Document XR45deVXa1JLDLGY9GgNnjprx

Safe Practices Pamphlet No. 52 Static Electricity Published by National Safety Council 425 North Michigan Avenue, Chicago 11 1. Sparks resulting from accumula tions of static electricity are a common cause of ignition in- accidental fires. Based upon investigation and experi ence, practical methods have been de veloped for the prevention or safe d;s charge of accumulations of static elec tricity. 2. Static electricity should not be confused with stray currents, although many of the things that can be done to eliminate static will also assist mate rially in reducing stray currents. Static is a type of electrical charge resulting from the contact and separation of two substances, one or both of which are non-conducting, and from electrostatic induction by a charged object. Its prin cipal hazard is that of fire or explosion due to discharges of sparks in the pres ence of flammable or explosive sub stances. 3. Static can be dangerous should a spark be discharged to or from a human body in that it may so startle a person that he may fall or come in contact with dangerous equipment. Relatively high potentials (300 volts and over) may produce discharges which will ig nite combustibie material or cause mus cular reactions in persons. 4. Hazards from static sparks are more severe in dry, cold weather than during humid, warm weather. During winter months, indoor surfaces in arti ficially heated buildings become dry be cause of low relative humidity. Conse quently, many surfaces decrease in their conductivity and may permit static elec tricity to accumulate. During summer months when relative humidity is high, most surfaces are coated with a film of moisture which makes them compara tively good conductors, and usually per mits the static to dissipate as it is generated. 5. Experiments have proved that static is generated by bringing together This pamphlet is one of more than 150 Safe Practices and Health Practices Pam phlets. it is a compilation of experience in accident prevention from many sources. It should not be assumed, however, that it includes every acceptable procedure in the field covered. It must not be confused with Amecican Standard safety codes; federal laws; insurance requirements; state law;, rules and regulations; and municipal ordinances. Additional copies of ibis pampbiei are available. Member price JO cents each. Quantity and nonmember prices on request. and separating two substances, one or both oi which are non-conductive. If a sheet of glass is coated with varnish and the varnish is allowed to dry, a static charge will be generated by peeling the dry varnish from the glass. The same is true in cleaning sheets of dried coat ing material from spray booths. Static is generated in this manner on belts and pulleys at the point the belt comes off the pulley. Experiments indicate that a Courteiy UnderwuUrj' Laboratories. Inc. Figure l. Gold-leaf electroscope. (See paragraph II.) tight pulley or one under load will gen erate as much static as a loose beit that slips on the pulley. 6. The generation of static elec tricity is difficult to prevent, but few hazards are involved in its generation. The hazard results when static accumu lates to such an extent that a spark is discharged. The hazard can be con trolled only by, preventing static accumulation. 7. Static charges of low potential (low voltage) are very common and fortunately are not always dangerous. They do not cause more trouble because their quantity and voltage are not al ways sufficient to cause hot sparks, and because an explosive mixture of com bustible substances is not always pres ent. The real danger from a static spark is largely a question of quantity, time (involving temperature of spark), and presence of something flammable which the spark can ignite. S. Tests made by the Underwriters' Laboratories, Inc., reported in Research Bulletin N'o. 8, "Generation of Static Electricity in Blower Systems," indicate that, "The static charge increases with increase in the concentration of dust or lint, decrease in particle size, velocity of air stream, and with decrease in the humidity of the atmosphere." This bul letin also states, "Not many data are available as to the minimum static charge required to cause ignition by spark discharges. Much depends upon the potential of the system and many other factors. Spark discharges from a circuit of very low electrical capacity charged by a small Wimshurst machine to a potential of 5,000 volts are capable of readily igniting gasoline vapor-air mixtures. It is likely, therefore, that in a blower system static charges of a still lower potential under favorable con ditions may cause ignition of readily combustible substances." (Copyright, 1942,1950, \ationol Safety Council. All rights reserved. Printed in US.A.) RSV 0005946 2 -- S.P.P. 52 SAFE PRACTICES PAMPHLET NO. 52 Courtesy Underwriters' Laboratories. Inc. Figure 2. Electrostatic voltmeter. (See paragraph 14.) 9. Voltages which result from mov ing rubber-tired vehicles, and from rapidly moving belts, conveyors, fabrics, paper and similar material, reach very high values under favorable conditions. Static charges are also produced when liquids, such as petroleum and pe troleum products, are pumped through pipe lines and hose. Also, high voltages may accumulate at nozzles or other openings of pipe lines and hose from rapidly issuing steam. However, many authorities believe that pure gas or air forced through pipes and orifices is not electrified and that for charges to be developed, liquid or solid particles must be present. Detecting Static Charges 10. One simple way to determine the presence of static electricity is to hold a piece of cotton thread near the object to be tested. If static is present, the fibres of the thread will stand out, giving the thread a bristle-like appear ance. 11. Another way to detect a static charge is through the use of a gold-leaf electroscope. The instrument is prac tical for industrial use but is compara tively delicate and will not withstand rough handling. Some persons object to its use because it is too sensitive to small charges and because it will indi cate the presence of a static charge but will give no indication of the strength of the charge except in an approximate degree. (See Figure 1.) 12. Another instrument that can be used to detect static charges is the neon tube tester. This device is frequently used fur testing spark plugs. It consists of a small cylindrical case containing a condenser at one end, a metallic top at the other, and between them a Geissler tube made of glass containing neon gas. When the tip is touched to a body containing a static charge, or even brought near it, part of the charge passes through the tube into the con denser, causing a bright, reddish-orange glow in the tube. These tests, however, are not always satisfactory. l.>. It should be borne in mind that in detecting static the neon tube will ordinarily glow only until the con denser in the handle becomes fully charged. Once the condenser has been charged, no more static will flow through the tube to produce a glow, is therefore necessary to discharge the condenser by grounding the. tip, after which it can be used ?g'air.. The neon tube tester should <iever be used if there is a possibility cnat contact may be made wi<h an electric power or lighting circii., inasmuch as there is danger of snock. 14. A fourth method of detecting a static charge is to use a static voltmeter. This instrument indicates not only whether an object is charged, but also the voltage of the charge. (See Figure 2.) Rubber-Tired Vehicles 15. Static electricity may be gen erated on rubber-tired vehicles by the rolling contact of the tires with road way surfaces. Such charges on passen ger vehicles are not only a nuisance, but may be hazardous. In the case of tank trucks carrying flammable liquids, such charges may be hazardous. 16. Static charges on vehicles have become such an annoyance to bridge toll collectors that grounded wires have been set in the paveihent on the approach to the toll collector's stand. These wires make contact with- the under-carriage of cars and discharge the accumulated static to ground. 17. The operation of motor buses over city pavements may cause such vehicles to become highly charged with static. This is borne out by the ex perience of a motor bus company in one of the larger cities. Numerous com plaints were made by patrons that in stepping off or onto a bus platform on certain days, slight shocks would be felt when they touched the hand-holds. The company grounded all of its buses by steel wire cables fastened to the brake and clutch levers which are electrically bonded to the chassis. When the brake is applied, the wire cable is thrown into contact with the ground, and if ground conditions are favorable, any static that may have coilected on the motor bus is dis charged. Since the buses have hep" grounder), there have been no f --j' complaints. 13. in addition to the use of steel cables for gro^cing on motor vehicles, strips of . inductive rubber and tires mad- -i conductive rubber have been ..-.1-0 successfully for grounding static electricity. Of course, the road surface must be conductive if this means of grounding is to be effective. Tank trucks 19. A number of fires and explo sions have been attributed to static dis charge on tank trucks hauling flam mable liquids. However, it is believed that in some cases the true sources of ignition were not determined and for lack of explanation were attributed to static. 20. There are two sources of static electricity to be considered in the trans portation of flammable liquids in tank trucks. One is the movement of liquids through hose and pipe lines and the drop through space into containers dur ing filling operations. The major source of static generation is the rolling con tact of rubber tires on road surfaces. The latter is particularly dangerous when the charge persists through the early part of the loading or unloading operation. 21. To prevent the accumulation of static charges and the dangers due to spark discharges during loading and un loading operations, it is essential that storage tanks, tank trucks, and con necting pipe lines and hose lines be elec trically interconnected before any of the gauge or fill openings are opened. It is important that the truck tanks, chassis, springs and axles be electrically bonded, if construction does not of itself provide electrical connection; also, that the chassis of trailer and semi-trailer units be electrically connected with the trac tor chassis. 22. When loading, it is essentia) that RSV 00059V? STATIC ELECTRICITY S.P.P. 52 -- 5 cawNteriow iw ciwuwo CMOi BOHDIHG a SflOUHPfHa TWOCKS a LOADING RJCK3 aOWPIWQ GWOWOtWC TtNK C*HS & UHLO*Q'G WI.CK& CPHWtCTtliq WOUNO CABLg TP LQuoma 3WXIT tank trucks be electrically intercon nected with the loading lines. (See Fig ure 3.) When unloading, it is essential that the tank truck be electrically bonded to the unloading connection, ot to the container to be filled. Particu larly is this true when pumping off fur nace oil, gasoline, etc. Such connections should be made before any of the gauge or fill openings are opened. 23. The unloading hose can be made to serve as the bonding connection by building a static wire in the hose and bonding the ends to the hose coupling. It is preferable to make use of tight connections between the unloading hose and the tank. When a iunnel is used, it should be provided with a good metalto-metai contact with the filling pipe and tank. 24, Example: At its loading plat forms one large oil company has con structed an overhead ground connection Figure 3. Grounding of tank cars and trucks. which hangs out over the trucks when loading, and which is connected to a metal contact lug welded on each truck. Workmen are required to attach the ground connection to the truck, thus equalizing the potential between the fill pipe and the truck. An overhead sign cautions workers- to "Ground Your Truck Before Loading." (Note: Grounding connections should be hung well clear of the tank; otherwise they provide a discharge point for static as the truck drives under the loading lines.) 25. Example: Another company, to ensure a continuous electrical connection between the tank truck and storage tanks, devised an interlock for the load ing cap of the truck. The loading cap is normally locked in a closed position. Before the truck can be filled, it is nec essary that the cap be unlocked by the grounding plug attached to one end of the storage tank grounding line. Courtesy The Pure Oil Company 26. The use of drag chains on tank trucks has been eliminated in the latest specifications of both the American Pe troleum Institute and the National Fire Protection Association. This elimination resulted from conclusive proof by vari ous agencies that a drag chain serves little purpose and, even under ideal road conditions, drains off oniy slightly more static charge than is drained from trans ports without chains. 2 7. It is felt that the drag chain on tank trucks might be relied on to drain off charges during loading or unloading operations and thus give a false sense of security. 28. Some states and municipalities require that drag chains be used, and such laws must be complied with. 29. Tank cars should be protected in filling and unloading by permanent bond between the track and the filling or unloading line and by a temporary RSV 0005948 n 4 -- S.P.P. 52 SAFE PRACTICES PAMPHLET NO. S2 All independent cargo tanks in wood hull tank vessels shall be electrically bonded to gether with stranded copper cable of not less than No. 4 B and S gage, and one end of this cable shall be grounded to a cop per or brass plate of not less than two square feet in area and it inch in thick ness, and this plate shall be securely fastened to the hull, on the outside, at a point where it shall be covered by water when the tank vessel is unloaded. DETAIL OP NOZZLE AND CONNECTIONS Groundtofrom /) ^~v>- 35. The grounding of shore pipe lines and electric bonding between pipe lines and the ship are important, and an established procedure should be fol lowed in making the bond. The Na tional Fire Protection Association Regu lations governing marine fire hazards require that: Grouii^ AM sci 3ordn4 cable >o some uninsulated metallic part pfthe oianem continuous metallic, contact .witn the Gas Tanfc. ^but not adjacent >a the fill opemne) before nstdinj nctil* nlo <hetn'. Oa not delach caoie until aflen nettle has been removed, and. caa replaced cn Tank., Before attaching hose for loading or dis charging, an electric bond shall be estab lished between the ship and shore by means of a No. 4 B and S gage copper cable. The cable shall be connected to the ship with switch or plug open, bond closed after at tachment, and left closed until all cargo movements have been completed and hose disconnected. The bond shall be broken by pulling the switch or plug and then discon necting from the ship. METHOD OF GROUNDING ANO BONDING TO PREVENT STATIC SPARKS DURING TANK TILLING OPERATION Courtesy National Fire Protection Association Figure 4. Method of grounding and bonding to prevent static sparks during tank ailing operations. (Sec paragraph 32.) bond between the filling stem and the car. The car tracks and the pipe lines should also be connected to ground. (See Figure 3.) 30. Ground connections of low re sistance should be installed. Many in dustrial concerns now require less than one ohm resistance for a ground to pre vent static accumulations. The main object is to provide a path just as short and easy as possible, from the point of generation of static to ground. 31. The Association of American Railroads (Circular No. 17-C) require ment for loading or unloading tracks is as follows: A permanent electrical connection shall be made between the rails on which rail equip ment may stand and the piping system used in connection with the transfer of inflam mable liquids. The electric connections shall not be less than one No. 4 or less than two No. 6AWG stranded copper, bronze or cop per-covered steel wire. Airplane Fueling Operations 32. The National Fire Protection Association (National Fire Codes for Flammable Liquids and Gases) recom mends the following precaution in air plane fueling operations (see Figure 4): (a) Gasoline hose shall be triplex metal lined or equivalent, with an adequate bond ing cable securely attached to the nozzle. (b) Before gasoline is transferred to airplane tank, the"clip on the bonding cable shall be attached to some uninsulated metal part of the airplane in continuous metallic contact with the tank but not adjacent to the tank filling opening. The bonding is abso lutely necessary to equalize static electrical charges and to prevent static sparks. 33. (See Data Sheet D-A. 1, "Elec trical Grounding of Airplanes.") Other Flammable Liquid Containers Tank vessels 34. Steel vessels are inherently grounded to the sea, but independent tanks in wooden vessels require sep arate grounding. General rules and reg ulations prescribed by the Board of Su pervising Inspectors, Bureau of Marine Inspection and Navigation, recommend; Portable gasoline containers 36. In filling a pail from a tank truck, sparks may occur if the container does not make a metal-to-metai contact with the truck. Wooden or painted han dles on the bails of cans serve to insu late them from the truck and permit a difference in potential to build up. Only cans with bare metal bails should be used. 37. In transferring flammable liquids from one metal container to another, the containers should be in contact with each other or electrically interconnected throughout the transfer so as to prevent accumulation of static charges. Where portable tanks, mixers or processing vessels are used for flammable liquids or compounds, they should be electri cally bonded and grounded at places where they are filled or emptied. (See Figure 5.) 38. If safety cans are filled with flammable liquids from drums or bar rels, the can and drum should be elec trically connected to each other and to ground. If gasoline filling station dis pensing hose is used, a connection be tween the drum and can is made by a metal-to-metal contact between the hose nozzle and the can. (See Figure 6.) Barrel or drum filling 39. In filling barrels with flammable RSV 0005949 STATIC ELECTRICITY S.P.P. 52 -- 5 liquids, provision should be made to es tablish metal-to-metal contact between the filling stem and the drum, or pref erably an electrical bond should be in stalled between the barrel and the filling stem. This can be done by placing a metal plate under the drum at the filling point and bonding the plate to the fill ing spout or stem. Another method is to use a cable with one end fastened per manently to the filling stem and the other end attached to the drum by means of a spring clamp. Tank steaming, cleaning 40.. Tank steaming and cleaning can be very dangerous operations due to the static accumulations if all steam lines are not connected and bonded to the tank. But if the bonding is intelligently done, a difference in potential cannot be built up. All nozzles or pipes through which steam and water are discharged into tanks should be effectively bonded to the grounded tank. Metallic structures 41. Oil companies have found it advisable to bond all parts of the permanent distribution and storage equipment. (See Figure 7.) One oil company makes a practice of bonding together not only all its metallic or metal-covered buildings, tanks, pipe lines, etc., but also all parallel or nearby railroad tracks, both main lines and spurs. 42. Numerous fires at first attrib uted to static have been traced to other causes. Stray currents leaking from electric railway tracks to steam railway tracks and sidings from them, set up a difference of potential between tank cars standing on the side tracks and the oil piping system provided for unloading the tank cars. This situation is aggra vated where the steam railway tracks make a direct crossing with electric rail way tracks. Insulating of side tracks and bonding of pipe lines to side tracks and all other equipment will help rem edy this condition. Belts 43. Static electricity is generated on drive belts by the contact and separa tion of the belt and pulley. As well as on drive belts, static charges are also found an conveyor and other types of ' belts. In one case, a charge of 45,000 ^ volts was reported on a rubber belt con.yeyor carrying grain. Courteiy National Fire Protection Aisocutign Figure 5. Grounding of portable flammable liquid containers. {Sec paragraph 37.) 44. In addition to the fire and ex plosion hazard, a heavy -static charge on a belt may cause -hect physic;-.! injury by attracting the hair of a person walk ing under or near it. Serious injuries have occurred where a girl's hair was attracted by static into contact with a belt and pulley. Grounding belts* 45. The static charge on a belt is not always eliminated by grounding the pulley or shaft; the belt itself should be effectively grounded. Investigators of the U. S. Department of Agriculture recommend the use of two wire combs, grounded and placed in contact with the belt at the points where the belt leaves the pulley. Satisfactory results have also been reported from placing metallic combs close to, but not touching, the surface of the belt where it runs off the pulley. The points of the comb should be needle sharp. (See Figure 8.) 46. A comb can be made by clamp ing No. 80 mesh bronze wire between brass or bronze strips and placing it so that the wire ends are not more than /l z inch from the belt. The comb is then connected to ground. In some cases, metallic tinsel wrapped on a wood base has been effective in collecting static charges. 47. In grounding belts in a drycleaning establishment or other places where explosive vapor may be present, the comb, if used, must be either in con tact with the belt or very dose to it, so there will be only a brush discharge. If placed too far away, there will be "hot" sparks between belt and comb which may ignite the vapor. Combs are not commonly used in the presence of ether and other highly explosive vapors. This hazard can be greatiy reduced by using metal pulleys and belts made of con ductive rubber or their equivalent. 48. Conductive rubber belting has proved satisfactory in the control of static charges. In many cases, such belting is preferable to combs and other RSV 0005950 6 -- S.P.P. 52 SAFE PRACTICES PAMPHLET NO. 52 devices. Static combs and brushes be come misplaced and damaged due to the breaking of belts. If they are not re paired and adjusted promptly and well maintained, they may cause flash fires in explosive atmospheres. 49. Various types of belt dressings have also been used successfully in con trolling the static hazard. One company reports success from periodically apply ing to the outer surface of belts an emulsion of one-half glycerin and onehalf water. The glycerin attracts and holds the moisture. The effect of this is to form a film of moisture on the belr which conducts the static charge through the pulleys which are grounded. This substance should be applied with a brush when the belt is not in motion. 50. Many companies have used ef fectively colloidal graphite and carbon tetrachloride as a belt dressing in con trolling static discharges. Another com pany uses castor oil, well worked into both sides of the belt, followed by a coating of graphite grease applied to the outside. Another secures satisfactory results on a 6-inch belt by cleaning the inside of the belt and applying ordinary belt dressing. In these cases, the dress ings act as a conductor of static. 51. One investigator has reported recently' testing the follovfing formula for an anti-static leather belt dressing, and found it satisfactory. The quanti ties given below suffice for about one quart of dressing: Fish glue ................................ Glycerin ................................... Sulphonatcd castor oil. ... Water ....................................... Lampblack .............................. Ammonia water (2% so lution) .............................. 0 ounces 7 ounces 7 ounces 12 ounces 6 ounces 2 ounces The ingredients of this formula are added individually, in the order given. The glue and glycerin are first heated together for two hours with frequent stirring at a temperature at which the mixture barely boils; and the other in gredients are stirred into the mixture. The requisite quantity of 2% ammonia water can be made by adding % ounce of ordinary household ammonia to 1 y ounces of water. This mixture should be applied while the belt is not in mo tion, and should be allowed to dry be fore the belt is placed in operation. The dressing should be applied to the pulley side of the belt. A weekly application is usually sufficient to keep the static down to a negligible amount. 52. A formula with similar ingre dients is recommended by the National Fire Protection Association in their bul letin, "Belt Dressing for Eliminating Static." It is as follows: Fisli glue....................... ......... 100 c.c. Glycerine....................... Sulphonated castor oil .... 100 c.c. Water ........................... .......... 170 c.c. Lampblack .................. . Ammonia water (2% solu- cion) ......................... .......... 20 c.c. This dressing is easily applied with a brush but should not be applied while the belt is moving. It is not necessary tor the dressing to dry before the belt is run. However, if the belt has not been properly maintained, this dressing will cause the belt to stretch slightly. 53. It has been reported that a mix ture of nine ounces of high grade spar varnish and one ounce of lampblack makes a good dressing for rubber and composition belts. If a thinner is re quired to facilitate the application of the dressing, one which is relatively non-hazardous may be made by mixing together equal parts of varnish maker's naphtha (70' to 72' Baume) and car bon tetrachloride. In most cases, only a small amount of thinner will be needed. This mixture should be applied while the belt is not in motion and should be allowed to dry before the belt is placed in operation. The dressing should be applied to the pulley side of the belt. Fipure 6. Grounding of solvent containers. Outdoor installation. (See paragraph 38.) Grounding shafting 54. It is also important that the shafting of all metal pulleys be effec tively grounded. Copper brushes used on electroplating dynamos are suitable for this purpose. Such brushes depre ciate very little over a long period of time and require a minimum of atten tion. Shafting is usually grounded by grounding the bearing. If there is any doubt about an effective ground due to the lubricant used in the bearing, then the shaft and bearing should be grounded separately. (See Figure 9.) RSV 0005951 STATIC ELECTRICITY S.P.P. 52 -- 7 Courtesy National fire Protection Ajjociation Figure 7. Grounding of solvent storage tanks. (See paragraph 41.) 55. There are many cases where the grounding of shafting is useless, even in connection with high speed belts, as in cases where wood, paper, or wood base pulleys are used. In such cases, the charge should be collected as fast as it is formed from the face of the pulley with a comb or its equivalent, mounted on the open side of the pulley. Also, it is well to bear in mind that in many cases the static problem can be elimin. -cj entirely by substituting chain or g.;vr drives. 56. The methods of collecting static accumulations from belts and shaftings and other equipment are not effective unless the connection is properly made. In buildings, it is usually sufficient to make a ground connection to the cold water piping. If grounding connection is made to a water line which includes a meter, an electric "jumper" should be installed around the meter, since some water meters are good insulators. A grounding connection should never be made to a gas or oil pipe line. Static grounds should be made in multiple as a safety precaution. Dry-Cleaning Establishments 57. The contact and separation of material being dry-cleaned generate static charges which accumulate on the machines and on the fabrics. Unless these charges are dissipated by means of grounding, by humid atmospheres, or some other manner, fires and explosions may result should sparks occur. Be cause fabrics are usually poor con ductors, grounding may not always be sufficient to prevent static from ac cumulating. Some companies make it a practice of allowing dryers and wash ers to stand for five minutes or more after the machines Have been stopped so that static charges can be drained off. 58. All dry-cleaning equipment, in cluding storage tanks, reclaiming tanks, pumps, piping, washers, drying tumblers and extractors, should be effectively bonded and grounded. Underground storage tanks located in dry, sandy soil or covered with corrosive-resisting sub stances or other insulating materials, should also be equipped with ground connections. 59. Pipe linefs containing flanges with gaskets should be bonded across the flange unless a good metal-to-meta! bond is secured by means of the flange bolts. Also, all piping, pipe outlets and pipe manifolds should be bonded to ground. Safety cans for transporting solvents should be grounded when they are being filled or emptied. 60. Cylinders of washing machines and tumblers should be grounded through shaft ends and bonded to the machine shells. Some drying tumblers are provided with a device which ad mits live steam during the first four or five minutes of operation; this keeps the contents damp, thus leading off the static charges, and also helps to extin guish any small fires that may occur. 61. Several explosions, fires, and at least two deaths have resulted from ig nition of residual vapors in cleaning fur garments. Linings of garments sponged with a mixture of carbon tetrachloride and a flammable solvent are first tum bled in sawdust in drums, and then cleaned in "cages." The tumbling process removes the greatest part of the carbon tetrachloride but leaves the slower-evaporating but more dangerous solvent vapors on the garments. Static electricity generated in this process is sufficient to cause ignition. The ac cumulation of static can -be prevented by grounding the cage barrel and grounding the operator by means of a metal floor plate. Also, thorough airin': of garments between processes will lessen the fire hazard, 62. Fire hazards due to static sparks are greatly reduced in modern drycleaning plants by use of closed sys tems in which volatile solvents are kept out of contact with air. Clothing and other materials to be cleaned are placed in a washer with an air-tight door. Air is exhausted from the washer by means of a vacuum pump before the cleaning fluid is introduced. After the cleaning operation is completed, the cleaning fluid is drawn off and the garments dried before the machine is opened. In this process, no flammable mixture can accumulate in the machine. Also, there is no appreciable vapor given off by the goods when removed from the machine. This process not only increases the RSV 00Q5952 8 -- S.P.P. 52 SAFE PRACTICES PAMPHLET NO. 52 and that static sparks may be sufficient to ignite the dust. Figure 8. Proper location of combs for removing the static from belts. (See paragraph 45.) 66 One company reports that granu lar or pulverized non-conducting mate rials, if dry, easily become charged with static electricity when they are moved. The motion of the materials need not be violent; a bag of thoroughly dry ma terial, sensitive to the formation of static, may on a dry day accumulate a charge if merely lifted from the ground and set down again. 67. Dry materials become charged when allowed to slide down a metal or wooden chute. The charge is distributed over the individual particles and can not be readily discharged. There is a tendency, however, for a spark to occur at the moment the last of the material, leaves the chute. 68. Static sparks are said to be pro duced by emptying a single tray of dry, smokeless powder. Static electricity does not accumulate, however, if the air is damp or the powder is "green.'' 69. Conductive rubber tubing has been found to be effective in reducing static charges in dust-collecting and conveyor systems. In some cases, inert gas has been used instead of air in conveyor tubes to avoid the accumula tion of an explosive mixture. Courtesy National Fire Protection Association Figure 9. Positive methods of grounding shafting or metal rolls. (See paragraph 54.) safety of a plant, but results in econom ical operation since the system is en tirely enclosed and the solvent vapors are reclaimed. 63. Provisions should be made for humidifying dry-cleaning rooms in cold, dry weather and in dry climates. Since cleaning establishments are com paratively small, they lend themselves readily to such air conditioning. It is advisable to provide automatically-con trolled humidifiers that will guarantee a high relative humidity at all times. Al though a relative humidity of 60 per cent or higher is desirable, it is difficult to obtain in northern climates in cold,weather except in buildings especially constructed for this purpose. If a min imum relative humidity of even 50 per cent is to be maintained, double win dows will be required. Otherwise, the moisture in the air will condense on the cold window glass. 64. Another method of lessening the static hazard is to mix a conducting ma terial with the fabrics being cleaned. This result is obtained when silks are lightly steamed before running through a dry-cleaning operation. In this case, the moisture in the. fiber of the silk conducts the charges away as fast as they are formed, to some grounded part of the machine. Static in Powdered Materials 65. Powdered material, if permitted to fall through the air or blown through a pipe, or ejected from small nozzles or orifices, as in spray drying or dust collecting and conveyor systems, may generate static electricity. Such a charge becomes dangerous if the pow dered material forms an explosive dust mixture with the air, thus furnishing fuel for a static spark to ignite. It has been proved that any flammable dust may explode under favorable conditions, 70. One authority states that static may be found on rust particles escaping through the needle valve of a com pressed gas tank. Experiments have shown, it is said, that hydrogen gas escaping from such a tank, rusted on the interior, may thu be ignited. 71. Machinery or equipment in which material is finely divided, or through which powdered or finely di vided material passes, should be elec trically bonded and grounded at a suf ficient number of points to ensure the collection of all charges. This method of grounding such machinery has been found very efficient in preventing the fires which formerly occurred from static sparks igniting cotton in a cotton gin mill. 72. Threshing machinery should be similarly grounded where there is any danger of static. One year when the weather was dry there was a serious epi demic of threshing machines fires caused by static in several western states. The trouble disappeared entirely when the apparatus was grounded. RSV 0005953 STATIC ELECTRICITY S.P.P. 52 -- 9 73. A manufacturer of a food prod uct had trouble with static where the finely ground grain was blown through cloth screens. This was eliminated by the use of fine copper wires woven into the screen and grounded. 74. Serious trouble in connection with collection of sulphur dust, in a filter-type collector, has likewise been eliminated by grounding the collector bags. Another company, however, re ports that after repeated experiments it was unable to remove, by grounding of apparatus, the static charges which oc curred on wood pulp in the process of drying and sifting. Rubber Industry "5. The high insulating properties of the solvents and materials handled in the rubber industry are conducive to the formation of static electricity. The separation of fabrics, rubber strips, etc., in process tends to create static charges. 76. Static hazards in the rubber in dustry may be guarded against by several safety measures, including the use of high humidity, grounding, and static eliminators. Where humidification is used, it should preferably be main tained at least at SO per cent or 60 per cent of saturation. While grounding the machinery will not always give 100 per cent protection against fires, it wiil pro vide some degree of safety. Occasion ally, a combination of humidity, ground ing and the use of- static eliminators is used to control this hazard. 77. Metal parts of buildings includ ing door and window frames, should be well-grounded. Grounding should in dude piping and all machinery, belts and other moving objects. Metallic platforms which are grounded should be provided in front of all such equipment as spreaders and chums, so located that employees are obliged to stand on the platforms. 78. Material passing through spreaders should be thoroughly grounded by a number of collecting points in con tact with the material or by thoroughly grounded metal rollers provided for that purpose. (See Figure 10.) 79. One company reports a fire that occurred on a work table about 60 feet long. Material which had been coated with rubber cement and was piled on a zinc-covered table was being separated by employees. This operation charged the zinc top of the table and the charge traveled along the table top to a point close to where the zinc coverings were about Yi inch apart. The spark ignited the vapors from the stock and started the fire, which opened 12 sprinkler heads. To prevent a recurrence, the zinc table covering has been soldered together, and ground wires securely soldered to the table to carry away any charges that may be generated. Printing and Textile Plants 80. Electrostatic attractions and re pulsions give rise to a great deal of trouble in the handling of such non-con ducting materials as paper and textile fibers. Although usually only the carry ing on of the required process so as to get a good product is affected, some times a hazard results. For example, sheets of paper handled rapidly in a dry atmosphere acquire a charge of static, which makes it difficult to pile or stack them. 81. When a stack of paper sheets Is placed upon the table of a guillotine paper cutter, it is necessary to damp it solidly just back of the point of oper ation of the knife. If the sheets are charged, there is difficulty in getting them properly clamped, and many in juries have occurred in an effort to smooth out such a pile of paper. 82. A similar hazard is found where paper is hand-fed to some printing presses. If the sheets are so charged that they will not readily slide into place against the stops, there is a temp tation to reach in and smooth them into Cooney Netioflil Fire Protection Association Figure 10. Grounding for static removal from non-conducting materials. (See paragraph 78.) RSV 0005954 10 --S.P.P. 52 SAFE PRACTICES PAMPHLET NO. S2 place. To do this is of course extremely dangerous and results in many serious hand and finger injuries. 83. The priming press hazard is largely eliminated by getting rid of static charges. The air may be kept verv humid, which will render the paper or fabric moist enough to be a conduc tor, so that the static is discharged to the machine or any other grounded con ductor over which the paper passes. Also, various methods of grounding may be employed, including the use of metal lic tinsel wrapped on a wooden bar and placed near the point where the static is generated. The tinsel, of course, must be grounded. 84. The static problem is often con trolled in paper conversion machines by maintaining a relative humidity of 50 per cent at 72 degrees F. When these conditions of humidity and temperature are maintained constantly, there is suf ficient moisture in suspension in the air to dispose of the minute static charges which accumulate in paper material. 85. The relative humidity is ob tained by installing a wet steam jet on each blower-type heater. Wet steam for the jet is secured from the return steam line from the heaters and is discharged into the intake side of the blower. The velocity of the air passing through the heater and the blower blades tends to atomize the moisture. The heated coils prevent condensation in the blower. The heated air readily takes up the moisture and the blower distributes it through out the room. The wet steam jet is con trolled by an electrically-operated valve which is operated by a humidistat when the relative humidity drops below 50 per cent, and closes when the relative humidity reaches 55 per cent. (See Figure 11.) 86. Another method of controlling the static in the printing industry is by the use. of a gas flame. The flame is also used to aid in drying the ink as the paper comes from the presses. Of course, the use of an open flame in the presence of flammable vapors is extremely dan gerous. This hazard is present in vari ous types of printing, especially roto gravure printing. It is preferable to use high humidity in place of the gas flame. 87. .Another method is the use of the static neutralizer or static eliminator, a device which has been successful in the paper and textile industries. It con sists of a bar over or under which the paper or fabric passes without actually touching it. The bar may be supple mented by a second bar so that the paper or cloth passes between the two. The bar or bars are connected to a source of fairly high alternating voltage, but low amperage. The bar is provided with points extended toward the surface of the material to be discharged, but not touching it. The charged points dis sipate the static on the paper or other material by carrying the static off or neutralizing it with a like charge of op posite sign. (See Figure 12.) 38. There has been a good deal of discussion as to the wisdom of employ ing the above kind of device in places where there is an exposure to flammable or explosive vapors and gases. There is no doubt that this device will dispel a large part of the troublesome static on a fabric, for instance, but the use of the device has not been universally approved. There is a feeling that it is unwise to expose a conductor, such as that of the neutralizing bar, at a voltage capable of producing a spark in such a location. Steam Jets 89. Static is also said to be gener ated during the escape and condensation of steam. In one experiment, a hori zontal jet of steam was shot into the air. Practically no charge of static elec tricity was found on the hose, but a wire placed 15 feet from the outlet of the hose collected a charge. This ex periment was made after there had been a serious explosion of benzol vapor in an empty tank car being cleaned by steam. 90. One authority refers to a simi lar experiment in which "Steam, issuing from a boiler, produces a cloud in a high state of electrification, resembling a thunder cloud on a small scale," and says that this "is probably dependent upon modification of surfaces or surface tension." Drops of water must be present in the steam to produce static charges. An English experimenter built a hydro-electric machine based on this principle 100 years ago for the definite purpose of producing static charges. Transmission Lines 91. A long electric transmission line, from which the current has been turned off, will retain a residual "static" charge. When an electric charge is turned into a long line, it takes a cer tain quantity of current to charge the line itself, just as it takes a certain amount of water to fill a pipe line. When the current is turned off, the elec trons remain on the line and may be drawn off as a static spark, the size RSV 0005955 I of the spark depending on the voltage and length of the line. 92. A transmission line may also become charged by induction and by wind friction. Cases have been reported where sufficient static has been built up on transmission lines during sandstorms to blow out the controls. 93. It is usual practice to ground transmission lines when they are out of service for any length of time and whenever any maintenance work is to be done. All important lines have permanent remote control grounding devices installed. Rules for grounding such lines are contained in the National Electrical Safety Code and the National Electrical Code. STATIC ELECTRICITY S.P.P. 52 -- n Paint Spraying 94. Paint spraying generates static electricity. The object being sprayed should be supported in such a way as to avoid insulating it from a conductive surface. In a test, charges of 8,000 to 10,000 volts were built up on a nonconductive plate, but no charge was ob served when a metal plate was used and grounded. Accumulation of Static on Persons 95. It is reported that a charge of 10,000 voits can be accumulated on a person's body by scuffing over a woolen rug or other insulated floor covering in a dry, cold atmosphere. Should flam mable vapors, dust or gases be present in explosive mixtures, an explosion and fire may result if a spark should occur when the charge goes to ground. 96. Static charges collected by work men can be controlled to some extent through the use of special-type footwear. Shoes that are well insulated by rubber soles, rubbers, or rubber boots, prevent the discharge of static electricity more so than leather, cord or other soles. Leather used in making soles tends to absorb moisture or perspiration and to some extent becomes a conductor and allows static to drain away. However, rubber cement used in manufacturing some leather shoes reduces the conduc tivity of leather soles. In some cases, shoes are made with non-sparking nails or rivets extending through the sole to increase conductivity. 97. Shoes with soles of electrically conductive rubber have been developed which are superior to ordinary leather shoes in respect to the dissipation of static charges. However, stockings made of wool or silk, or talcum powder in shoes, tend to nullify the value of a conductive shoe. Thin cotton socks are preferable to others in reducing body static. Conductive shoes are of most value where there is a conductive floor surface. Wax or other non-conductive floor finishings reduce the value of such a grounding measure. 98. Also, the' effect of conductive shoes may be impaired by accumula tions of foreign material on the bottom of the soles, as might happen should an employee walk across a waxed floor or work in an area where the floor is cov ered with French chalk or similar sub stances. 99. To determine the effectiveness of footwear in the control of static sparks, daily inspections are important. Such inspections may be visual to see that employees are wearing the correct type of socks or to observe the condition of the soles. Preferably, however, the inspection should be made with instru ments. (.See Figure 13.) This also ap plies to checking the conductive rubber tires on dollies and trucks. 100. It is good practice to provide some means of automatically grounding employees before they enter areas that may contain explosive mixtures. One ar rangement consists of a short flight of steps provided with a grounded hand rail, so located that the employees will use it to aid themselves in ascending the stairs. Another method is to install a grounded metaL self-closing gate that the employee must open to enter the area. 101. To guard against accumulation of static charges by workmen; a) Employees' footwear should be of con ductive type. b) Electrically conductive flooring, metal plates or floor^mats of conductive ma terial should be used in and around the hazardous area, extending a distance sufficient to permit draining of any static charges from the workmans hotly before they reach the point where they are dangerous. c) Increase the relative humidity of the room to SO per cent or higher. d) All metal equipment with which the workman may come in contact .should be electrically grounded. RSV 0005956 12 -- S.P.P. 52 SAFE PRACTICES PAMPHLET NO. 52 Humidity 102. It is important to remember that in heated buildings the air is al most always very dry in winter. If the outdoor temperature is 30 F, for exam ple, the air cannot possibly contain more than about 2 grains of moisture per cubic foot, representing 100 per cent humidity. If this air is taken into a building and warmed to 70 F without the addition of moisture, the relative humidity becomes only 25 percent, which is bad from the standpoint of health as well as static. The heating and venti lating equipment of every building should contain means for humidifying the air. Otherwise, open pet cocks can be fitted into the steam pipes or radiators. (See Figures 11 and 14.) 103. It takes a surprisingly large amount of moisture to humidify air properly in cold weather. If outside air at 0 F and 50 per cent humidity is heated to 70 F, about Zf/i grains of moisture per cubic foot must be added to maintain the same relative humidity. In a room 100 feet square and 12 feet high, with four changes of air per hour, over 2$ gallons oi water per hour are required. 104. The report of the N.F.P.A. Committee on Static Electricity states that the maximum relative humidity in the workroom which can be maintained without fogging or condensation on the inside of window panes is as follows: Outdoor Temperatures *F 0" .10' 20* 30 40* 50* 60g Relative Humidity Single Double Windows Windows 15% 2095, 27* 36* 47* 60% 78% 48% 53% 59% 66% 73% 81% 90% 105. It is easy to measure the hu midity of any room with a wet and dry bulb thermometer. Safe Practices Pam phlet No. 37, "Industrial Ventilation," contains a chart showing the relative humidity corresponding to various wet and dry bulb temperatures, and the cor responding quantity of moisture in grains per cubic foot of air. 106. The degree of conductivity of surfaces having low conductivity de pends on the relative humidity. One authority states that four grains of moisture per cubic foot of air disposed of static electricity in a rubber factory. Assuming a room temperature of about 70 degrees F,, this quantity of moisture means a relative humidity of 50 per cent. This degree of humidity was like wise found sufficient to keep down static difficulties by another company handling flammable material. Preventing Explosions 107. Sometimes it is impossible to prevent the occurrence of static sparks. Hence, it is always desirable, if possible, to remove the hazard by making it im possible for an explosive mixture of Courtesy X. 1. Humidifier Sc Ventilating Co. Figure 14. Humidifier--coupled to water and air lines. (See paragraph 102.) vapors, gases, or dusts to be present, as well as to eliminate all sources of ignition. 108. In the textile, dry-cleaning, and other industries where volatile sol vents are used, a process has been developed for drying out flammable solvents in an atmosphere of carbon dioxide and other gases instead of air, the oxygen being kept below the per centage necessary to form an explosive mixture. 109. In handling pulverized or finely divided materials, the explosion hazard may be reduced by providing equipment to exhaust such materials. This, like other methods mentioned in this section, removes the hazard, whether caused by static or by the presence of some foreign object, by pre venting the accumulation of an explo sive concentration. Fires and explosions are sometimes ascribed to static, which actually were caused by some hard or metallic object striking a spark when passing through grinding rolls, convey ors tubes, exhaust ducts, etc. Courtesy Tlie Procter & Gamble Defense Cnrp. Figure 13. Testing shoes with conductivity indicator. (See paragraph 99.) A CKNO WLEDGMEN T This pamphlet has been revised by the Industrial Department, National Sajety Council, and has been reviewed by the Safe Practices Conference Com mittee and a special committee on static electricity tinder the chairmanship of A. H. Nuckolls, Underwriters' Labora tories, Inc. Grateful acknowledgment is made to the National Fire Protection Association for permission to use data and illustrations from the report of its committee on static electricity. Rep. 2M-- 1150--WHP RSV 0005957