electrical wires: use proper insulation
Insulation is made o...
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This document was uploaded on 03/14/2014 for the course ECE 482 at University of Tennessee.
- Spring '09