If both utility sources are lost generators backup

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transformer fails or needs maintenance. If both utility sources are lost, generators backup essential electrical loads via transfer switches. Transformer sizing is critical if all secondary loads are to be serviced from one transformer. This is usually accomplished by either loading the transformers to 50%, or by using the transformer forced air (fan) rating with temperature controllers. This system may require complex ground fault solution if both 480 Y/277 V transformers have separate grounds or if 3 pole transfer switches are used for 4 wire loads. This causes circulating current on the neutral busses. This system is common in large cities where the utility companies use transformers, located in vaults, to service hospitals. Dual Source and Secondary Selective – Generators Secondary Selective – Generators This system combines the advantages of both primary sources and secondary selective systems used with backup generators. It not only provides the more reliable system, but also one of the most costly system. Evaluation of the probability of total downtime costs will be necessary to justify the additional first cost. This system may require complex ground fault solution if both 480 Y/277 V transformers have separate ground points or if 3 pole transfer switches are used for 4 wire loads. This causes circulating current on the neutral busses. This power system is popular for large hospital complexes. Advantages: Normal operation as radial system with stand-by generators Isolation of cable or transformer for faults or normal maintenance Feed other side with use of transfer scheme and electrically operated breakers Main and tie breakers can be interchanged for maintenance to keep outages to a minimum Disadvantages: Additional cost Transformer load monitoring May require complex ground fault system if neutrals are tied together and sources have multiple grounding points Advantages: High Reliability Combined advantages of both sources and generators Disadvantages: Higher initial cost May require complex ground fault system if neutrals are tied together and sources have multiple grounding points
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12 Selective Coordination Selective coordination of overcurrent protective devices is very important in hospitals when patients lives are at stake. Downstream overcurrent protection devices, closest to a fault, trip or clear is the focal point of this section. The NFPA 90, 2005 National Electrical Code ® (NEC ® ) issue on selective coordination is addressed in sections 700.27 (Emergency Systems), 701.18 (legally required Standby Systems), 517.26 (Health Care Facilities) and have made it mandatory that emergency power systems overcurrent protective devices are designed to selectively coordinate. This increases the reliability of power systems for critical healthcare facilities.
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