Standby vs. emergency power in mission critical facilities

System designers must interpret the requirements of NFPA 110, ensure their designs follow them, and educate their clients about how the standard affects their operations.


This article has been peer-reviewed.Learning objectives

  • Understand which codes and standards dictate standby and emergency power systems, including NFPA 101, NFPA 110, and NFPA 70.
  • Learn about the criteria for EPSS classification.
  • Know how to better educate the owner concerning ongoing operational and testing requirements of NFPA 110.

Recently, an electrical contractor on-site at a mission critical project was having difficulties with the integration of the emergency lighting inverters with the power monitoring system. He looked at the large uninterruptible power supply (UPS) sitting adjacent to it and remarked about unnecessary pieces of equipment and why he should mess around with the pesky little 10 kW units when there were 2-MW generators supporting 800-kW UPS systems.

Surely, the contractor understands the differences in the code and listing requirements of the emergency and optional standby systems. However, his point was also clear. Due to the bulk of attention on and the much higher cost of the mission critical power standby system, it sometimes seems the emergency systems lag behind.

In this example, the life safety system was segregated from the standby system to simplify the overall system design and operation. It was also segregated to prevent the need to have the mission critical loads served from the same generator as life safety loads. That segregation was to simplify the overall power system while meeting the necessary codes. This article explains that decision-making process.

Reviewing applicable codes and standards

It is important to understand how different sources of information interact and affect the overall design of a facility. This information includes: codes, design and industry standards, and owner requirements.Understanding the individual sources of information provides a basis for understanding how to bridge the gaps between them, and it minimizes confusion and conflicts. This discussion focuses primarily on National Fire Protection Association (NFPA) codes, especially NFPA 110: Standard for Emergency and Standby Power Systems, 2013 Edition.

The following select codes and standards required to properly specify, design, and install emergency and standby systems typically involve these three primary areas:

NFPA 101 defines the minimum requirements for equipment needed to support life safety systems, such as elevators, exit signs, and emergency lighting. This code relies on other codes, such as the NEC and NFPA 110, to provide information on how those systems should be installed and how they should perform.

The NEC is primarily concerned with installation requirements and safety. While it defines some operational requirements, it does not list specific requirements for performance, testing, or maintenance. In some places the codes overlap. For example, the NEC has requirements for emergency lighting to support certain types of installations, but it does not have specific requirements for when general emergency, exit signs, or elevator power are required to be on emergency power. The NEC also informs engineers about the overall system design criteria by presenting the different options as well as the minimum installation requirements.

NFPA 110 provides guidance on the performance, operation, and maintenance of emergency and standby power systems. Knowing how to apply it properly requires an understanding of the terminology used in the standard as well as its application.

NFPA 110, Chapter 3, Section 3.3.3 defines the electric power source for the emergency power system as an "emergency power supply (EPS)." This is the actual generator producing the power used by the system. Section 3.3.4 defines the overall standby system as the "emergency power supply system(EPSS)." The EPSS is the entirety of the emergency power supply to the load terminals of the transfer equipment.

Understanding EPSS classification criteria

NFPA 110 defines EPSS levels. Level 1 is defined as "where failure of the equipment to perform could result in loss of human life or serious injuries." This is typically interpreted as an emergency system in other codes, such as the NEC. Level 2 is defined as "where failure of the EPSS to perform is less critical to human life and safety." NFPA 110 further defines classification and types of EPSSs. NFPA 110,Section 4.2 defines classification as the amount of time, in hours, that the EPS is designed to operate atits rated load without being discharged or refueled. NFPA 110, Section 4.3 defines type as the maximum time, in seconds, that the EPSS will permit the load terminals to be without acceptable power.

The balance of NFPA 110 provides requirements for the two levels of EPSS. Included with the code are annexes that provide further guidance on the code's intent. While this information is not considered part of the code's language, it is useful to review to provide clarity of intent. Annex A is keyed to specific articles in the code, and Annex B provides diagrams of typical EPSSs.

Level 1 EPSSs are clearly defined. Per NFPA 110-A.4.4.1, Level 1 systems are intended to automatically supply illumination, power, or both to critical areas and equipment essential for safety of human life. This equipment includes: life safety illumination, fire detection and alarm, elevators, fire pumps, public safety communications systems, industrial processes where the interruption would produce serious life safety or health risks, and essential ventilation or smoke removal.

When a system is defined as a Level 1 EPSS, NFPA 110 triggers a number of requirements that must be considered during the design. Most of the requirements are simply good design practices. These include sizing the unit for ambient temperature and elevation, installation per the NEC and manufacturer's requirements, and so on. Others include oversizing the fuel storage tank (Section 5.5.3), physically separating the EPS with a 2-hr fire rating from the rest of the facility (Section, and physical isolation of all EPSS equipment from the normal service equipment (Section 7.2.3). These concerns mayor may not be constraints, depending on the facility. However, they are always considerations for Level 1 EPSSs. Also included in NFPA 110 are operation, maintenance, and testing requirements, which are applicable to both Level 1 and Level 2 EPSSs.

The Level 2 EPSS designation more subtle. Per NFPA 110-A.4.4.2, Level 2 systems are intended to supply power to selected loads and are typically installed to serve loads that could create hazards or hamper rescue or fire-fighting operations. These include: heating and refrigeration systems,communication systems, ventilation and smoke removal systems, sewage disposal, lighting, and industrial processes. The list of equipment that, upon loss of power, could "create hazards" is extensive,which complicates this designation. This is further complicated by the need to determine if the standard actually applies to the system. NFPA 110, Section 1.1.3, item 6 states "equipment of systems that are not classed as Level 1 or Level 2 systems in accordance with Chapter 4 of this standard" are not covered by NFPA 110. This subtlety manifests by having similar systems in different jurisdictions having different designations.

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