Module 4: Electrical Hazards Electricity is the flow of negatively charged particles called electrons through an electrically conductive material. Electrons orbit the nucleus of an atom, which is located approximately in the atom’s center. The negative charge of the electrons is neutralized by particles called neutrons, which act as temporary energy repositories for the interactions between positively charged particles called protons and electrons.
Figure 1: Basic structure of an atom
Sources of Electrical Hazards Short circuits are one of many potential electrical hazards that can cause electrical shock. Another hazard is water, which considerably decreases the resistance of materials, including humans. The resistance of a wet skin can be as low as 450 ohms, whereas dry skin may have an average resistance of 600,000 ohms. According to ohm’s law, the higher the resistance, the lower the current flow. When the current flow is reduced, the probability of an electrical shock is also reduced. The major causes of electrical shock are • • • •
Contact with a bare wire carrying current. Working with electrical equipment that lacks the UL label for safety inspection. Electrical equipment that is not properly grounded. Working with electrical equipment on damp floors or other sources of wetness. 1
• • • •
Static electricity discharge. Using metal ladder to work on electrical equipment. Working on electrical equipment without first switching it off. Lightning strikes
Electrostatic Hazards Electrostatic hazards may cause minor shock. Shock from a static electricity may result from a single discharge or multiple discharges of static. Sources of electrostatic discharge include the following: • • • • • •
Briskly rubbing a nonconductive material over a stationary surface Moving large sheet of plastic, which may discharge sparks. The explosions of organic and metallic dust, which have occurred from static, build up in farm grain silos and mine shafts. Conveyer belts. Vehicles tires rolling across a road surface. Friction between a flowing liquid on a solid surface.
Arcs and Sparks Hazards Combustible and Explosive Materials High currents through contaminated liquids may cause the contaminants to expand rapidly and explode. This situation is particularly dangerous with contaminated oilfilled circuit breakers of transformers. A poor match between current or polarity and capacitors can cause explosion. In each of these cases, the conductor is not capable of carrying a current of such high magnitude. Overheating from high currents can also lead to short circuits, which in turn may generate fires and explosions.
Lighting Hazards Lightning is static charges from clouds following the path of least resistance to the earth, involving very high voltage and current. If this path to the earth involves humans, serious disability may result, including electrocution. Lightning may also damage airplanes from intracloud or cloud-to-cloud flashes. Electrical equipment and building structures are commonly subject to lightning hazards. Lightning tend to 2
strike the tallest object on the earth below the clouds. A tree is a common path for lightning.
Figure 2 Improper wiring permits equipment to operate normally but can result in hazardous conditions. The section of this chapter on detection of electrical hazards discusses tests to identify unsafe wiring practices. One common mistake is to â€˜jumpâ€™ the ground wire to the neutral wire. In this case, the ground wire is connected to the neutral wire. Equipment usually operates in a customary way, but 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. Improper wiring can cause other hazards. When the ground is connected improperly, the situation is referred to as open ground. Usually the equipment with this miswiring will operate normally. If a short occurs in the equipment circuitry without proper grounding, anyone touching that equipment may be severely shocked. 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. If a short between the on/off switch and the load occurred, the equipment may run indefinitely, regardless of the switch position.
Insulator Failure • • • • •
Direct sunlight or other sources of ultraviolet light, which can induce gradual breakdown of plastic insulation material. Sparks or arcs from discharging static electricity, which can result in burned-through holes in insulation. Repeated exposure to elevated temperatures, which can produce slow but progressive degradation of insulation material. Abrasive surfaces, which can result in erosion of the material strength of the insulation. Substance incompatibility with the atmosphere around the insulation and the insulation material, which can induce chemical reactions. Such reactions may include oxidation or dehydration of the insulation and eventually breakdown. Animals such as rodents or insects chewing or eating the insulation material, leading to exposure of the circuit. Moisture and humidity being absorbed by the insulation material, which may result in the moisture on the insulation carrying a current.
Equipment Failure • • • •
Wet insulation can become a conductor and cause an electric shock. Portable tool can result in the device’s housing carrying an electric current. Broken power line carries great amperage and voltage and can cause severe disability. When equipment is not properly grounded or insulated.
Hazardous Locations for Electrical Equipment Class I for flammable vapours and gases, class II for combustible dusts, and class III for ignitable fibres. There are also two divisions of hazard categories. Division I have more stringent requirements for electrical installation than Division II does.
Electrical Hazards to Humans Detection of electrical hazards Several items of test equipment can be used to verify electrical equipment safety. A circuit tester is an inexpensive piece of test equipment with two wire leads capped by probes and v=connected to a small bulb. 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). Infrared thermal imaging is another technique that can be used for detecting electrical hazard.
Reduction of Electrical Hazards Electrical system grounding is achieved when one conductor of the circuit is connected to the earth. Power surges are attenuated and usually eliminated with power system grounding. Bonding is used to connect two pieces of equipment by a conductor. Bonding can reduce 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 are used for entire electrical systems. A ground fault circuit interrupter (GFCI) can detect the flow of current to the ground and open circuit, thereby interrupting the flow of current. When the current flow in the hot wire is greater than the current in the neutral wire, a ground fault has occurred. The GFI provides a safety measure for a person who becomes part of the ground fault circuit. The GFI cannot interrupt current passing between two circuits or between the hot and neutral wires of a three-wire circuit. To ensure safety, the equipment must be grounded as well as protected by a GFI. A GFI should be replaced periodically based on the manufacturerâ€™s recommendations. Humidification is another mechanism for reducing electrical static; it was discussed in the section on sources of electrical hazards. Raising the humidity above 65% reduces charges accumulation. However, when the relative humidity exceeds 65%, biological agents can begin to grow in heating, ventilation, and air conditioning (HVAC) ducts and unventilated areas. 5
Ionizers and electrostatic neutralizers ionize the air surrounding a charged surface to provide a conductive path for the flow of charges. 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. Fuses consist of a metal strip or wire that melts if a current above a specific value is conducted through the metal. Melting the metal causes the circuit to open at the fuse, thereby stopping the flow of current. Some fuses are designed to include a time lag before melting to allow higher currents during startup of the system or as an occasional event. Double insulation is another means of increasing electrical equipment safety. Most double-insulated tools have plastic nonconductive housings in addition to standard insulation around conductive materials. Another means of protecting workers is isolating the hazard from the worker or vice versa. Interlocks automatically break the circuit when an unsafe situation is detected. Interlocks may be used around high-voltage areas to keep personnel from entering the area.
Lightning Hazard Control: • • • • • • • • • • • •
Avoid standing in higher places or near tall objects. Be aware that trees in an open field may be the tallest object nearby. Do not work with flammable liquids or gases during electrical storms. Ensure proper grounding of all electrical equipment. If inside an automobile, remain 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. Do not work touching or near conducting materials. Avoid using the telephone during an electrical storm. Do not use electrical equipment during the storm. Avoid standing near open doors or window where lightning may enter the building directly. Ensure that power has been disconnected from the system before working with it. 6
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Allow only fully authorized and trained people to work on electrical systems. Do not wear conductive material such as metal jewellery when working with electricity. Screw bulbs securely into their sockets. Ensure that bulbs are matched to the circuit by the correct voltage rating. Do not use water to put out electrical fire. Check the entire length of electrical cord before using it. Discharge capacitors before working on the equipment. Use fuses and circuit breakers for protection on all structures.
Prevention of Arc Flash Injuries An arc flash is an electrical short circuit that travels through the air rather than flowing through conductors, bus bars, and other types of equipment. The uncontrolled energy released by an arc flash can produce high levels of heat and pressure. It can also cause equipment to explode, sending dangerous shrapnel flying through the air. Arc flashes are sometimes produced by electrical equipment malfunctions, but a more common cause is accidental human contact with an electrical circuit or conductor. For example, a person working near a piece of energized electrical equipment might accidently drop a tool that then makes contact with an electrical circuit or conductor. The result is an arc flash that can injure or even kill the worker, not to mention the equipment damage. Arc flashes become even more hazardous when workers are wearing flammable clothing instead of appropriate personal protective equipment (PPE). Arc flashes can produce sufficient heat to easily ignite clothing, cause severe burns, and even damage hearing. The best and most obvious way to prevent arc flash injuries is to deenergize maintenance or service work on it.