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Maintenance and construction workers alike are usually required to wear certain articles of protective clothing, dictated by the environment of the work area and the job being performed.
Head Protection Some type of head protection is required on almost any work site. A typical electrician's hard hat, made of nonconductive plastic.
It has a pair of safety goggles attached that can be used when desired or necessary.
--6 Typical electrician's hard hat with attached safety goggles.
Eye protection is another piece of safety gear required on almost all work sites. Eye protection can come in different forms, ranging from the goggles to the safety glasses with side shields. Common safety glasses may or may not be prescription glasses, but almost all provide side protection. Sometimes a full face shield may be required.
Section III, Chap. 5, of the OSHA Technical Manual includes requirements concerning hearing protection. The need for hearing protection is based on the ambient sound level of the work site or the industrial location. Workers are usually required to wear some type of hearing protection when working in certain areas, usually in the form of earplugs or earmuffs.
Special clothing made of fire-retardant material is required in some areas, generally certain industries as opposed to all work sites. Fire-retardant clothing is often required for maintenance personnel who work with high-power sources such as transformer installations and motor-control centers. An arc flash in a motor-control center can easily catch a person's clothes on fire. The typical motor-control center can produce enough energy during an arc flash to kill a person 30 feet away.
--7 Safety glasses provide side protection.
--8 Leather gloves with rubber inserts.
--9 Kevlar gloves protect against cuts.
--10 Typical safety harness. SHOULDER STRAPS; CHEST STRAP; LEG STRAPS; D-RING LANYARD
Another common article of safety clothing is gloves. Electricians often wear leather gloves with rubber inserts when it’s necessary to work on energized circuits. These gloves are usually rated for a certain amount of voltage.
They should be inspected for holes or tears before they are used. Kevlar gloves help protect against cuts when stripping cable with a sharp blade.
Safety harnesses provide protection from falling. They buckle around the upper body with leg, shoulder, and chest straps; and the back has a heavy metal D-ring. A section of rope approximately 6 feet in length, called a lanyard, is attached to the D-ring and secured to a stable structure above the worker. If the worker falls, the lanyard limits the distance he or she can drop.
A safety harness should be worn:
1. When working more than 6 feet above the ground or floor
2. When working near a hole or drop-off
3. When working on high scaffolding
A safety harness is shown.
--11 Safety harness.
Ladders and Scaffolds
It’s often necessary to work in an elevated location. When this is the case, ladders or scaffolds are employed. Scaffolds generally provide the safest elevated working platforms. They are commonly assembled on the work site from standard sections. The bottom sections usually contain adjustable feet that can be used to level the sections. Two end sections are connected by X braces that form a rigid work platform. Sections of scaffolding are stacked on top of each other to reach the desired height.
--12. Typical section of scaffolding.
--13. X braces connect scaffolding sections together.
Rolling scaffolds are used in areas that contain level floors, such as inside a building. The major difference between a rolling scaffold and those discussed previously is that it’s equipped with wheels on the bottom section that permit it to be moved from one position to another. The wheels usually contain a mechanism that permits them to be locked after the scaffold is rolled to the desired location.
Hanging or Suspended Scaffolds
Hanging or suspended scaffolds are suspended by cables from a support structure. They are generally used on the sides of buildings to raise and lower workers by using hand cranks or electric motors.
Ladders can be divided into two main types, straight and step. Straight ladders are constructed by placing rungs between two parallel rails. They generally contain safety feet on one end that help prevent the ladder from slipping. Ladders used for electrical work are usually wood or fiberglass; aluminum ladders are avoided because they conduct electricity. Regardless of the type of ladder used, you should check its load capacity before using it. This information is found on the side of the ladder. Load capacities of 200 pounds, 250 pounds, and 300 pounds are common. Don’t use a ladder that does not have enough load capacity to support your weight plus the weight of your tools and the weight of any object you are taking up the ladder with you.
--14 Straight ladder.
--15 A ladder should be placed at an angle of approximately 76°.
Straight ladders should be placed against the side of a building or other structure at an angle of approximately 76°. This can be accomplished by moving the base of the ladder away from the structure a distance equal to one-fourth the height of the ladder. If the ladder is 20 feet high, it should be placed 5 feet from the base of the structure. If the ladder is to provide access to the top of the structure, it should extend 3 feet above the structure.
Step ladders are self-supporting, constructed of two sections hinged at the top. The front section has two rails and steps, the rear portion two rails and braces. Like straight ladders, step ladders are designed to withstand a certain load capacity. Always check the load capacity before using a ladder.
As a general rule, ladder manufacturers recommend that the top step not be used because of the danger of becoming unbalanced and falling. Many people mistakenly think the top step is the top of the ladder, but it’s actually the last step before the ladder top.
--16 Typical step ladder. HIGHEST USABLE STEP TOP STEP LADDER TOP SPREADER
For a fire to burn, it must have three things: fuel, heat, and oxygen. Fuel is any thing that can burn, including materials such as wood, paper, cloth, combustible dusts, and even some metals. Different materials require different amounts of heat for combustion to take place. If the temperature of any material is below its combustion temperature, it won’t burn. Oxygen must be present for combustion to take place. If a fire is denied oxygen, it will extinguish.
Fires are divided into four classes: A, B, C, and D. Class A fires involve common combustible materials such as wood or paper. They are often extinguished by lowering the temperature of the fuel below the combustion temperature. Class A fire extinguishers often use water to extinguish a fire. A fire extinguisher listed as Class A only should never be used on an electrical fire.
Class B fires involve fuels such as grease, combustible liquids, or gases.
A Class B fire extinguisher generally employs carbon dioxide (CO2), which greatly lowers the temperature of the fuel and deprives the fire of oxygen. Car bon dioxide extinguishers are often used on electrical fires, because they don’t destroy surrounding equipment by coating it with a dry powder.
Class C fires involve energized electric equipment. A Class C fire extinguisher usually uses a dry powder to smother the fire. Many fire extinguishers can be used on multiple types of fires; For example, an extinguisher labeled ABC could be used on any of the three classes of fire. The important thing to remember is never to use an extinguisher on a fire for which it’s not rated.
Using a Class A extinguisher filled with water on an electrical fire could be fatal.
Class D fires consist of burning metal. Spraying water on some burning metals can actually cause the fire to increase. Class D extinguishers place a powder on top of the burning metal that forms a crust to cut off the oxygen supply to the metal. Some metals cannot be extinguished by placing powder on them, in which case the powder should be used to help prevent the fire from spreading to other combustible materials.
--17 The current in both the "hot" and neutral conductors should be the same, but flowing in opposite directions.
--18 A ground fault occurs when a path to ground other than the intended path is established.
Ground-Fault Circuit Interrupters
Ground-fault circuit interrupters (GFCI) are used to prevent people from being electrocuted. They work by sensing the amount of current flow on both the ungrounded (hot) and grounded (neutral) conductors supplying power to a device. In theory, the amount of current in both conductors should be equal but opposite in polarity. In this example, a current of 10 amperes flows in both the hot and neutral conductors.
A ground fault occurs when a path to ground other than the intended path is established. Assume that a person comes in contact with a defective electric appliance. If the person is grounded, a current path can be established through the person's body. In the example, it’s assumed that a current of 0.1 ampere is flowing through the person. I.e., the hot conductor now has a current of 10.1 amperes, but the neutral conductor has a current of only 10 amperes. The GFCI is designed to detect this current difference to protect personnel by opening the circuit when it detects a current difference of approximately 5 milliamperes (0.005 ampere). The National Electrical Code (NEC ) 210.8 lists places where ground-fault protection is required in dwellings. The National Electrical Code and NEC are registered trademarks of the National Fire Protection Association, Quincy, MA.
--19 Ground-fault circuit breaker.
--21 Ground-fault extension.
--20 Ground-fault receptacle.
Several devices can be used to provide ground-fault protection, including the ground-fault circuit breaker. The circuit breaker provides ground fault protection for an entire circuit, so any device connected to the circuit is ground-fault protected. A second method of protection, ground-fault receptacles, provide protection at the point of attachment. They have some advantages over the GFCI circuit breaker. They can be connected so that they protect only the devices connected to them and don’t protect any other outlets on the same circuit, or they can be connected so they provide protection to other outlets. Another advantage is that, because they are located at the point of attachment for the device, there is no stray capacitance loss between the panel box and the equipment being protected. Long wire runs often cause nuisance tripping of GFCI circuit breakers. A third ground-fault protective device is the GFCI extension cord. It can be connected into any standard electric outlet, and any devices connected to it are then ground-fault protected.
Arc-Fault Circuit Interrupters (AFCIs)
Arc-fault circuit interrupters are similar to ground fault circuit interrupters in that they are designed to protect people from a particular hazard. Where the ground fault interrupter is designed to protect against electrocution, the arc-fault interrupter is intended to protect against fire. Studies have shown that one-third of electrical related fires are caused by an arc-fault condition. At present, the National Electrical Code requires that arc-fault circuit interrupters be used on all 120-volt, single-phase, 15- and 20-ampere circuits installed in dwelling units supplying power to family rooms, dining rooms, living rooms, parlors, libraries, dens, bedrooms, sunrooms, recreation rooms, closets, hallways, or similar rooms or areas.
An arc-fault is a plasma flame that can develop temperatures in excess of 6000°C (10,832°F). Arc faults occur when an intermittent gap between two conductors or a conductor and ground permits current to "jump" between the two conductive surfaces. There are two basic types of arc faults, the parallel and the series.
--22 Parallel arc faults are caused by two conductors touching.
Parallel Arc Faults
Parallel arc faults are caused by two conductors becoming shorted together. A prime example of this is when the insulation of a lamp cord or extension cord has become damaged and permits the two conductors to short together. The current in this type of fault is limited by the resistance of the conductors in the circuit. The current in this type of fault is generally much higher than the rated current of a typical thermomagnetic circuit breaker. A continuous short will usually cause the circuit breaker to trip almost immediately because it will activate the magnetic part of the circuit breaker, but an intermittent short may take some time to heat the thermal part of the circuit breaker enough to cause it to trip open. Thermal/magnetic type circuit breakers are generally effective in protecting against this type of arc fault, but cords with small-size conductors, such as lamp and small extension cords, can add enough resistance to the circuit to permit the condition to exist long enough to produce sufficient heat to start a fire.
Parallel arc faults can be more hazardous than series arc faults because they generate a greater amount of heat. Arc faults of this type often cause hot metal to be ejected into combustible material. Parallel arc faults, however, generally produce peak currents that are well above the normal current rating of a circuit breaker. This permits the electronic circuits in the arc-fault circuit interrupter to detect them very quickly and trip the breaker in a fraction of a second.
--23 Series arc-faults are generally caused by bad connections.
Series Arc Faults
Series arc faults are generally caused by loose connections. A loose screw on an outlet terminal, or an improperly made wire nut connection, is a prime example of this type of problem. They are called series arc faults because the circuit contains some type of current-limiting resistance connected in series with the arc ( --23). Although the amount of electrical energy converted into heat is less than that of a parallel arc fault, series arc faults can be more dangerous. The fact that the current is limited by some type of load keeps the current below the thermal and magnetic trip rating of a common thermo/magnetic circuit breaker. Because the peak arc current is never greater than the normal steady current flow, series arcing is more difficult to detect than parallel arcing.
When the current of an arc remains below the normal range of a common thermo-magnetic circuit breaker, it cannot provide protection. If a hair dryer, e.g., normally has a current draw of 12 amperes, but the wall outlet has a loose screw at one terminal so that the circuit makes connection only half of the time, the average circuit current is 6 amperes. This is well below the trip rating of a common circuit breaker. A 6-ampere arc, however, can produce a tremendous amount of heat in a small area.
--24 Current spike produced by turning a light on or off.
--25 Waveform produced by typical arc fault.
There are conditions where arcing in an electric circuit is normal, such as:
++ Turning a light switch on or off
++ Switching on or off of a motor relay
++ Plugging in an appliance that is already turned on
++ Changing a light bulb with the power turned on
++ Arcing caused by motors that contain a commutator and brushes
The arc-fault circuit interrupter is designed to be able to distinguish between normally occurring arcs and an arc fault. An arc caused by a toggle switch being used to turn a light on or off will produce a current spike of short duration. An arc fault, however, is an intermittent connection and will generally produce current spikes of various magnitudes and lengths of time. In order for an arc-fault circuit interrupter to determine the difference between a normally occurring arc and an arc fault, a microprocessor and other related electronic components are employed to detect these differences. The AFCI contains current and temperature sensors as well as a microprocessor and nonvolatile (retains its information when power is switched off) memory. The current and temperature sensors permit the AFCI to operate as a normal circuit breaker in the event of a circuit overload or short circuit. The microprocessor continuously monitors the current and compares the wave form to information stored in the memory. The microprocessor is monitoring the current for the magnitude, duration, and length of time between pulses, not for a particular waveform. For this reason, there are some appliances that can produce waveforms similar to that of an arc fault and may cause the AFCI to trip. Appliances containing motors that employ the use of brushes and a commutator, such as vacuum cleaners and hand drills, will produce a similar waveform.
--26 Arc-fault circuit breaker.
--27 The arc-fault interrupter connects in the same manner as a ground-fault interrupter.
Connecting an Arc-Fault Circuit Interrupter
The AFCI is connected in the same manner as a ground fault circuit breaker.
The AFCI contains a white pigtail that is connected to the neutral bus bar in the panel box. Both the neutral and hot or ungrounded conductors of the branch circuit are connected to the arc-fault circuit breaker. The circuit breaker contains a silver-colored and a brass-colored screw. The neutral or white wire of the branch circuit is inserted under the silver screw, and the black wire is inserted under the brass screw. A rocker switch located on the front of the AFCI permits the breaker to be tested for both short and arc condition. In addition to the manual test switch, the microprocessor performs a self-test about once every 10 minutes.
Grounding is one the most important safety considerations in the electrical field. Grounding provides a low resistance path to ground to prevent conductive objects from existing at a high potential. Many electric appliances are pro vided with a three-wire cord. The third prong is connected to the case of the appliance and forces the case to exist at ground potential. If an ungrounded conductor comes in contact with the case, the grounding conductors conduct the current directly to ground. The third prong on a plug should never be cut off or defeated. Grounding requirements are far too numerous to list in this section, but NEC 250 covers the requirements for the grounding of electrical systems.
Also see: Electrical safety systems
Monday, February 25, 2013 15:15