SE-I COMMITTEE PERSONNEL AND VOTE STATEMENT GENERAL 77-5 Recommended Practice on Report of the Committee on Static Electricity Static. Electricity- NFPA No..77--i977 Richard D. Stalker, Chairman Xerox Corp., Stamford, CT 06904 (rep. NFPA Industrial Fire Protection Section) Chapter 1 General R. D. Boddorff, Institute of Makers of E. L. Litchtield, 13ureau of Mines, U.S. / Explosives Department of the. Interior K. M. Carlisle, Manufacturing Chemists A. C. Lloyd, Int'l Fabricare Institute "1-1 Purpose. Assn. J. F. McKenna, American Petroleum C. W. Conaway, Industrial Risk Insurers Institute 1-1.1 Thepurpose of this recommended practice is to assist R. B. Fuller, American Mutual Insurance J. E. Owens, Instrument Society of America • in reducing the fire hazard of static electricity by presenting a Alliance Dr. A. F. Robertson, Fire Protection Section, -discussion of the nature and origin of static charges, the general H. F. George, Technical Assn. of the Nat'l Bureau of Standards methods of mitigation and recommendations in certain specific Graphic Arts E. J. Sehl~r, Underwriters Laboratories operations for its dissipation. D. L Ha/l, Millers Nat'l Federation Inc. C. F. Hedlund, Factory Mutual Research "B. Sperm,an, M & M Protection Consultants 1-i.2 Static electricity is often the i.gnition source for an Corp. J. It. warren, American Petroleum Institute ignitible mixture, an operating problem in industry or an an- O. W. Johnson, Palo Alto, CA Dr. J.F.I. ~ick, American Society of noyance to some individuals. _ • R.J. Kunz, 3M Company Anesthesiologist~ 1-2 Scope. 1-2.1 This publication covers methods for the control of Alternates static electricity for the purpose of eliminating or mitigating its W. L. Bulkley, Amoco Research Center Lawrence H. Foamier, American Mutual fire hazard, except as provided in 1-2.3 below. (Alternate to J.H. Warren) Insurance Alliance (Alternate to R. B. Fuller) 1-2.2 Chapters 5 through 8 cover the common commercial and industrial operations where the presence of static is a hazard. W. A. Rand, Industrial Risk Insurer's Static is generated in many other circumstances where its accumu- (Alternate to C. W. Conaway) lation does not constitute a direct fire or explosio n hazard. Some of these occurrences may'be mentioned in passing but they are not the primary concern of this publication. ThA list ~eptesent* the membershtp at the lime the Committee u~ts balloted on the text o/this edition. Since that time, change.s in the membership may have occurred. 1-2.3 The prevention and control of static electricity in hospital operating rooms or in areas where flammable anesthetics are administered are not covered by this publication but are covered in the Standard for the Use of lnhalation Anesthetics, NFPA No. 56A. The Committee on Static Electricity presents for official adop-. 1-2.4 Lightning is not covered by this publication but is tion the report on revisions to the Recommended Practice on Static covered in the Lightning Protection Code, NFPA No. 78. Electricity, NFPA No. 77. This report has been .submitted to letter ballot of the Committee which • 1-3 D~finitigns (see also Appendix A). consists of l 9 voting members, Ofwhom 15 have voted affirmatively, and 4 1-3.1 Ignitible Mixture shall mean a vapor-air, gas-air, ,IL have not returned ballots. Messrs. Boddorff, Carlisle, Hail and Warren dust-air mixture or combinations of these mixtures which can be have not returned ballots. ignited by a static spark. 77-6 ,STATIC ELECTRICITY "GENERAL 77-7 1-3.2 Static Spark shall mean an impulsive discharge of (b) Steam, air, ,or gas flowing from any opening in a electricity across a gap between two points not in contact. pipe or hose, when the steam is wet or the air or gas 1-3.3 Bonding and Grounding are defined in Chapter 3 as stream contains particulate matter, the act or process of applying or affixing a bond or ground (c) " Nonconductive power or conveyor belts in motion, connection between two objects. (d) Moving vehicles, and 1-3.4 The words Bonded or Grounded, as they are used in (e) Motions of all sorts that involve changes in relative the text, inust be understood to mean either that a bond or position of contacting surfaces, usually of dissimilar ground as defined has been deliberately applied, or that'an- liquids or solids. electrically conductive path having a resistance adequately low for the intended purpose (usually 10 6 ohms or'less) is inherently 1-4.5 The object of most static-corrective measures is to present by the nature of the installation. provide a means whereby charges separated by whatever cause' may recombine harmlessly before sparking potentials are at- tained, or ~o avoid spark gaps where harmful discharges could . occur. 1-4 Introduction. 1-4.6 If hazardous static conditions cannot be avoided in 1-4.1 The term "static electricity," as usi~d in this publica- certain operations, means must be taken to assure that there are. tion, "shall mean electrification of materials through physical no ignitible mixtures ~at points where sparks may occur.. contact and separation, and the various effects that result from the positive and negative charges so formed -- particularly where the y..constitute, a fire or ex p losion hazard. The generation of static electricity cannot be prevented, absolutely, because its intrinsic origins are present at every interface. 1-4.2 The development of electrical charges may not be in itself a potential fire or explosion hazard. There must be a discharge or sudden recombination of separated positive and negative charges. In order for static to be a source of ignition, four conditions must be fulfilled: (a) There must first 0fall be an effective means of static generation, (b) There must be a means of accumulating the sepa- rate charges and maintaining a suitabl~ difference of electrical potential, (c). There must be a spark discharge of adequate- energy, and (d) The spark must occur in an ignitible mixture. 1-4.3 The accumulation of static charges may be prevented under many circumstances by grounding or bonding, by humid- ification, or by ionization. These means and their functions will be discussed in Chapter 3. 1-4.4 Common sources of static electricity include: (a) Pulverized materials passing through chutes or pneumatic conveyors,- • 77-8 STATIC ELECTRICITY GENERATION AND ACCUMULATION 77-9 2-1.4 The charge may be either positive (+) or negative (-). At one time it was thought that the two charges were two kinds of Chapter 2 Generation and Accumulation electricity and that in a neutral' (uncharged) body they were of Static Electricity present in exactly equal amouots. Now it Is known that there is actually only one kind of electricity', although it is described by many adjectives. It is manifested when some force has abnormally 2-1 General. separated a few of its positive and negative constituents.'These 2-1.1 TO the average person the words "static electricity" entities are components of all atom~, the outer electrons (-) and .wmit~ mean either a noise in the radio receiver which interferes the inner, nuclear,protons (+). Curiously, a surface that has an good reception or the electric shock experienced when excess or deficiency of one electron in every 100,000 atoms is very touching a metal object after walking across a carpeted floor or strongl), charged. sliding across the plastic seat cover in an automobile. Some people also have experienced mysterious crackling noises and a tendency 2-1.5 It is true, however, that in a neutral Or uncharged for some of their clothing to cling or st.ick tightly together.when body the tWO entities are presentin exactly equal amounts. Work wool, silk, or synthetic fiber garments are worn. Nearly eyeryone is required to separate positive and negative charges. Electricity, recognizes that these phenomena occur mainly when the atmo- therefore, is sometimes referred to as a form of energy produced sphere is very dry. To most people they are simply an annoyance. by expenditure of energy in some other form, such as mechanical, chemical, or thermal. Likewise, when electrical energy (a better 2-1.2 The word "electricity" is derived from the ancient term) is expended, its equivalent appears in one or the other of Greek work "elektron" -- meanirig amber -- for'it was with this these other torms. substance that the phenomenon of electrification was first ob- served. For centuries "electricity" had no other meaning than the 2-1.6 Electrons are free to move from one molecule to property exhibited by some substances, after being rubbed with a another in conductors but the proton, in the nucleus of the atom, material like silk or w0ol, of being able to attract or repellight- cannot move appreciably unless the atom moves. Therefore, in weight objects. Stronger electrification accompanied by luminous solids, only the electrons are mobile; in gases and liquids both are effects and small sparks was first observed about 300 years ago by free to move. yon Guericki~. In comparatively recent times, when the properties. of flowing eleiztricity were discovered, the word "static" came into 2ol.7 The stable structure of the atom shows that unlike "use as a-means of distinguishing the old from the new. The charges attract; conversely, like charges repel. It follows that a implication that such electricity is always at rest is erroneous; it is separated charge will be self-repellent-and will reside only on the when it ceases to rest that it causes the most concern. surface of a charged body. If the body were a perfect-insulator or 2-1.3 F6r the sake of simplicity, one may imagine electricity perfectly insulated, the charge would remain indefinitely.