Transcript document
Chapter 18 Electrical Hazards Major Topics • • • • • • Electrical hazards Sources of electrical hazards Detection of electrical hazards Reduction of electrical hazards OSHA’s electrical standards Electrical safety program Zero potential • The earth (ground) is considered to have zero potential. The path of electric current includes the source of electrical power, a conductor to act as the path, a device to use the current (called load), and a path to the ground. Volt, Amp, Ohm, Hertz, and Watt • Volt: The potential difference between two points in a circuit is measured by voltage. Voltage is measured in volts [V]. • Amp: Electrical current is produced by the flow of electrons. The unit of measurement of current [I] is amperes [or amps]. • Ohm: The higher the resistance to the flow of electrons, the lower the current. Resistance [R] is measured in ohms. • Hertz: Most industrial and domestic users of electricity are supplied by alternating current [AC current]. In the United States standard AC circuits cycle 60 times per second. The number of cycles per second id called frequency [ f ] and is measured in hertz [Hz]. • Ohm’s Law: describes relationship among volts, ohms and amps. V=I*R where V= potential difference in volts, I = current flow in amps, and R = resistance to current flow in ohms. • Watt: Power is measured in wattage [or watts] and can be determined from Ohm’s Law: W = V * I or W = I2R DC and AC and Effective Current • Most industrial and domestic use of electricity is supplied by alternating current [AC]. Because voltage cycles in AC current, an effective current for AC circuits is computed which is slightly less than peak current during a cycle. • A direct current [DC] has been found to generate as much heat as a AC current that has a peak current 41.4% higher than the DC. • Effective current = Peak current * 0.707 • Effective voltage = 110 V AC = 110/0.707 = 157 V peak. NEC and UL • The National Electric Code [NEC] is published by the National Fire Protection Agency [NFPA]. This code specifies industrial and domestic safety precautions. • The National Board of Fire Underwriters sponsors Underwriters Laboratories [UL]. The UL determines whether equipment and material for electrical systems are safe. Potential Difference, Lightning, and Grounding • Potential Difference: The potential difference between two points in a circuit is measured by voltage. The higher the voltage, the more likely is that the electricity will flow between the negative and positive points. • Lightning: Lightning is static charges from clouds following the path of least resistance to the earth, involving very high voltage and current. Lightning tends to strike the tallest object on the earth below the clouds. • Grounding: Typically a 110V circuit wiring has a hot wire [red or black insulation] carrying current, a neutral wire [white], and a ground wire [green or bare]. The ground wire is connected to the ground. Open Ground • When the ground wire is connected improperly (may be connected to neutral wire), the situation is referred to as open ground. Usually the equipment with this wiring will operate normally. If a short occurs in the equipment circuitry without proper grounding, anyone touching that equipment might be severely shocked. Lightning Hazard Control Measures • Place lightning rods so that the upper end is higher than nearby structures. • Avoid standing in high places or near tall objects. Be aware that trees in a open field may be the tallest object nearby. • Do not work with flammable liquids or gases during electric storms. • Ensure proper grounding of all electrical equipment. • If inside an automobile, stay inside the automobile. • If in a small boat, lie down in the bottom of the boat. • If in a metal building, stay in the building and do not touch the walls of the building. • Wear rubber clothing if outdoors. • Avoid using a telephone during an electrical storm. • Do not use electrical equipment during the storm. • Avoid standing near open doors or windows where lightning may enter the building directly. Circuit Load and Short Circuit • A short circuit is a circuit in which the circuit load has been removed or bypassed. The ground wire in a standard three-wire circuit provides a direct path to the ground, bypassing the load. Short circuits can be another source of electrical hazard if a human is the conductor to the ground, thereby bypassing the load. Ionizers, Radioactive neutralizers, Antistatic Material, and Humidification • Ionizers: and electrostatic neutralizers ionize the air surrounding a charged surface to provide a conductive path for the flow of electrons. • Radioactive neutralizers: include a radioactive element that emits positive particles to neutralize collected negative electrical charges. Workers need to be safely isolated from the radioactive particle emitter. • Antistatic material: have been used effectively to reduce electrical static hazards. Such material either increase the surface conductivity of the charged material or absorb moisture, which reduces resistance and the tendency to accumulate charge. • Humidification: is a method of reducing electrical static. Raising the humidity above 65% reduces charge accumulation. Bonding and Grounding • Electrical system grounding is achieved when one conductor of the circuit is connected to the earth. Power surges and voltage changes are attenuated and usually eliminated with proper system grounding. • Bonding is used to connect two pieces of equipment by a conductor. Bonding can reduce the potential differences between the equipment and thus reduce the possibility of sparking. • Grounding in contrast, provides a conducting path between the equipment and the earth. Bonding and grounding together are used for entire electrical systems. Continuity testers, circuit testers, and receptacle wiring testers • A circuit tester is an inexpensive piece of test equipment with two wire leads capped by probes and connected to a small bulb. This simple tester can ensure that power has been turned off before electrical maintenance begins. When one lead makes contact with a hot wire and the other lead connects to the ground conductor, the bulb lights, if there is power in the circuit. • A receptacle wiring tester is a device with two standard plug probes for insertion into an ordinary 110 volt outlet and a probe for the ground. Indicator lights show an improperly wired receptacle [outlet]. • A continuity tester may be used to determine whether a conductor has a break in the circuit. Continuity is checked on circuits that are disconnected from the power source. Freeze and Let-go current • Some levels of current “freeze” a person to the conductor; the person cannot voluntarily release his or her grasp (10 to 50 milliamps). • Let-go current is the highest current level at which a person in contact with the conductor can release the grasp of the conductor (6 to 9 milliamps). • See page 390 fig 18-5. Structure of an atom • The basic structure of an atom has a positively charged nucleus in the center. The nucleus consists of positively charged protons and electrically neutral particles called neutrons. The electrons are energy bands of orbiting negatively charged particles. Each ring of electrons contains a particular quantity of negative charges. • Electricity is the flow of negatively charged particles called electrons through an electrically conductive material. Proper wiring of a 3 wire circuit • Typical 110 volt circuit has a hot wire [110V] carrying current [red or black insulation], a neutral wire [nearly zero voltage] [white insulation], and a ground wire [grounded][green or bare]. The neutral wire may be called ground conductor while the ground wire is called a grounding conductor. Fig 18-2 page 385 shows a typical three-wire circuit. Why is Jumping the Ground Wire a Hazard • One common mistake is to jump the ground wire to the neutral wire. • Equipment usually operates in a customary way, but the hazard occurs when low voltages are generated on exposed parts of the equipment, such as the housing. If the neutral circuit becomes corroded or loose, the voltage on the ground wire increases to a dangerous level. Reversed Polarity • With reversed polarity, the hot and neutral wires have been reversed. A worker who is not aware that the black lead [hot] and white lead [neutral] have been reversed could be injured or cause further confusion by connecting the circuit to other apparatus. In a reversed polarity light bulb socket, the screw threads become the conductor. Warning devices are less effective than designed in safety precautions • It is better to design safety into equipment and system than to rely on human behavior such as reading and following the label. • Warning devices to alert personnel about detected hazards may include lights, colored indicators, on/off blinkers, audible signals, or labels. • Designed-in safety precautions such as interlocks automatically break the circuit when an unsafe condition is detected. Elevator doors typically have interlocks to ensure that the elevator does not move when the doors are open. Establishing an effective electrical safety program • • • • • • • • • • • Electrocution accounts for approximately 6% of all workplace deaths in the United States every year. National Institute of Safety and Health [NIOSH] recommends the following: Develop and implement a comprehensive safety program and when necessary revise existing programs to address thoroughly the area of electrical safety in the workplace. Ensure compliance with existing OSHA regulations [Subpart S of 29 CFR 1910.302 through 1910.399]. Provide all workers with adequate training in the identification and control of the hazards associated with electrical energy in the workplace. Provide additional specialized electrical safety training to those working with or around exposed components of electrical circuits. Develop and implement procedures to control hazardous electrical energy that include lockout and tagout procedures. Ensure that workers follow these procedures. Provide testing or detection equipment for those who work directly with electrical energy that ensures their safety during performance of their assigned tasks. Ensure compliance with the National Electrical Code and the National Electrical Safety Code. Conduct safety meetings regularly. Conduct scheduled and unscheduled inspections at work sites. Importance for Safety Personnel to help employees and supervisors conduct self assessments • • • • • • • • • Even the best safety professional cannot be everywhere at once. Safety personnel should develop checklists that supervisors can use to undertake periodic self assessments: Are all electricians up-to-date with the latest requirements of the National Electrical Code [NEC]? Does your company specify compliance with the NEC as part of its contract for electrical work with outside personnel? Do all electrical installations located in the presence of hazardous dust or vapors meet the NEC requirements for hazardous locations? Are all electrical cords properly strung? Is all conduit properly attached to supports and tightly connected to junction boxes and outlet boxes? Are all electrical cords free of fraying? Are rubber cords free of grease, oil, chemicals, and other potentially damaging material? Are all metallic cables and conduit systems properly grounded? Summary • Electricity is the flow of charge through an electrically conductive material. • Current is measured in amps, voltage in volts, resistance in ohms, power in watts, and frequency in hertz. • The UL symbol certifies if equipment is safe to use. • Lightning tends to strike the tallest object on earth. • Above 6 to 9 milliamps of current, people freeze to conductors and are unable to let go of the conductor. • With electrocution accounting for approximately 6% of all workplace deaths, it is better to design in safety for the electrical system than to deal with accidents. Home Work • Answer questions 2, 7, 10, 12, 18 and 19 on pages 400401. • 2. Explain what each of the following term measure: volt, amp, ohm, hertz, and watt. • 7. List at least 5 lightning hazard control measures. • 10. Discuss how bonding and grounding work together to increase electrical safety. • 12. Explain freeze and let go current. • 18. Explain 5 strategies for establishing an effective electrical safety program. • 19. Explain why it is important for safety personnel to help employees and supervisors conduct selfassessments.