ELECTRICALPOWERDISTRIBUTIONSYSTEMS
IEEE srd 602-2007
electrically connected,then both power supply neutrals will have to be effectively grounded. 3.6.3 System and load characteristics The type of grounding system and the arrangement.of system and equipment grounding conductors will affect the service continuity. Engineers should design grounding conductorsand connectionsso that objectionable stray neutral currents will not exist and ground-fault currentswill flow in low-impedance,predictablepathsto protect people from electrical shock and to assureproper operationofthe circuit protective equipment. Where systemsservephase-to-neutral loads,codesrequire them to be solidly grounded. However,600 v and 480 v systernssupplyingonly phase-to-phase loadsmay be solidly grounded, high-resistancegrounded, or ungrounded. High-resistance grounded systems may provide a higher degree of service continuity than solidly grounded systems and might be useful in servingphase-to-phase loadsrequiring very high reliability. Any such system,howeve¡ needsto be carefully monitored to detectphase-to-groundfaults early enough so that the fault(s) can be isolated and repaired promptly. Do not use ungrounded systemssince they may experienceovervoltagesdue to resonanceconditions, or "neutral" shifts occurringduring a ground-faultcondition. For safetyreasons,proüde only solidly groundedsystemsbelow 600 V. When the alternate power supply [generator(s)or separateutility service] and its wiring system electrically interconnect with the normal power supply by the neutral or phase conductors,then the NEC doesnot considerthe alternatepower supply to be a separately derived system.Thus, its neutral is cannot be grounded.In this case,only the not-ul power supply neutral can be groundedas shown in Figure 3-1. This requirementwill prevent objectionableneutral load currentsfrom flowing in the systemground conductors. Note that the equipment grounding conductor defined as EGC in Figure3-l is still required; thus the generatorneutral must be insulatedfrom the machine frame. When the alternate power supply [generator(s)or separateutility service] and its wiring system are not electrically interconnectedwith the normal power supply wiring system either by the neutral or phase conductors, or both, then the NEC considersthe alternate power supply to be a separatelyderived system. In this case,the alternatepower supply neutral must be grounded [in addition to the neutral(s) of the normal power supply]. Figure 3-2 depicts a systemwhere the neutral and phaseconductorsbetweenthe alternati and normal power supply wiring systemsare not electrically interconnectedby the use of a four-pole transfer device. Since the neutral conductors are isolated between the two power systems,grounding of the generator neutral will not proüde an altemate path (to ground) for load cu¡rentson the normal power source. Engineersshould carefully considerwhether or not to switch the neutral. See IEEE Std 446 (IEEE orange Book) and IEEE std 242 (IEEE Buf Book).for more details on these lssues.
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3.7 System protection and coordination The system and equipmentprotective devicesguard the health care facility power system from the ever presentthreat ofdamage causedby overcurrentsthat can result in equipment loss,systemfailure, andhazardsto patientsand otherpeopleAll protectivedevicesshould be applied within their ratings of voltage, frequency, current intemrpting rating, and current withstand rating. In addition, the site where they will serve needsto be taken into account(i.e., if it is at a higher altitude,if seismicactivity is common,if temperaturesare extrem€,if humidity is high, etc.).Many referencesand standardsprovide guidelinesasto the various device descriptions, their ratings and application limits, and rating factors if required.Theserequirementsare best documentedin IEEE std l4l (EEE Red Book), IEEE std 241 (IEEE Gray Book), and the other ANSI, NEMA, and IF,EE standardslisted in 3.10. The readershouldrefer to thesefor details.The following summarizessomeof the information in those references.
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