Fonte (Source): Consulting – Specifying Engineer
Por (By): Brian Martin, PE, and Jeremy Taylor, PE, CH2M
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Design engineers must consider the implications of combining emergency, legally required, and optional standby systems to ensure code compliance, maintainability, and economics.
- Interpret the requirements of NFPA 110 and NFPA 70.
- Describe how to design mission critical facilities to meet these NFPA requirements.
- Identify potential alternative designs to meet the intent of NFPA 110.
Design engineers have many factors to consider when designing a backup system for a facility. Safety, maintainability, code compliance, and economics play crucial roles in determining the topology of a backup system for a critical facility. In large facilities where electrical system downtime results in significant economic loss, a backup power system usually is employed. Owners frequently desire to use their backup systems to support their emergency and legally required standby loads. Due to the requirements of NFPA 110-2013: Standard for Emergency and Standby Power Systems, and NFPA 70-2014: National Electrical Code (NEC), the design engineer must carefully consider the implications of combining emergency, legally required, and optional standby systems to ensure code compliance with maintainability and economics in mind.
NFPA 110 provides requirements, but is not meant to be a design guide. The annexes provide example topologies that meet the intent of the standard, but these examples do not address the complexities of designing a system for a large facility with multiple system types.
NFPA 110 defines terms used throughout this article. NFPA 110-3.3.3 defines the electrical power source for the emergency power system as the emergency power supply (EPS). This includes the actual generator, turbine, or other source producing the power used by the system. NFPA 110-3.3.4 defines the emergency power supply system (EPSS) as the distribution system from the EPS to the load terminals of the transfer equipment. NFPA 110-4.4 defines two levels of EPSSs. Level 1 is defined as “where failure of the equipment to perform could result in loss of human life or serious injuries.” Level 2 is defined as “where failure of the EPSS to perform is less critical to human life and safety.” There are numerous articles that further discuss the code requirements and implications of NFPA 110and its relationship with other codes. As such, this article does not focus on the details of NFPA 110 definitions. Instead, it concentrates on ways to meet NFPA 110 and 70 while providing the owner with a system that meets expectations.
Major challenges to meeting NFPA 110
The first major challenge to meeting the requirements of NFPA 110 is properly defining system levels. This requires careful evaluation of the loads you are serving and coordination with your authority having jurisdiction (AHJ). According to Annex A.4.4.1, “Level 1 systems are intended to automatically supply illumination or power, or both, to critical areas and equipment … Essential electrical systems can provide power for the following essential functions: life safety illumination, fire detection and alarm systems, elevators, fire pumps, public safety communications systems, industrial processes where current interruption would produce serious life safety or health hazards, and essential ventilating and smoke removal systems.” Some jurisdictions have interpreted the text of this annex to mean that any electrical system that includes these types of loads is a Level 1 system.
The next significant challenge to meeting NFPA 110 is fuel storage requirements. According to Annex A.4.2, 96 hr of fuel may be required in certain seismic zones. In summary, “Where the seismic design category is C, D, E, or F, as determined in accordance with ASCE/SEI 7: Minimum Design Loads for Buildings and Other Structures, the EPS supplying a Level 1 EPSS should be capable of a minimum 96 hr of operation without refueling if it is determined that EPS operation is necessary for this period.” This is a change from the 2010 standard where the 96-hr fuel requirement was called out explicitly in the body of NFPA 110. Some jurisdictions have interpreted this as a requirement to provide 96 hr of fuel any time you have a Level 1 system in a high seismic zone.
In addition, Section 5.5.3 requires that the main fuel tank carry 133% of the fuel required to meet the class requirements of the EPSS. In other words, if you require 20,000 gal of fuel to run a large generator for 96 hr, you must actually store 26,600 gal of fuel. In a large facility with large generator sets, these two requirements can result in hundreds of thousands of gallons of fuel storage. In addition to the obvious cost and real estate issues with this requirement, fuel recirculation and stabilization quickly becomes an issue.
Another challenge to NFPA 110 compliance is serving the relatively small code-required loads in a mission critical facility such as a data center. A data center is certainly a major example of mission critical facilities that have spawned publications and organizations to support them, but there are other types of mission critical facilities. Other examples of mission critical systems are those that support research where the failure can result in millions of dollars of loss, or response centers where power failure could hinder the response of a company to a crisis. Based on NFPA definitions, mission critical loads are generally classified as optional standby loads. Despite the fact that these types of loads are not life safety loads, in the owner’s perception, they are no less critical to maintain. As such, the electrical distribution that supports them can be as robust, and many times are more robust than the Level 1 EPSS that supports life safety loads.
Finally, it can be challenging to economically scale NFPA 110 on a large system for a large system load. The examples given in Annex B of NFPA 110 are well-suited for applications lower than 600 Vac (see Figure 1). Large power systems are typically designed at system voltages of 12 kV and higher. Large loads will lead you toward system designs that include medium-voltage transfers. This may not meet the requirements of section 6.1.6, which states that only “medium-voltage transfer of central plant or mechanical equipment not including life safety, emergency, or critical branch loads shall be permitted.”
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