MAINTAINABILITY DESIGN TECHNIQUES METRIC - AcqNotes.com

Downloaded from http://www.everyspec.com on 2011-10-29T14:56:01.DOD-HDBK-791(AM)frequency spectrum allotted to the communications bandgenerally are not believed to cause significant effects onpersnnnnel. On the other hand, it is documented and wellknownthat the effects of X rays, lightning strikes, highintensity light pulses, and even microwaves under theright combination of conditions can produce hazardouseffects on personnel and can affect materiel (Ref. 6).Accordingly, the electromagnetic effects on personnelmust be considered in the design of electronic equipment.Basically, the effects produced by electromagnetic fieldson personnel are classified into thermal and nonthermal.Certain parts of the human physiology are particularlysusceptible to certain frequencies of electromagneticenergy. One of the prime areas of concern involves theeffects of microwaves. As an example, it is well documentedthat microwaves produce cataracts in the eyes ofpersons who are subjected to strong microwave fields forlong periods of time. This is believed to be a thermal effectbecause there is no blood supply to the lens of the eye tocarry away the heat of absorption. Other effects occur inhumans. but cataracts seem to appear first. The importanceof nonthermal effects is a source of scientificdiscussions; sufficient evidence is not yet present tospecify the nonthermal effects that are both importantand originate within the electromagnetic environment.Accordingly, since there is no consensus on the effects ofelectromagnetic radiation on personnel, the maintainabilityengineer should seek the expert advice of healthphysicists and medical personnel. Areas in which electromagneticenergy is present should be posted as indicatedin Chapter 8.10-2.2.4 Enemy and Friendly FireEnemy and friendly fire produce detrimental effectsthat are both psychological and acoustic. Psychologicaleffects are related solely to personnel; acoustic effects canaffect both personnel and materiel. By far the mostsignificant aspect of sound to personnel is its relationshipto communication by speech and hearing. Speech and itsaccurate perception are absolutely essential to the normalexistence of humans; the fact that speech perception isaffected adversely by excessive noise or by hearing loss isobvious. Consequently, every reasonable effort must bemade to insure that the inadvertent hearing loss fromfriendly fire is minimized. Above certain sound intensitylimits, exposure to sound has physical and physiologicaleffects in addition to its effects on hearing. Sufficientlyintense airborne sound can destroy materiel and killexposed personnel (Ref. 6).Sound pressure levels (SPL) of interest in the Army’sacoustic environment cover roughly 20 decibel (dB)orders of magnitude, i.e., a 200-dB range. Any commonArmy small arms—all weapons up to and including cal. 50and shotguns—produce impulse noise levels in excess of140 dB. Most, if not all, of the mechanized equipment inthe Army produces steady state noise environments thatcan interfere with direct person-to-person communication(Ref. 6). Table 10-4 (Ref. 6) relates sound levels to weaponsources. A detailed discussion of noise can be found inAppendix B, Ref. 7. Ref. 8 is the basic regulatorydocument governing maximum noise levels in Armyequipment. Ref. 9 officially defines dangerous noise levelsfor the Army and specifies methods for controlling noiseexposures and for conserving hearing in high-noiseenvironments.10-3 EFFECTS OF ENVIRONMENTS10-3.1 EFFECTS ON PERSONNELEach of the modified environments described in par.10-2.2 has an adverse effect on personnel performingmaintenance operations. Some affect comfort, someincrease the time required to perform a maintenanceaction, and all may introduce a higher level of error thanwould an optimum maintenance environment. Successfulmaintainability design must also consider the normalenvironmental factors that affect the ability of personnelto perform at an optimum level of proficiency. Humanerror, comfort level, and some of the psychological effectsof noise were discussed in Chapter 9.10-3.1.1 TemperatureAlthough the effects of temperature on human performanceare not completely understood, it is known thatcertain temperature extremes are detrimental to workefficiency. As the temperature increases above the comfortzone, mental processes slow down, motor responses areslower, motivation is reduced, and the likelihood of errorincreases. Fig. 10-1 illustrates the increased error ratewith increasing effective temperature. Dry-bulb temperatureis not the sole criterion in relating temperature tocomfort or efficiency; humidity and airspeed are importantconsiderations. The discussion of effectivetemperature, Wet-Bulb Global Index, and WindchillIndex in the paragraphs that follow presents this interrelationship.TABLE 10-4. SOUND LEVELS ASSOCIATED WITH WEAPON TYPES (Ref. 6)Sound Level Typical Source Significance or Actiondb bar Required140-170 0.010-0.012 Small arms Hearing protection required forrepeated exposure175-190 0.0125-0.014 Artillery Hearing protection essential; bodyprotection desirable10-5