Consulting-Specifying Engineer - November 2007 - (Page S12)

on-peakperformance >> In critical operations power systems, risk assessment shall be performed to identify hazards, the likelihood of their occurrence, and the vulnerability of the electrical system to those hazards. real, everyday types of risk from hype will be the practical challenge facing facility managers and systems designers in implementing a COPS. In the words of Timothy Croushore, a professional engineer from Allegheny Power and member of Code-Making Panel 20, “If the risks of a particular catastrophic event are realistic or probable at some level, then protection from that risk should be considered and implemented. However, if the risk of a particular event is very low, then protection from that risk needs to be considered appropriately.” (NEC Digest, Feb. 2007) Reliable backup An on-site backup generator is a complex mechanical and electrical system. The reliability of this equipment is strongly affected by system design, installation, and preventive maintenance. Article 708 recognizes this issue by requiring a commissioning plan, baseline testing, and witness testing by the authority having jurisdiction (AHJ) at start-up and periodically afterward. Article 708 also requires that the system have a documented preventive maintenance program, written records of testing, and means for testing all critical power systems during maximum anticipated load conditions, as required by NEC 708.6(E). The use of the word “during” seems to imply that the generators may need to be tested with the defined DCOA as the load vs. a load bank. The testing practice of transferring critical operations onto a generator during peak usage is something that most facility managers typically avoid. Though a single backup generator can be welldesigned, and properly installed, maintained, and tested, as a complex system it still has hundreds of possible single point failures. Article 708 does not directly require parallel generation but does require single generator configurations to include a means for connecting a portable generator and requires the portable to “be available for use” whenever the generator is out of service for maintenance or repairs. The AHJ will need to determine if “available for use” means connected to the power system. Given the focus on reliable power and the failure risk presented by a single generator configuration, the facility managers and system designers will have to convince the AHJ that this risk is adequately mitigated per the requirements of NEC 708.4(C). The case for using paralleled generation in the COPS design is further strengthened by current market innovations. Various manufacturers offer integrated generator paralleling solutions that are cost-effective and feasible in smaller kW configurations, as small as 100 kW. For example, fuel is a major concern when evaluating a facility’s ability to defend in place. Through the grid failure of 2003, the Florida hurricanes and the flooding in New Orleans, it has become apparent that during significant events the generator’s ability to be refueled is significantly impacted. In certain situations, facility managers can’t just call up for another tank of fuel or expect it to be available upon demand. It may be days or even weeks before a diesel fuel truck can reach the destination. NEC 708.22(C) requires the COPS to have a minimum operating time of 72 hours with the DCOA at full load. In addition to this requirement, NEC 708.20(F)(1) requires that the generator may not be solely supplied by the public gas utility. On first evaluation, many system designers will approach these fuel requirements by configuring the system with a diesel generator and utilizing main storage tanks to assure a minimum of 72 hours of run time. Is this the best approach? Will this mitigate the failure risks as required by NEC 708.4(C)? Is this enough fuel or is this too much fuel? There are a number of competing issues that are in conflict. Too little fuel and the generator runs out prior to refueling; too much fuel and the fuel goes bad. The NEC does not directly reference any other NFPA standard but it does use fine print notes (FPN) to reference key related codes. NFPA 110 is a repeated FPN reference within Article 708 and will probably be adopted by statute for COPS. NFPA 110 has numerous requirements related to fuel: the system design shall provide for a supply of clean fuel (7.9.1.2); the fuel must be consumed in its storage life or provisions shall be made to replace stale fuel 12 november2007

Table of Contents Feed for the Digital Edition of Consulting-Specifying Engineer - November 2007

Consulting-Specifying Engineer Contents Editor's Viewpoint Letters In The News M/E Roundtable Emergency Power for Healthcare 2007 Products of the Year Industrial-strength Lighting Hazardous HVAC Codes and Standards Management Report Jobs/Classifieds Specifier's Notebook On-Peak Performance Contents Best Battery Selector in Engine Starting Applications NEC 708: Practical Impact on Backup Power Systems Protecting Power: Specifying Outdoor Generator Enclosures Investing in Backup Power Systems Coes, Consultants, Manufacturers, and Standby Power Systems

Consulting-Specifying Engineer - November 2007

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