Consulting-Specifying Engineer - January 2008 - (Page 41) BY KEITH LANE, PE, LEED AP A n isolation power system provides an ungrounded electrical service for various applications within a hospital or a medical office building. These isolation power systems remain in operation in the event of a single lineto-ground fault situation. These systems also eliminate the danger of an electric shock to patients who may be more susceptible to leakage current and unable to move in their beds. If there is a fault, the system alarm in the isolation panel activates. When the alarm is activated, the critical medical equipment remains operational, because no ground fault protection or overcurrent protective device trips. The triggering of an alarm from a single ground fault must be rectified as soon as possible at a “safe” time, as a second ground fault could trigger the short circuit protection and take an entire operating room offline. The time current curves for the breakers in Figure 1 represent a 60-amp main breaker and a 20-amp sub-breaker in an operating room. If the panel in the operating room experiences a short circuit current that caused more than 250 amps to flow, the main breaker in the panel could trip—in addition to the 20-amp subbreaker—and leave the entire operating room without power. The example does not comply with the requirements of selective coordination required by National Electrical Code (NEC) 517.26, which indicates the following: • Application of other articles: Essential electrical system shall meet the requirements of Article 700, except as amended by Article 517. • NEC 700.27 coordination: Emergency system(s) overcurrent devices shall be selectively coordinated with all supply side overcurrent protective devices. • NEC 100 definitions—coordination (Selective): Localization of an overcurrent condition to restrict outages to the circuit or 1000 Current in amperes 100 60 A Isolation 20 Amp 10 Time in seconds 1 0.10 0.01 0.5 1 10 100 1K 10K Isolation panel.tcc Ref. voltage: 480 Current scale x10A0 Figure 1: A short circuit current causing more than 250 amps to flow could leave this entire operating room without power. Source: Lane Coburn & Assocs. 60-amp main breaker is associated with a 10-kVA transformer—the largest allowed by NEC Section 517 – 160 (6). The 60-amp breaker is sized based on a maximum of 125% of the output current of the transformer: 10 kVA / 0.208 = 48 amps x 125% = 60 amps If the transformer has 4% impedance, than the maximum fault When does a healthcare facility require an isolated power system? equipment affected, accomplished by the choice of overcurrent protective devices and their ratings and settings. If the available fault current at the point of the fault were less than the instantaneous setting of the upstream 60-amp breaker, then the system would be coordinated selectively. In the example shown by the time current curve, the instantaneous of the 60amp main breaker starts at approximately 250 amps (there is no adjustment in the instantaneous setting; this is a fixed setting). The current would be approximately 1,200 amps (48 amps / 0.4 = 1,200 amps). This far exceeds the instantaneous setting of the 60-amp main breaker and would therefore not be coordinated selectively according to the definition in the 2005 NEC. This is an interesting situation. Isolated power supply systems that incorporate fuses into the system in lieu of breakers are unusual. Typically, fuses applied in a 2:1 ratio are required to obtain a completely coordinated system. If the system was selectively Consulting-Specifying Engineer • JANUARY 2008 41
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