Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... Vol. 47, No. 4 • May-June 2011 In the Spotlight Electrical Equipment in Fueled Areas President's Message From the Editor's Desk TBD In the Spotlight: Electrical Equipment in Fueled Areas A Case of ALARP not being Applied to Automotive Traffic System Safety Safety Heuristics for Hospitals Book Review: How Reliable is Your Product?: 50 Ways to Improve Product Reliability, by Mike Silverman Chapter News Mark Your Calendar About this Journal Advertising in eJSS Contact Us Puzzle by Harvey C. "Chuck" Dorney, P.E., C.S.P. Pages 1 | 2 | 3 | 4 | 5 At the last International System Safety Conference (ISSC), Terry Osborn presented a version of "Electrical Safety 101" for hazardous areas. This paper will continue what Osborn started with version "201." The U.S. Air Force (USAF) has completed extensive work to determine the safety aspects of using electrical equipment around an aircraft containing fuel or undergoing fueling operations. This paper will describe the methods of evaluating and controlling the hazards associated with electrical power arcing, static electricity and radio-frequency (RF) radiation hazards. Although the primary hazards are associated with igniting fuel products, other hazard effects will also be discussed. Controlling these hazards has led to several safety policy and equipment changes, which will also be described. Introduction Electrical energy is all around us, and is used for many things, such as transmitting power (in our homes and offices) to operate devices and for communication (as in radio waves and data transmission). Because it involves energy, many hazards can be present. The most common is ignition, along with damage to personnel from electrical shocks. This paper focuses on ignition effects of electrical energy on ignitable substances, mainly aircraft fuels. It also describes efforts to provide confidence that portable electronic devices can be safely used around fueled aircraft. The Ignition Phenomenon To create a fire or explosion, certain things must be present: an ignition source, an ignitable material and an oxidizer. A classical fault tree approach can be used to identify how these conditions can co-exist. Figure 1 shows such an example. In addition to the items described, the proper conditions must be present. The ignitable material must be in the proper form and environment. For example, most hydrocarbon fuels must be in a proper fuel-air mixture to be ignited. Figure 1 — Ignition Fault Tree 1 of 3 6/17/11 3:43 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... Click to enlarge Electrical energy causes ignition in several ways: electrical arcing (including static discharges), heating a conductor (hot wire) and radio-frequency radiation. Each of these will be discussed separately. Electrical Arcing Whenever electrical contacts are made or broken, as through a switch, arcing occurs at the points of contact. The amount of arcing depends on four factors: the amount of electrical current (amperes), the electrical pressure (voltage), the size and shape of the contacts, and the surrounding material (air, water, oil, etc.). An example of significant low-voltage, but high-current, arcing can be found in an automotive battery. An example of high-voltage, low-current arcing would be static electricity discharge. Finally, lightning is a well-known example of a high-voltage and high-current arcing phenomenon. ...it can be seen that a circuit having 50 volts and 200 milli-amperes does not have sufficient spark energy to ignite a Group D substance, such as aviation jet fuel. However, a circuit with 50 volts and two amperes can cause ignition of the same substance. Not surprisingly, there have been attempts to define the range of ignition requirements. For example, when attaching a jumper cable to start a car, are those sparks capable of igniting flammable materials, such as gasoline or hydrogen? Apparently so, because we have always been cautioned about igniting materials when jump-starting a car. Underwriters Laboratories Standard UL 913 [Ref. 1] includes several graphs that show ignition ranges. Figure 2 is an example of such a chart, showing ignition ranges for an electrical resistance circuit. (Similar charts exist for inductance and capacitance circuits.) In the Figure 2 example, it can be seen that a circuit having 50 volts and 200 milli-amperes does not have sufficient spark energy to ignite a Group D substance, such as aviation jet fuel. However, a circuit with 50 volts and two amperes can cause ignition of the same substance. It must be noted that such charts are for the arcing from making or breaking electrical contacts; these charts do not address heating of a conductor from high electrical currents. Figure 2 — Ignition from Electrical Arcing Click to enlarge These charts have been used by a number of U.S.A.F. organizations to determine the relative safety of using electrical devices in hazardous areas. Generally, these charts show that the vast majority of devices powered by household 110- or 220-volt circuits are easily capable of serving as ignition sources for a variety of materials. On the other hand, many low-voltage battery circuits (e.g., below 10 volts) would need substantial electrical currents to create ignition sources. A standard batterypowered wristwatch is not considered an electrical ignition source. These watches have no safety labels or certifications, yet we are never cautioned against wearing them while refueling a car at a gas station. next page » 2 of 3 6/17/11 3:43 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... Vol. 47, No. 4 • May-June 2011 In the Spotlight Electrical Equipment in Fueled Areas President's Message From the Editor's Desk TBD In the Spotlight: Electrical Equipment in Fueled Areas A Case of ALARP not being Applied to Automotive Traffic System Safety Safety Heuristics for Hospitals Book Review: How Reliable is Your Product?: 50 Ways to Improve Product Reliability, by Mike Silverman Chapter News Mark Your Calendar About this Journal Advertising in eJSS Contact Us Puzzle by Harvey C. "Chuck" Dorney, P.E., C.S.P. Pages 1 | 2 | 3 | 4 | 5 Reference 2 provides another method of determining the capability of an electrical arc to be an ignition source. Past experiments have shown that a spark with an energy level of 0.25 milli-joules or more can be capable of igniting hexane gas. The U.S.A.F. is not aware of any similar experiments for automotive or aviation fuels, but considers that energy level to be valid for such fuels, due to the molecular similarity between hexane and the fuels. The 0.25 milli-joule level is not very high — this is approximately the spark level that is seen when touching a metal doorknob after walking on a carpet. In mechanical terms, this level is achieved by dropping a quarter from a height of one inch. One must keep in mind that the above values are for ideal conditions. The fuel-vapor mixture must be stoichiometric and the electrical contacts must be relatively small in the contact area. (Imagine trying to achieve a high spark level when using two large plates as contacts, compared to two button or needle contacts.) This latter consideration is due to the quenching effect from the contacts. If two contacts are close together, they can actually cool the spark to a point that the spark will not have sufficient energy to cause ignition [Ref. 2]. For example, a 0.25 milli-joule spark has a minimum quenching distance of approximately 0.06 inch between flat plates. Hot Wire Ignition A second cause of ignition is a hot surface (usually a wire) from excessive current. Just place a wire across the terminals of a car battery, and you can see the immediate effects. For a given voltage source, the current through a conductor depends on the resistance in the circuit. Ohm's Law states that the current multiplied by the circuit resistance is equal to the voltage. This can work both ways. For a given voltage and a resistor, one can directly determine the electrical current in amperes. In the other direction, given a current and a resistance, one can then determine the voltage drop across the resistance. Going back to our car battery, let us assume that it is capable of a constant 13.5 volts. If we place a wire having a one-ohm resistance across the battery terminals, the resulting current will be 13.5 amperes, which can generate considerable heating power. Electrical power (in watts) is simply the voltage multiplied by the current, which, in this case, is 188.25 watts. Unless the wire is considerably large, it will readily heat up, possibly to the point of failure or igniting a nearby flammable material. Past tests [Refs. 2 & 3] have shown that aviation and automotive fuels have a minimum hot surface ignition temperature of 900 degrees Fahrenheit (F) or higher. Other tests [Ref. 2] have shown that when a steel wire begins to glow from excessive current, the temperature is approaching 990 degrees F (A yellow wire glows at 1,800 degrees F, and a white-hot wire glows at 2,220 degrees F). In other words, if a conductor is visibly glowing, it is easily capable of igniting automotive or aviation fuels. Accordingly, circuits with high current capabilities must be controlled to prevent excessive heating of a conductor. Interestingly, hydrocarbon fuels cannot be easily heated in a microwave oven, nor by other means of generating RF radiation. The molecular structures (non bi-polar) of these fuels do not lend themselves to easy RF heating. It was once thought, these fuels could be easily heated by radio transmissions, so there was a universal prohibition against transmitting on aircraft radios during refueling. Radio-Frequency Heating Anyone who has used a kitchen microwave oven has witnessed a form of heating due to radiofrequency (RF) radiation. Basically, RF radiation can act as an ignition source in two ways: It can 1 of 2 6/17/11 3:43 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... directly heat a material, or it can induce voltages in conductive objects until arcing occurs. Both phenomena can occur in microwave ovens. Depending on its molecular structure, a substance can be quickly heated by RF radiation. In the low gigahertz range (3 to 5 billion Hertz or cycles per second), water will heat quickly, depending on the amount of RF power. However, other substances, such as cardboard and dry cereal, cannot be heated in a microwave oven. Interestingly, hydrocarbon fuels cannot be easily heated in a microwave oven, nor by other means of generating RF radiation. The molecular structures (non bi-polar) of these fuels do not lend themselves to easy RF heating. It was once thought these fuels could be easily heated by radio transmissions, so there was a universal prohibition against transmitting on aircraft radios during refueling. Once it was discovered that the fuels could not be directly heated, additional investigations were conducted; hence the second method of ignition was examined - inducing voltages into conductors so that they arced between them. Anyone who has placed aluminum foil in a kitchen microwave oven has probably seen this phenomenon. The exact amount of RF radiation needed to create an ignition source is not totally certain, but early tests [Ref. 4] showed that an RF power density of five watts per square centimeter was sufficient to generate sparks between metal shavings. That value was later adopted by U.S.A.F. technical Order 31Z-10-4 [Ref. 5] as the minimum safe power level to prevent ignition of hydrocarbon fuels. The manual states that an RF power density of five watts per square centimeter is necessary to cause a spark with sufficient ignition energy potential to ignite fuel vapors. For an omni-directional antenna, the RF power density is calculated by dividing the peak-radiated power by the area of the sphere whose radius is the distance from the antenna. For example, if a fuel line is 7.5 feet (225 cm) from a high-frequency (HF) antenna radiating at 1,000 watts, the RF power density (pd) at the vent would be: pd = Power/Area = 1000 w/4 (225)2 (1) = 0.00157 w/cm2 This is far too low to present an ignition source hazard at the fuel line. To reach a five w/sq cm density, a fuel line would need to be less than two inches from the 1,000-watt antenna. For a 10-watt antenna, the hazard distance becomes 0.2 inches. Normally, fueling equipment is not close enough to any antenna to create an ignition problem. As a result, UHF and VHF cockpit radios, hand-held radios and cellular telephones can be operated in a fuel environment. Cockpit radios are 10 to 30 watts, while hand-held radios and cellular telephones transmit at five watts or less. « previous page | next page » Copyright © 2010 by the System Safety Society. All rights reserved. The double-sigma logo is a trademark of the System Safety Society. Other corporate or trade names may be trademarks or registered trademarks of their respective holders. 2 of 2 6/17/11 3:43 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... Vol. 47, No. 4 • May-June 2011 In the Spotlight Electrical Equipment in Fueled Areas President's Message From the Editor's Desk TBD In the Spotlight: Electrical Equipment in Fueled Areas A Case of ALARP not being Applied to Automotive Traffic System Safety Safety Heuristics for Hospitals Book Review: How Reliable is Your Product?: 50 Ways to Improve Product Reliability, by Mike Silverman Chapter News Mark Your Calendar About this Journal Advertising in eJSS Contact Us Puzzle by Harvey C. "Chuck" Dorney, P.E., C.S.P. Pages 1 | 2 | 3 | 4 | 5 Directional antennas (e.g., radar, SATCOM) have higher RF power densities because they concentrate their power into a narrow beam instead of in an omni-directional manner. The amount of beam concentration depends on the antenna gain, which is an expression of the power ratio of a concentrated beam compared to an omni-directional beam. In other words: Gain (dB) = 10 log(P2/P1) (2) where P2 is the concentrated power and P1 is the omni-directional power. For example, if the antenna gain is 30 dB, the concentrated RF power density is 1,000 times greater than an omni-directional beam. Ignition hazards are then controlled by two methods: prohibiting the transmission (e.g., radar) or elevating the antenna (e.g., using SATCOM only when the beam is aimed at least 10 degrees above the horizon). Pulsed transmissions (e.g., identification friend or foe (IFF)) are time-averaged to determine their RF power densities. IFF units can have power spikes up to 500 watts, but their effective radiating power is actually less. Hand-held radios, cordless intercoms and cellular telephones do not present a significant ignition hazard. Transmitting at five watts or less, they are not likely to ignite fuel vapors by RF radiation. In addition, under normal operating conditions, these units are naturally safe due to their low battery voltages. In general, batteries having less than 10 volts cannot generate sufficient spark energy levels to ignite hydrocarbon fuel vapors. Some can, however, generate enough current to heat a conductor to an ignition temperature. As a result, some units could be potential ignition sources if they fail internally (i.e., short circuit), but these would present a problem only if near a fuel vapor area. Potential ignition sources should always be minimized, but there are several areas where the sources absolutely must be prevented. Prime examples of these areas are aircraft and automotive fuel tanks, and the immediate areas around fueling operations. Accordingly, several safety "zones" need to be defined. The Ignitable Material The automotive and aviation fuels addressed in this paper have many similar ignition properties. They have similar minimum hot surface ignition temperatures (900 to 1,100 degrees F), auto-ignition temperatures (where a heated vessel of fuel will self-ignite) of 450 degrees F and up, and the same minimum spark energy of 0.25 milli-joules. A major difference between some fuels is the flash point, i.e., the minimum temperature where an ignitable vapor can be found. For example, the flash points of gasoline and JP-4 jet fuel are below zero degrees F, while the minimum flash points of JP-5 and JP-8 jet fuels are 140 degrees F and 100 degrees F, respectively. (Commercial aviation fuels, e.g., Jet A-1, have flash points of approximately 100 degrees F.) If these temperatures are not present, the ignition source will not likely cause ignition of the fuel. However, this case applies only to fuel vapors; liquid fuel in a fine mist can be ignited at lower temperatures. This is evidenced by the ease of starting aircraft jet engines at temperatures that are well below 100 degrees F. Nevertheless, when electrical arcing or RF heating occurs, any fuel that is present is usually in vapor form, so the higher flash-point fuels will offer a considerable increase in safety. 1 of 2 6/17/11 3:44 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... One other point worth noting is that the fuel must be in the proper fuel-air ratio range to be ignited. The ideal ratio, usually approximately 14 to 1 by weight, is termed the stoichiometric ratio. An automotive carburetor or fuel injection computer has a primary function of ensuring the correct fuel-air ratio (mixture) for proper ignition. If the ratio becomes leaner (too much air) or richer (too much fuel), the minimum ignition properties change greatly. For example, a 40 percent leaning of a stoichiometric mixture of methane will triple the minimum spark energy ignition requirement. The Oxidizer The third part of the classic "fire triangle" (after an ignition source and an ignitable material) is the oxidizer. This paper deals with aircraft environments on the planet Earth, so the presence of an oxidizer (air) is a given. However, with increasing altitudes and, to some effect, increasing temperatures, the air becomes less dense, so its oxidizing capability is somewhat reduced. For example, at an altitude of 18,000 feet, the minimum spark energy level can be quadrupled or more. On the other hand, a higher altitude can improve the chances of ignition. On the ground, an aircraft fuel tank containing Jet A fuel will usually have an overly lean mixture in the ullage (the air above the fuel in the tank). However, at higher altitudes, the overly-lean mixture can approach a stoichiometric mixture as the air gets thinner and the mixture becomes richer. « previous page | next page » Copyright © 2010 by the System Safety Society. All rights reserved. The double-sigma logo is a trademark of the System Safety Society. Other corporate or trade names may be trademarks or registered trademarks of their respective holders. 2 of 2 6/17/11 3:44 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... Vol. 47, No. 4 • May-June 2011 In the Spotlight Electrical Equipment in Fueled Areas President's Message From the Editor's Desk TBD In the Spotlight: Electrical Equipment in Fueled Areas A Case of ALARP not being Applied to Automotive Traffic System Safety Safety Heuristics for Hospitals Book Review: How Reliable is Your Product?: 50 Ways to Improve Product Reliability, by Mike Silverman Chapter News Mark Your Calendar About this Journal Advertising in eJSS Contact Us Puzzle by Harvey C. "Chuck" Dorney, P.E., C.S.P. Pages 1 | 2 | 3 | 4 | 5 Applications Potential ignition sources should always be minimized, but there are several areas where the sources absolutely must be prevented. Prime examples of these areas are aircraft and automotive fuel tanks, and the immediate areas around fueling operations. Accordingly, several safety "zones" need to be defined. The National Electrical Code (NEC) [Ref. 6] defines Class I locations as those where flammable gasses or vapors may be present in sufficient quantities to produce ignitable mixtures. Class II and III locations are for ignitable dusts and fibers, respectively. A Class I location is sub-divided into Division 1 and Division 2 areas. Class I, Division 1 areas are those where ignitable vapors are expected to be present during normal operations; Class I, Division 2 areas are those where ignitable vapors are not normally expected to be present, but can be if a failure occurs. To add to this "alphabet soup," Class I, Division 1 and 2 locations have assigned groups A, B, C and D, depending on the ignitable material that is present. Automotive and aviation fuels fall into Group D, a lesser ignitable group (Group A is acetylene, which is very easily ignited). Examples of these locations can be found in and near an aircraft. The space inside an aircraft fuel tank is considered a Class I, Division 1 location, while the area outside a hangared aircraft, but within five feet of its fuel tanks and engines, is considered a Class I, Division 2 location (as long as the fuel temperature is above its flash point). If an aircraft is inside a hangar, then areas below the hangar floor level are considered Class I, Division 1 locations, and areas above the floor up to a height of 18 inches are considered Class I, Division 2 locations. For aircraft ground refueling operations, the area inside a 50-foot bubble around the aircraft is termed the fuel servicing safety zone (FSSZ) and is treated as a Division 2 location. During these operations, fuel entering the tank forces air outside the aircraft vent outlet, so this vapor-laden air is treated like a Division 1 location for 10 feet around the vent. (A similar, but much smaller, bubble is present around the tank filler port on an automobile being refueled. However, this bubble is almost non-existent if the gas station pump has a vapor recovery system that vacuums the fuel vapors.) Hazard Control Methods Given that air is present during all operations, the presence of an ignition source or a fuel source must be prevented. In some cases, such as in Division 1 locations, the presence of fuel cannot be prevented, so the ignition source must be prevented. There are several methods of preventing electrical ignition sources: Something to ponder: Many automobiles have electrical fuel pumps that are submerged in the fuel tanks, and use the fuel (gasoline) as a pump lubricant and a coolant. These pumps use quite a bit of energy. How do you suppose they keep from starting fuel tank fires? Using explosion-proof housings (usually used in Division 1 areas). Such housings are strong enough to contain potential explosions resulting from electrical sparks. Using sealed, or vapor-proof, housing (usually used in Division 2 areas). Such housings have gaskets or sealants to prevent fuel entry. Purging or filling with an inert material, such as nitrogen, to prevent entry of an ignitable material. Some oils have also been used for this application. 1 of 3 6/17/11 3:44 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... Using non-incendive circuits, which, under normal use, have insufficient energy to be capable of serving as an ignition source. A battery-operated wristwatch is a good example. Using intrinsically-safe circuits, which are similar to non-incendive circuits, but also are considered low-energy even if certain faults occur, such as a short circuit. Such devices are usually tested in an explosion chamber and are certified and labeled as such. However, there are many applications where none of the above conditions apply to an electrical device around a fueled area, so the U.S.A.F. has undertaken risk management procedures. Again, for a high ignition risk to be present, there must be a likely ignition source and a likely presence of fuel in an ignitable form (near stoichiometric and above its flash point.) With the exception of the spaces inside fuel tanks and around fuel vent outlets, the presence of such fuel is rare in aircraft operations. Accordingly, the U.S.A.F. has approved the operation of several non-certified or non-compliant electrical devices around an aircraft containing fuel or being refueled. A general summary of U.S.A.F.-accepted practices is shown in Table 1. Table 1 — Accepted Practices Battery voltage less than 12 volts No making or breaking connections Secured connections (locking rings on plugs) Do not open the case Do not change or charge the battery No evidence of physical damage Do not use dropped unit Do not use in Class I, Division 1 area Do not use near aircraft fuel vent Continuous vigilance for presence of fuel RF power density less than 5 watts/sq cm Something to ponder: Many automobiles have electrical fuel pumps that are submerged in the fuel tanks, and use the fuel (gasoline) as a pump lubricant and a coolant. These pumps use quite a bit of energy. How do you suppose they keep from starting fuel tank fires? Special Tests This paper identified several sources of safety data and criteria. However, there were still some cases where the data or criteria were non-existent. It became necessary to conduct some tests to verify the above-listed accepted practices. These tests were accomplished at the author's personal residence, but should not be tried at home without some safety precautions, such as eye protection and a standby fire extinguisher. Gasoline in Kitchen Microwave Oven Earlier in this paper, it was mentioned that automotive and aviation jet fuels could not be directly ignited by radio frequency radiation. This skeptical author placed a few grams of gasoline inside a sealed plastic bag, and inserted the bag into the household microwave oven. The oven was turned on at the "high" setting for a few seconds at a time, until about 30 seconds of cooking time had passed. The gasoline did not increase its temperature, thus verifying, to a small effect, that it is not susceptible to RF heating. (Incidentally, the sealed bag was used to ensure that the gasoline vapors did not reach the electrical portions of the oven.) Shorting an Automotive Battery with Gasoline Present Some of the previously mentioned UL ignition curves imply that electrical energy sources that have 12 volts or less might not have sufficient energy to be capable of spark ignition. However, anyone who has jump-started a car battery has probably observed substantial sparking that appears to be easily capable of igniting fuel. This once-again skeptical author placed a car battery on the ground and applied a very small amount of gasoline at both bare terminals. He then placed a steel shorting bar across the terminals, and, not surprisingly, ignition immediately occurred. Due to the very small amount of gasoline present, the small flame was quickly extinguished before it had an opportunity to ignite hydrogen from the battery. Shorting AA-, C-, and D-Cell and 9-Volt Batteries This test was also conducted at the author's home to determine the effects of a short circuit on standard household batteries, many of which are used to power portable electronic devices around fueled aircraft. For this test, standard AA-, B-, D-cell and square 9-volt batteries were held in a long shorted condition by using a C-clamp to see if they would rupture, explode, vent gasses or even elevate their temperatures. In all cases, no ruptures, explosions or venting occurred after being shorted for 30 minutes. However, each battery case temperature reached approximately 130 degrees (as determined subjectively by the author). These tests provided additional confidence in the safety of battery-operated portable electronic equipment around fueled aircraft. 2 of 3 6/17/11 3:44 PM eEdition Journal of System Safety http://www.system-safety.org/ejss/past/mayjune2011ejss/spotlig... Vol. 47, No. 4 • May-June 2011 In the Spotlight Electrical Equipment in Fueled Areas President's Message From the Editor's Desk TBD In the Spotlight: Electrical Equipment in Fueled Areas A Case of ALARP not being Applied to Automotive Traffic System Safety Safety Heuristics for Hospitals Book Review: How Reliable is Your Product?: 50 Ways to Improve Product Reliability, by Mike Silverman Chapter News Mark Your Calendar About this Journal Advertising in eJSS Contact Us Puzzle by Harvey C. "Chuck" Dorney, P.E., C.S.P. Pages 1 | 2 | 3 | 4 | 5 Conclusion Several U.S.A.F. organizations have been able to safely use portable electronic equipment around fueled aircraft without expensive and time-consuming testing for each item. In these cases, the risk of ignition is low. The equipment evaluated herein does not create a likely source of ignition. Further, the likelihood of having ignitable fuel present is quite low, due to existing standard procedures for preventing fuel leaks and spills. Finally, there is only a small likelihood of having any present fuel in an ignitable form, i.e., near a stoichiometric ratio and above its flash point. Combining these three factors allows the U.S.A.F. to operate portable electronic equipment at a low risk around fueled aircraft. By determining ignition potentials for families of electrical devices, the U.S.A.F. has determined appropriate electrical safety criteria, and has used these criteria to save taxpayers money by purchasing commercial off-the-shelf equipment, instead of relying on expensive "military rugged" equipment. About the Author Chuck Dorney was born an Air Force "brat" in Olney, Illinois, on June 13, 1944. He attended grade school in many locations and graduated from high school from the International School of Brussels, Belgium. He has a B.S. in aerospace engineering and an MBA. He has worked in system safety for 32 years and is currently the chief of system safety for the U.S.A.F. Materiel Command. He is also responsible for developing and publishing MIL-STD-882. Chuck is a registered professional engineer (Ohio) and a certified safety professional (CSP). He is currently president of the Ohio Chapter of the ISSS. References 1. Underwriters Laboratory Standard UL 913. Intrinsically Safe Apparatus and Associated Apparatus, May 20, 1988, p. 41. 2. Clodfelter, R. and J. Kuchta. Aircraft Mishap Fire Pattern Investigations, Air Force Wright Aeronautical Laboratories Technical Report (AFWAL-TR-85-2057), August, 1985, p. 176. 3. Knezek, C. et al, General Dynamics, Forth Worth Division, Fueling Ignition Study/Test, F-16, Report No. 16PR1360, April 31, 1980, p. 336. 4. Burkett, V. et al. General Electric Co., Final report of the Radio Frequency Radiation Arcing Hazard in Refueling, Contract AF 30 (602) -1419, Feb. 29, 1956, p. 72. 5. USAF Technical Order 31Z-10-4, Electromagnetic Radiation Hazards, McClellan AFB CA, Oct.15, 1981, p. 110. 6. National Electrical Code, Article 500. « previous page 1 of 2 6/17/11 3:44 PM