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1、NFPA 77 Recommended Practice on Static Electricity 2000 Edition NFPA, 1 Batterymarch Park, PO Box 9101, Quincy, MA 02269-9101 An International Codes and Standards Organization NFPA License Agreement This document is copyrighted by the National Fire Protection Association (NFPA), 1 Batterymarch Park,
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27、index of this document. (For further explanation, see the Policy Concerning the Adoption, Printing, and Publication of NFPA Documents, which is available upon request from the NFPA.) 771 Copyright 2000 NFPA, All Rights Reserved NFPA 77 Recommended Practice on Static Electricity 2000 Edition This edi
28、tion of NFPA 77, Recommended Practice on Static Electricity, was prepared by the Tech- nical Committee on Static Electricity and acted on by the National Fire Protection Associa- tion, Inc., at its World Fire Safety Congress and Exposition held May 1417, 2000, in Denver, CO. It was issued by the Sta
29、ndards Council on July 20, 2000, with an effective date of August 18, 2000, and supersedes all previous editions. This edition of NFPA 77 was approved as an American National Standard on August 18, 2000. Origin and Development of NFPA 77 An NFPA project addressing static electricity was initiated in
30、 1936, and a progress report was presented to the NFPA in 1937. A tentative edition of NFPA 77 was adopted in 1941. This tentative edition was further revised and officially adopted by the NFPA in 1946. Revisions were adopted in 1950, 1961, 1966, 1972, 1977, 1982, 1988, and 1993. The 2000 edition of
31、 NFPA 77 is a completely new presentation of the subject of static elec- tricity and its hazards. It reflects the current level of understanding of static electricity and presents considerable new information that explains the fundamental aspects of the phenom- enon and recommendations for evaluatin
32、g and controlling potential hazards. New informa- tion has been provided to address specific hazards of flammable gases and vapors and combustible dusts and also to address specific industrial processes and operations. The 2000 edition also provides the user with relevant properties of numerous comm
33、ercially significant materials, a glossary of terms, and diagrams that show acceptable methods of bonding and grounding. 772STATIC ELECTRICITY 2000 Edition Technical Committee on Static Electricity C. Charles Snow, Jr., Chair 3M Company, MN M Laurence G. Britton, Union Carbide Corp., WV U C. James D
34、ahn, Safety Consulting Engineers Inc., IL SE Vahid Ebadat, Chilworth Technology Inc., NJ SE Steven J. Gunsel, Marsh USA, Inc., OH I Francisco N. Nazario, Exxon Research possessing a conductivity greater than 104 pS/m or a resistivity less than 108 -m. 3.1.9 Conductor. A material or object that allow
35、s an electric charge to flow easily through it. 3.1.10 Grounding. The process of bonding one or more con- ductive objects to the ground, so that all objects are at zero (0) electrical potential; also referred to as “earthing.” 3.1.11 Ignitible Mixture. A gasair, vaporair, mistair, or dustair mixture
36、, or combinations of such mixtures, that can be ignited by a sufficiently strong source of energy, such as a static electric discharge. 3.1.12 Inert Gas. A nonflammable, nonreactive gas that ren- ders the combustible material in a system incapable of sup- porting combustion. 3.1.13 Nonconductive. Th
37、e ability to resist the flow of an electric charge. 3.1.14 Nonconductor. A material or object that resists the flow of an electric charge through it. 3.1.15 Semiconductive. Possessing a conductivity between 102 pS/m and 104 pS/m or a resistivity between 108 -m and 1010 -m. 3.1.16 Static Electric Dis
38、charge. A release of static electricity in the form of a spark, corona discharge, brush discharge, or FUNDAMENTALS OF STATIC ELECTRICITY775 2000 Edition propagating brush discharge that might be capable of causing ignition under appropriate circumstances. 3.1.17 Static Electricity. An electric charg
39、e that is significant only for the effects of its electrical field component and that manifests no significant magnetic field component. 3.2 Symbols and Units. The following symbols are used throughout this recommended practice and are defined as follows. A = ampere (coulombs per second). Electric c
40、urrent; the quantity of charge passing per second through a given point. C = capacitance (farads) d = diameter (meters) e = 2.718 base of Napierian (natural) logarithms E = V/m = electric field strength (volts per meter) = dielectric constant of a material 0 = 8.845 1012 farads per meter (electrical
41、 permittivity of a vacuum) 0 = electrical permittivity of a material (farads per meter) Is = streaming current (amperes) = liquid conductivity (picosiemens per meter) = ion mobility (square meters per volt-second) m = micrometers (microns) = 106 meters -m = electrical resistivity (ohm-meter) P = pre
42、ssure (millimeters of mercury) Q = quantity of electrical charge (coulombs) R = electrical resistance (ohms) = volume resistivity (ohm-meters) S = electrical conductance (siemens) t = elapsed time (seconds) = charge relaxation time constant (seconds) v = flow velocity (meters per second) V = electri
43、cal potential difference (volts) W = energy or work done (joules) Chapter 4 Fundamentals of Static Electricity 4.1 General. 4.1.1 The most common experiences of static electricity are the crackling and clinging of fabrics as they are removed from a clothes dryer or the electric shock felt as one tou
44、ches a metal object after walking across a carpeted floor or stepping out of an automobile. Nearly everyone recognizes that these phenom- ena occur mainly when the atmosphere is very dry, particularly in winter. To most people, they are simply an annoyance. In many industries, particularly those whe
45、re combustible materi- als are handled, static electricity can cause fires or explosions. 4.1.2 The word electricity is derived from the ancient Greek word for amber, elektron. The phenomenon of electrification was first noticed when pieces of amber were rubbed briskly. For centuries, the word elect
46、ricity had no meaning other than the ability of some substances to attract or repel lightweight objects after being rubbed with a material like silk or wool. Stronger electrification accompanied by luminous effects and small sparks was first observed about 300 years ago by von Guericke. In comparati
47、vely recent times, when the properties of flow- ing (current) electricity were discovered, the term static came into use as a means of distinguishing a charge that was at rest from one that was in motion. However, today the term is used to describe phenomena that originate from an electric charge, r
48、egardless of whether the charge is at rest or in motion. 4.1.3 All materials, whether solid or fluid, are composed of var- ious arrangements of atoms. The atoms are composed of posi- tively charged nuclear components that give them mass, surrounded by negatively charged electrons. Atoms can be consi
49、dered electrically neutral in their normal state, meaning that there are equal amounts of positive and negative charge present. They can become charged when there is an excess or a deficiency of electrons relative to the neutral state. Electrons are mobile and of insignificant mass and are the charge carri- ers most associated with static electricity. 4.1.4 In materials that are conductors of electricity, such as metals, electrons can move freely. In materials that are insula- tors, electrons are more tightly bound to the nuclei of the ato