electrical safety design n.
Skip this Video
Loading SlideShow in 5 Seconds..
Download Presentation


348 Vues Download Presentation
Télécharger la présentation


- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

  1. ELECTRICAL SAFETY DESIGN General Requirements Wiring Design and Protection Wiring Methods and Components Equipment for General Use.

  2. Introduction • This module deals with Occupational Safety and Health Administration (OSHA) standards for electrical safety design. • These standards cover only electrical system parts that an employee would use or with which she/he could make contact. • The module’s purpose is to minimize potential workplace hazards by specifying electrical equipment and systems design characteristics. © 2003 Seton Identification Products

  3. Development of The Code • The OSHA electrical standards were based on the National Fire Protection Association's standard NFPA 70E, "Electrical Safety Requirements for Employee Workplaces." The NFPA 70 Committee derived Part I of their document from the 1978 edition of the National Electrical Code (NEC). • The standards extracted from the NEC were those considered the most applicable to employee safety and least likely to change with each new edition of the NEC. • OSHA's electrical standards are performance oriented; therefore they contain few direct references to the NEC. However, the NEC contains specific information as to how the required performance can be obtained. © 2003 Seton Identification Products

  4. Introduction - Objectives • At the end of this module, you should be able to: • Identify the general requirements for electrical safety design • Define proper wiring design and methods • Define proper installation and protection of electrical equipment and components • Identify unsafe wiring design, methods, equipment, protection, and components. © 2003 Seton Identification Products

  5. General Requirements - Introduction • The General Requirements section, based on standard 1910.303, describes the general requirements for electrical safety design. • The topics for this section are listed on the left. • The conductors and equipment required or permitted by 1910.303 shall be acceptable only if approved. • Electrical equipment shall be safe from recognized hazards that are likely to cause death or serious physical harm to employees. • Examination and proper installation are important methods for ensuring safety from hazards. © 2003 Seton Identification Products

  6. Equipment Examination, Installation, and Use • The following considerations are examined to determine if equipment is safe: • Suitability for installation and use, in conformity with electrical standard provisions. Suitability of equipment for an identified purpose may be evidenced by listing or labeling for that identified purpose • Mechanical strength and durability, including the adequacy of the protection by parts designed to enclose and protect other equipment • Electrical insulation © 2003 Seton Identification Products

  7. Equipment Examination, Installation, and Use • Heating effects under conditions of use • Arcing effects • Classification by type, size, voltage, current capacity, and specific use • Other factors which contribute to the practical safeguarding of employees using or likely to come in contact with the equipment. • Listed or labeled equipment shall be used or installed in accordance with any instructions included in the listing or labeling and with the requirements on the following pages. © 2003 Seton Identification Products

  8. Splices and Arcing Parts • Conductors shall be spliced or joined with suitable splicing devices. • Another option is for conductors to be spliced by brazing, welding, or soldering with a fusible metal or alloy. • Soldered splices should be mechanically and electrically secure before soldering. • All splices, joints, and the free ends of conductors shall be covered with an insulation equivalent to that of the conductors, or covered with an insulating device suitable for the purpose. • Parts of electric equipment which in ordinary operation produce arcs, sparks, flames, or molten metal shall be enclosed or separated and isolated from all combustible material. © 2003 Seton Identification Products

  9. Marking • Electrical equipment may not be used unless the manufacturer's name, trademark, or other descriptive marking by which the organization responsible for the product may be identified is placed on the equipment. • Other markings indicating voltage, current, wattage, or other ratings shall be provided as necessary. • Marking is almost always provided on the name-plate of reputable manufacturers’ equipment. • The markings shall be of sufficient durability to withstand the operating environment. • Violations usually occur when the nameplate is covered, removed, or obliterated by painting or other abuse. • One example of abuse is shown to the left. • Importance of Marking © 2003 Seton Identification Products

  10. Disconnecting Means and Circuit Identification • Identification (as used in OSHA standards) means equipment is labeled to indicate its purpose. • Required disconnecting means for motors and appliances shall give legible indication of its purpose. • If the purpose of the circuit is obvious, no identification of the disconnect is required. • A disconnecting means is a switch used to disconnect the circuit conductors from the electric current source. • Disconnect switches protect workers and equipment by enabling a circuit to be opened to stop the flow of electricity. • Each service, feeder, and branch circuit shall have its purpose legibly identified (unless clearly evident) at its disconnecting means or over current device. • Identification should be located at the circuit origination point. • Identification Example © 2003 Seton Identification Products

  11. Identification Example • On a panel that controls several motors or on a motor control center, each disconnect must be clearly marked to indicate the motor to which each circuit is connected. • In this figure, the Number 2 circuit breaker in the panel box supplies current only to disconnect Number 2, which in turn controls the current to motor Number 2. • This current to motor Number 2 can be shut off by the Number 2 circuit breaker or the Number 2 disconnect. © 2003 Seton Identification Products

  12. 600 Volts, Nominal or Less • 1910.303 also addresses the safety of persons qualified to work on the equipment, but who do not remove guards and enclosures in order to work on live parts. • Table S-1 of 1910.303(g)(1)(i) lists the minimum dimensions for working space in the direction of access to live parts operating at 600 volts or less and likely to require examination, adjustment, servicing, or maintenance while energized except where required or permitted in Subpart S. • In addition, work space may not be less than 30 inches wide in front of the electrical equipment measured from exposed live parts, or from the enclosure front or opening if the live parts are enclosed. © 2003 Seton Identification Products

  13. 600 Volts, Nominal or Less • Concrete, brick, or tile walls are considered to be grounded. • Working space is not required in back of assemblies such as dead-front switchboards or motor control centers where there are no renewable or adjustable parts such as fuses or switches on the back and where all connections are accessible from locations other than the back. • Working space shall not be used for storage. • In addition the working space access, illumination, and headroom shall be adequately maintained. © 2003 Seton Identification Products

  14. 600 Volts, Nominal or Less • Except as required or permitted elsewhere in this standard, live parts of electric equipment operating at 50 volts or more shall be guarded against accidental contact by approved cabinets, enclosures, or by: • Location in a room, vault, or similar enclosure only accessible to qualified persons • Suitable permanent, substantial partitions or screens so arranged that only qualified persons will have access to the space within reach of the live parts and that any openings shall be so sized and located to prevent accidental contact with the live parts or to bring conducting objects into contact with them • Location on a suitable balcony, gallery, or platform • Elevation of 8 feet or more above the floor or other working surface. • In locations where electric equipment would be exposed to physical damage, enclosures or guards shall be so arranged and of such strength as to prevent such damage. • The purpose of guarding equipment is to protect any person, not just qualified electricians, against accidental electrical equipment contact. © 2003 Seton Identification Products

  15. Over 600 Volts, Nominal • Electrical equipment operating above 600 volts, nominal creates extremely dangerous working environments. Therefore, it is important to heed the following requirements. • The entrances to all buildings, rooms, or enclosures containing exposed live parts or exposed conductors operating at over 600 volts, nominal shall be under the observation of a qualified person or locked at all times. © 2003 Seton Identification Products

  16. Over 600 Volts, Nominal • Electrical installations in the following areas where access is controlled by lock and key or other approved means are considered sufficient to prevent unqualified access: • Area surrounded by a wall, screen, or fence 8 feet or higher unless other features provide a degree of isolation equivalent to an 8 foot fence • Vault • Room • Closet © 2003 Seton Identification Products

  17. Over 600 Volts, Nominal • Electrical installations operating over 600 volts, nominal having exposed live parts shall be accessible to qualified persons only. • Sufficient space shall be provided and maintained about electric equipment to permit ready and safe operation and maintenance of such equipment. • Where energized parts are exposed, the minimum clear work space may not be less than 6 feet 6 inches high (measured vertically from the floor or platform), or less than 3 feet wide (measured parallel to the equipment). © 2003 Seton Identification Products

  18. Over 600 Volts, Nominal • The minimum depth of clear working space in front of electric equipment is given in Table S-2 of 1910.303(h)(3)(i). • Unguarded live parts above working spaces shall be maintained at elevations not less than specified in Table S-3 of 1910.303(h)(3)(iii). © 2003 Seton Identification Products

  19. Over 600 Volts, Nominal • At least one entrance not less than 24 inches wide and 6 feet 6 inches high shall be provided to give access to the working space about electric equipment. • On switchboard and control panels exceeding 48 inches in width, there shall be one entrance at each end of such board where practicable. Where bare energized parts at any voltage or insulated energized parts above 600 volts are located adjacent to such entrance, they shall be suitably guarded. • Adequate illumination shall be provided for all working spaces about electric equipment operating over 600 volts, nominal. • The lighting outlets shall be arranged so persons changing lamps or making repairs on the lighting system will not be endangered by live parts or other equipment. • The points of control shall be so located that persons are not likely to come in contact with any live part or moving part of the equipment while turning on the lights. • Permanent ladders or stairways shall be provided to give safe access to the working space around electric equipment installed on platforms, balconies, mezzanine floors, or in attic or roof spaces. © 2003 Seton Identification Products

  20. Quiz Question: • One of the primary purposes of soldering is to mechanically and electrically secure two conductors. • True • False © 2003 Seton Identification Products

  21. Quiz Question: • Identification of "disconnecting means" is used to: • Mark the power rating of the device • Mark the purpose of the device • Mark the manufacturer of the device • Mark the current and voltage rating of a device © 2003 Seton Identification Products

  22. Quiz Question: • A wall, screen, or fence less than 8 feet in height is not considered sufficient to prevent access to electrical equipment operating at over 600 volts, nominal. • True • False © 2003 Seton Identification Products

  23. Wiring Design and Protection • Circuits connected improperly can pose serious shock or burn hazards. • The Wiring Design and Protection section, based on standard 1910.304, describes the requirements for proper wiring design and protection for worker and equipment safety. • The topics covered in this section are listed on the left. © 2003 Seton Identification Products

  24. Grounded and Grounding Conductors • Grounded/grounding conductors of an electrical circuit must be marked or color coded in a way that allows employees to identify and distinguish them from each other and from the other conductors in the circuit. • This section describes identification procedures for two such conductors. • Circuit Grounded Conductors • Equipment Grounding Conductors © 2003 Seton Identification Products

  25. Grounded and Grounding Conductors • Color coding is one means by which conductors are identified from other circuit conductors. • Acceptable color coding includes the method required by the National Electrical Code, Section 210-5. • Color Coding Example © 2003 Seton Identification Products

  26. Color Coding Example • The graphic illustrates color coding on a distribution panel board. • The National Electric Code, Section 210-5 states: • "The circuit grounded conductor of a branch circuit shall be identified by a continuous white or natural gray color. The equipment grounding conductor of a branch circuit shall be identified by a continuous green color or a continuous green color with one or more yellow stripes unless it is bare." © 2003 Seton Identification Products

  27. Color Coding Example • Bare copper or aluminum wire is permitted for use as a grounding conductor. • Distinct identification is important to maintain correct polarity between the ungrounded (hot) and grounded (neutral) conductor when plugs, receptacles, and connectors are used in an electrical branch circuit. © 2003 Seton Identification Products

  28. Wiring Design and Protection - Grounded and Grounding Conductors • The correct polarity between the ungrounded (hot) and grounded (neutral) conductors when using plugs, receptacles, and connectors is important because incorrect polarity can create dangerous situations. There are two basic guidelines: • No grounded conductor may be attached to any terminal or lead which would reverse designated polarity • A grounding terminal or grounding-type device on a receptacle, cord connector, or attachment plug may not be used for purposes other than grounding. • Many pieces of equipment will operate properly even though the supply wires are not connected in the order designated by design or the manufacturer. Improper connection is most prevalent on the smaller branch circuit typically associated with standard 120 volt receptacle outlets, lighting fixtures, and cord- and plug-connected equipment. © 2003 Seton Identification Products

  29. Wiring Design and Protection - Grounded and Grounding Conductors • In a common 120-volt outlet with a portable hand tool attached, the black conductor supplies power to the motor. • The grey (or white) conductor is used as a circuit grounded conductor for the motor. • This ground is needed to trigger the circuit breaker, thus stopping the motor if a fault occurs. • The green wire is used as the equipment grounding conductor to protect employees handling metal enclosures that may be energized by an internal fault. © 2003 Seton Identification Products

  30. Wiring Design and Protection - Grounded and Grounding Conductors • The graphic at the left illustrates the proper grounding for a hand tool. • An incorrectly connected circuit can cause dangerous situations. • You can inspect three examples and their outcomes by clicking on the examples below. • Reversed Polarity with Circuit Ground • Reversed Polarity with Equipment Ground • Reversed Circuit and Equipment Ground © 2003 Seton Identification Products

  31. Wiring Design and Protection - Service Entrance • The service entrance is where the serving conductors enter a building. • Means shall be provided to disconnect all conductors in a building or other structure from the service entrance conductors. • One disconnecting means for conductors is required to be located at a point near the service entrance. • The disconnecting means can be a switch or circuit breaker, and must be capable of interrupting the circuit from the source of supply. © 2003 Seton Identification Products

  32. Wiring Design and Protection - Service Entrance • The disconnecting means shall plainly indicate whether it is in the open or closed position. • Additional service entrance requirements exist for services over 600 volts, nominal: • Service-entrance conductors installed as open wires shall be guarded to make them accessible only to qualified persons • Signs warning of high voltage shall be posted where other than qualified employees might come in contact with live parts. © 2003 Seton Identification Products

  33. Conductors and Overcurrent Devices • Electric current is the flow of electrons through a conductor. • The size of the wire is the main determinate in deciding how much current can safely flow through a conductor. • A larger wire can carry more current safely. If too much current flows through a conductor, excess heat is produced. • If the circuit is not protected, the heat may continue to build until it reaches a temperature high enough to destroy the insulation and cause a fire. • The National Electrical Code specifies the allowable current flow permitted in certain-sized conductors. • Ampacity is the term used to describe the current-carrying capacity of a conductor. The size of the fuse or circuit breaker required to provide protection is determined by the ampacity of the conductor in the circuit to be protected and the type of load that is on the circuit. • Fuses and circuit breakers are designed to protect equipment and facilities against overcurrent and overheating, but they also provide considerable protection against shock in most situations. • The basic idea of a protective device is to make a weak link in the circuit. A fuse acts as a weak link that is the first piece of the system to be destroyed. • In the case of a circuit breaker, a set of contacts open the circuit and stops the flow of current. Unlike a fuse, a circuit breaker can be re-used by re-closing the contacts. © 2003 Seton Identification Products

  34. Wiring Design and Protection - Overcurrent Protection • Except for motors running overload protection, overcurrent devices may not interrupt the continuity of the grounded conductor unless all conductors of the circuit are opened simultaneously. • Unless excepted, overcurrent devices are always placed in the "hot" side of a circuit (usually a black wire) and in series with the load, so that all the current in the circuit must flow through overcurrent devices. © 2003 Seton Identification Products

  35. Wiring Design and Protection - Overcurrent Protection • Except for service fuses, all cartridge fuses which are accessible to other than qualified persons and all fuses and thermal cutouts on circuits over 150 volts to ground shall be provided with disconnecting means. • This disconnecting means shall be installed so that the fuse or thermal cutout can be disconnected from its supply without disrupting service to equipment and circuits unrelated to those protected by the overcurrent device. © 2003 Seton Identification Products

  36. Wiring Design and Protection - Overcurrent Protection • Overcurrent devices require proper operation and maintenance due to the tremendous amounts of energy they are likely to handle during a fault. • Device location must allow for immediate access by all employee or authorized building management personnel when trouble occurs. • Overcurrent devices may not be located where they will be exposed to physical damage. • Physical damage often results when they are located where they can be struck by lift trucks or other vehicles, by crane hooks, by materials being handled, etc. © 2003 Seton Identification Products

  37. Wiring Design and Protection - Overcurrent Protection • Most fuses and circuit breakers have the potential to emit flashes or sparks. • As a result, easily ignitable material should not be stored nearby. • Since fuses and overcurrent devices can sometimes operate at elevated temperatures, they shall be located or shielded so that employees will not be burned or otherwise injured by their operation. © 2003 Seton Identification Products

  38. Wiring Design and Protection - Overcurrent Protection • Circuit breakers shall clearly indicate whether they are in the open (off) or closed (on) position. • Where circuit breaker handles on switchboards are operated vertically rather than horizontally or rotationally, the up position of the handle shall be the closed (on) position. • If used as switches in 120-volt, fluorescent lighting circuits, circuit breakers shall be approved for the purpose and marked "SWD." © 2003 Seton Identification Products

  39. Quiz Question: • The circuit grounded conductor should be colored : • Green with yellow stripes • Blue • White • Green © 2003 Seton Identification Products

  40. Quiz Question: • Most equipment will NOT operate if the supply and grounding wires are not connected correctly. • True • False © 2003 Seton Identification Products

  41. Wiring Design and Protection - Grounding Requirements • Grounding electrical circuits and electrical equipment is required to protect employees against electrical shock, safeguard against fire, and protect against damage to electrical equipment. • There are two kinds of grounding which correspond to the two types of grounding conductors (circuit grounding conductors and equipment grounding conductors) described earlier in this module. • Electrical Circuit or System Grounding • Electrical Equipment Grounding • Circuit and Equipment Grounding Schematic © 2003 Seton Identification Products

  42. Wiring Design and Protection - Grounding Path • The path for electrical circuit and equipment grounding shall be permanent and continuous. • This requirement was extracted from NEC 250-51, Effective Grounding Path. • NEC 250-51, Effective Grounding Path © 2003 Seton Identification Products

  43. Wiring Design and Protection - Grounding Path • Remember the following items regarding safe grounding paths. • The fault current in AC circuits will be limited by the sum of resistance and reactance. The only low- reactance path is that which closely follows the circuit conductors. • If a metallic raceway system is used, make sure that the metallic system is continuous and permanent. • In cases where a metallic raceway system is not used, provide a green or bare equipment grounding conductor close to the supply conductors to assure that all enclosures are bonded together and to the source. © 2003 Seton Identification Products

  44. Support, Enclosure, and Equipment Grounding • Proper grounding is important to prevent shock, stabilize voltage, and channel fault currents. • Metal enclosures for service equipment shall be grounded. • Metal cable trays, metal raceways, and metal enclosures for conductors shall be grounded, except for: • Metal enclosures such as sleeves that are used to protect cable assemblies from physical damage © 2003 Seton Identification Products

  45. Support, Enclosure, and Equipment Grounding • Metal enclosures for conductors added to existing installations of open wire, knob-and-tube wiring, and non-metallic-sheathed cable which meet the following conditions: • Runs are less than 25 feet • Enclosures are free from probable contact with ground, grounded metal, metal laths, or other conductive materials • Enclosures are guarded against employee contact. © 2003 Seton Identification Products

  46. Support, Enclosure, and Equipment Grounding • Frames of electric ranges, wall-mounted ovens, counter-mounted cooking units, clothes dryers, and metal outlet or junction boxes which are part of the circuit for these appliances shall be grounded. • Exposed non-current carrying metal parts of fixed equipment which may become energized and meet the following conditions shall be grounded if: • Within 8 feet vertically or 5 feet horizontally of ground or grounded metal objects and subject to employee contact • Located in a wet or damp location and not isolated • In electrical contact with metal • In a hazardous (classified) location • Supplied by a metal-clad, metal-sheathed, or grounded metal raceway wiring method. © 2003 Seton Identification Products

  47. Support, Enclosure, and Equipment Grounding • If exposed non-current carrying metal parts of fixed equipment operates with any terminal at over 150 volts to ground which may become energized it must be grounded. • However, there are exceptions to this situation. These exceptions are: • Enclosures for switches or circuit breakers used for other than service equipment and only accessible by qualified person • Metal frames of electrically heated appliances which are permanently and effectively insulated from ground • The cases of distribution apparatus such as transformers and capacitors mounted on wooden poles at a height exceeding 8 feet above ground or grade level. © 2003 Seton Identification Products

  48. Support, Enclosure, and Equipment Grounding • Exposed non-current-carrying metal parts of cord- and plug-connected equipment which may become energized shall be grounded: • If in a hazardous (classified) location • If operated at over 150 volts to ground, except for guarded motors and metal frames of electrically heated appliances when the appliance frames are permanently and effectively insulated from ground • If the equipment is of the following types: • Refrigerators, freezers, and air conditioners • Clothes-washing, clothes-drying, dishwashing machines, sump pumps, and electrical aquarium equipment • Hand-held motor-operated tools • Motor-operated appliances of the following types: hedge clippers, lawn mowers, snow blowers, and wet scrubbers • Cord- and plug-connected appliances used in damp or wet locations or by employees standing on the ground or on metal floors or working inside of metal tanks or boilers • Portable and mobile X-ray and associated equipment • Tools likely to be used in wet and conductive locations • Portable hand lamps. © 2003 Seton Identification Products

  49. Support, Enclosure, and Equipment Grounding • If metal parts or tools are likely to be used in wet or conductive locations, they do not have to be grounded if their power is supplied through an isolating transformer with an ungrounded secondary winding of not over 50 volts. • Listed or labeled portable tools and appliances protected by an approved system of double insulation, or its equivalent, need not be grounded. • The equipment shall be distinctively marked to indicate that the tool or appliance uses an approved system of double insulation. © 2003 Seton Identification Products

  50. Support, Enclosure, and Equipment Grounding • The metal parts of the following non-electrical equipment shall be grounded: • Frames and tracks of electrically operated cranes • Frames of non-electrically driven elevator cars to which electric conductors are attached • Hand operated metal shifting ropes or cables of electric elevators • Metal partitions, grill work, and similar metal enclosures around equipment of over 750 volts between conductors. © 2003 Seton Identification Products