Designing an emergency power supply system

Engineers and owners must consider NFPA requirements when designing and maintaining alternate power supply systems to ensure reliable service

By Mario Vecchiarello, Jeff Donaldson and Tyler Roschen November 20, 2020
Figure 2: This demonstrates a typical medium voltage one-line diagram consisting of two paralleled standby generators. Courtesy: CDM Smith


Learning Objectives

  • Understand the differences between emergency and standby and between Level 1 and Level 2 emergency power supply systems (EPSS).
  • Learn the NFPA 70 NEC (2020) and NFPA 110 (2019) requirements in emergency and standby power supply system design.
  • Understand the importance of maintenance and testing to comply with both Level 1 and Level 2 systems.

Electrical and mechanical engineers should identify and apply NFPA 110: Standard for Emergency and Standby Power Systems and NFPA 70: National Electrical Code requirements when designing alternate power supply systems.

The first step to design an emergency power supply system is to identify the operational requirements of the essential loads to properly classify the EPSS and select the appropriate type of equipment. Engineers must determine what the EPSS is required to power in the event of a normal power failure. This typically requires the input of the architect and the client (i.e., building occupant) and a review of the applicable state or jurisdictions adopted building codes. This review will determine if the alternative power source is considered a Level 1 or Level 2 system as defined by NFPA 110.

In addition to the requirements of NFPA 110, the engineer needs to determine if the emergency power supply is characterized as an “emergency,” “legally required standby” or “optional standby” system per Articles 700, 701 and 702 of the 2020 edition of NFPA 70, respectively. These articles include specific requirements for the system performance, ancillary equipment arrangement, fire protection measures, etc.

According to Section 3.3.4 of NFPA 110, an EPSS is “a complete and functioning EPS system coupled to a system of conductors, disconnecting means and overcurrent protective devices, transfer switches and all control, supervisory and support devices up to and included the load terminals of the transfer equipment needed for the system to operate as a safe and reliable source of electric power.”

Defining a system in terms of standby or emergency sometimes causes confusion because NFPA 110 uses the term “emergency power system” for both emergency and standby systems.

Classification of EPSS

The classification of the EPSS depends on the minimum time in which the EPSS is designed to operate at rated load without needing to be refueled or recharged. See NFPA 110 Table 4.1(a) for classifications of EPSSs. The designated type of EPSS defines the maximum time in which the load terminals of the transfer equipment can be without acceptable electrical power based on the essential load requirements. See NFPA 110 Table 4.1(b) for types of EPSSs.

Both the classification and the type of EPSS are dependent on the level the EPSS is classified as and based on a review of applicable state and local codes, discussions with the authority having jurisdiction and coordination with the owner or end user of the EPSS.

NFPA 110 delineates the EPSS into two distinct levels that dictates installation, performance and maintenance requirements:

  • Level 1 systems are required where the failure of the equipment to perform could result in loss of human life or serious injuries. For instance, these systems are typically provided for life safety, emergency or critical loads as defined in NFPA 99: Health Care Facilities Code, NEC Article 700, NFPA 110, NFPA 101: Life Safety Code and applicable building codes. Section 700.12 of the NEC requires the power to be “available within the time required for the application but not to exceed 10 seconds.” This would designate this EPSS as Type 10 or less.
  • Level 2 systems are required where failure of the EPSS to perform is less critical to human life and safety. These systems are provided for both legally required standby or optional standby systems as defined in NEC Articles 701 and 702, respectively. The time that is required for power to be restored to Level 2 installations is dependent on the applicable codes and application for each installation:
    • Level 2 systems that are legally required by NEC Article 701 must have standby power available within 60 seconds or less after normal power loss in accordance with Section 701.12.
    • Level 2 systems designated as optional standby EPSS is dependent on the end users’ needs and applicable codes. The end users’ essential load needs will determine the acceptable time from loss of utility power to when the EPSS provides adequate power.

A combination of different types and classifications of EPSSs are sometimes provided in a facility to economically meet the operational requirements of the various essential loads. For instance, there may be a an EPSS requirement as follows: All the essential loads require a minimum time of 48 hours in which the EPSS must operate at its rated load without being refueled or recharged; a small portion of the essential loads, such as supervisory control and data acquisition, cannot tolerate any power interruption between normal power loss and the time it takes the standby generator to start and accept load.

However, the majority of the essential loads can tolerate a 60-second power interruption between normal power loss and EPSS power restoration. A combination EPSS system consisting of a Class 48, Type 60 EPSS provides power to the essential loads that can tolerate the 60-second power restoration and also provides power to a Class 0.083, Type U EPSS to power the uninterruptable essential (SCADA) loads. The Class 0.083, Type U EPSS will continue to provide power to the uninterruptable loads for a period of up to five minutes to bridge the 60-second power restoration time of the Class 48, Type 60 EPSS.

If there is a need for a centralized emergency system to provide power to life safety loads, such as emergency egress lighting, a third type of EPSS would be required to comply with Level 1 requirements of NFPA 110, NFPA 101 and Article 700 “Emergency Systems” of the NEC. NEC Article 700 requires that the emergency circuits be kept entirely independent of all other wiring and equipment unless otherwise permitted in Sections 700.10(B)(1) through (B)(5).

For this situation, a Class 1.5, Type U, UL 924 rated uninterruptible power supply system located in a two-hour rated space would be an acceptable solution. Because the UPS provides continuous power to the emergency loads during loss of normal power, it meets the NEC Section 700.12 requirement, which states that on loss of normal power, power shall be available within the time required for the application but not to exceed 10 seconds.

The Class 1.5 rating also meets the NEC Section 700.12 (C) requirement of 90 minutes of battery capacity to supply the load during loss of normal power. It is very important to note that when combining Level 1 and Level 2 EPPSs and normal power equipment, the physical separation requirements defined in NEC Article 700 must be considered. See Figure 1 for an example of this type of installation.

Figure 1: An emergency power supply system provides generator backup with uninterruptible power supply for uninterruptable loads. Courtesy: CDM Smith

Figure 1: An emergency power supply system provides generator backup with uninterruptible power supply for uninterruptable loads. Courtesy: CDM Smith

Transfer switch equipment

Transfer switches are required to transfer electric loads from one power source, typically called the “normal” power source, to another, typically called the “emergency” or “standby” power source and back again. Where life safety, emergency or critical branch loads are supplied, the automatic transfer switch shall be listed (UL 1008 for ATSs rated 1,000 volts and less) for emergency service as a completely factory-assembled and factory-tested apparatus. The transfer equipment shall be provided with either mechanical interlocking or with an approved alternate method to prevent the interconnection of the two separate power sources. NFPA 110 requires the ATS to be capable of electrical operation and mechanical holding, transfer and retransfer of the load automatically and visual annunciation when “not-in-automatic.”

The ATS is required to have undervoltage-sensing devices that monitor all the ungrounded lines of the normal source of power to initiate the engine startup and the process to transfer power to the EPS on loss of adequate power on the normal power source. These voltage sensing devices are also used to initiate the process of transferring the essential loads back to the normal source of power. The transfer to normal power typically occurs after a predetermined time delay after the normal source returns to within an acceptable voltage to support the loads.

In addition to voltage sensing all ungrounded lines of the normal source of power, at least one ungrounded line of the EPS shall be monitored for both voltage and frequency. These sensing devices inhibit the transfer of the essential loads to the EPS until both the voltage and frequency of the EPS are within a predetermined acceptable range to support the essential loads.

NFPA 110 identifies time delay devices within the automatic transfer scheme of the EPS to provide reliable power to the essential loads through properly transitioning the loads from one power source to the other. Sections 6.2.5 through 6.2.11 of NFPA 110 provide the detailed requirements and significance of the various time delay devices to ensure reliable power to the essential loads

If an EPSS uses two or more paralleled generator sets as the alternate power source, the transfer of loads to the EPS shall be sequenced by load priority to provide power to the essential loads. To ensure power will be available to the essential loads within the required time from normal power loss, consideration needs to be given to the time it takes the system to detect under voltage, initiate the starting of the generator(s), time for generators to reach acceptable voltage and frequency and breakers opening and closing. Therefore, the designer and the owner need to identify and categorize the essential loads by priority.

For example, first-priority loads are the most critical and are therefore energized as soon as power within acceptable voltage and frequency limits is available. Each time an additional generator is paralleled to the emergency/standby bus, the next priority load shall be energized until all emergency and standby loads are powered. Upon failure of one or more engine generator sets, the loads shall automatically shed starting with the least priority load first and continuing in ascending priority so that the highest priority load is the last load to be affected.

See Figure 2 for a typical Level 2 medium-voltage one-line diagram consisting of two paralleled standby generators using electrically interlocked medium voltage breakers, which are permitted for use in accordance with NFPA 110 Section 6.1.6 if the loads being served do not include life safety, emergency or critical branch loads.

An EPS is permitted to serve optional loads provided the EPS has adequate capacity or if the system is equipped with automatic selective load pickup and load shedding. The automatic selective load pickup/shedding shall provide priority to the Level 1 loads, Level 2 loads and any optional loads in that order to ensure power is provided to the most critical loads. Although the purpose of the EPS is to provide backup power to essential loads when utility power is available, it is acceptable to use the EPS for other purposes such as peak load shaving and utility load curtailment. However, additional air permitting issues may need to be addressed under these scenarios.

Figure 2: This demonstrates a typical medium voltage one-line diagram consisting of two paralleled standby generators. Courtesy: CDM Smith

Figure 2: This demonstrates a typical medium voltage one-line diagram consisting of two paralleled standby generators. Courtesy: CDM Smith

Installation and environmental considerations

Chapter 7 of NFPA 110 establishes minimum requirements and considerations relative to the installation and environmental conditions. The performance of the EPSS can be affected by geographic location, building type, classification of occupancy and hazardous contents.

An important aspect of the EPSS installation is to minimize the probability of equipment or cable failure within the EPSS. Some methods used to minimize the probability of equipment of cable failures are to:

  • Locate the EPS as close to the critical loads as possible.
  • Segregate the emergency feeders and circuits from the normal ones to prevent failure of one from taking out the other.
  • Locate critical EPS equipment in locations that are protected from natural conditions such as floods, storms, fire, earthquakes, etc. and human causes such as vandalism, sabotage and similar occurrences.

The EPS design team must also consider minimizing single points of failure within the EPS to reduce the probability of power disruption to the protected loads due to material and equipment failures.

For Level I EPS installations located indoors, NFPA 110 requires that the EPS be installed in a separate two-hour fire resistant rated room separated from the rest of the building. This room shall only include the EPS equipment and equipment that serves this space. Level 1 EPSS equipment shall not be installed in the same room with normal service equipment that is rated over 150 volts to ground and rated 1,000 amperes or greater.

Outdoor Level 1 or Level 2 EPSS equipment shall be installed in a suitable enclosure that protects the equipment from the environmental conditions as required by the local building codes. This enclosure shall only include the EPS equipment and equipment that serves this space. There are additional requirements for mounting the energy converter, lighting, heating, cooling, ventilating, EPS cooling system, fuel system, exhaust system, protection and distribution for the EPSS.

As shown in Figure 3, the exhaust system shall be installed so that exhaust gases cannot reenter the building. NFPA 110 indicates that the exhaust system equipment shall be installed in accordance with other applicable standards, such as NFPA 37: Standard for the Installation and Use of Stationary Combustion Engines and Gas Turbines.

Testing and maintenance

Routine maintenance and operational testing for the EPSS is critical to ensure reliability of the system. A suggested EPSS maintenance schedule is included in NFPA 110 Chapter 8. EPSS maintenance and testing should be customized based on manufacturer’s instruction manuals, minimum requirements of Chapter 8 in NFPA 110 and the AHJ.

An important aspect of routine maintenance is to inspect the generator set weekly and exercise it monthly under load for a minimum of 30 minutes. The ATS is used to exercise the generator automatically. The exercising shall follow one of two methods:

  • The EPS shall be loaded so that it maintains a minimum exhaust temperature based on manufacturer recommendations.
  • The EPS shall be exercised with a minimum load no less than 30% of the rated kilowatts of the EPS under operating temperatures.

Loading under the recommended 30% of rated load can cause wet stacking, which is a buildup of unburned fuel or carbon in the exhaust system of the EPS that could prevent the engine from delivering its rated load when needed.

Storage batteries used in conjunction with the system require weekly inspections and shall be maintained per the manufacturer’s recommendations. Properly maintained batteries are crucial for any EPSS. Batteries that have not been maintained or tested regularly are susceptible to failure, which will cause the EPS to not start and provide emergency power to the system when needed.

Figure 3: Shown is an indoor level 2 emergency power supply diesel generator installation. Exhaust systems must be installed so that exhaust gases cannot reenter the building. Courtesy: CDM Smith

Figure 3: Shown is an indoor level 2 emergency power supply diesel generator installation. Exhaust systems must be installed so that exhaust gases cannot reenter the building. Courtesy: CDM Smith

Testing of the EPSS is critical to ensure that each component is working correctly. Every component in conjunction with the EPSS shall be included in the required inspections and testing. This includes the EPS itself, transfer switches, circuit breakers, batteries, etc. The testing should also ensure EPSS complies with the maximum time that the generator is required to accept load after power loss, based on the EPSS Level, Class and Articles 700 and 701 of the NEC.

Testing the operation of the EPS consists of simulating a power outage. Simulation is typically initiated by using the test switches on the ATS or by opening a normally closed breaker. For more complex systems where multiple ATSs are used, the ATSs should be rotated monthly for testing. The testing consists of electrically operating the transfer switch from the normal power contact to the emergency power contact and then returning to the normal power contact. The test should also be initiated from a cold start.

The entire Level 1 EPSS shall be tested at least once every 36 months in addition to the inspections and testing of the individual components indicated above. This requirement is to ensure the EPSS is capable of running for the duration of its EPSS classification. However, if the EPS is designated as a class greater than four hours, the test shall be allowed to terminate after four hours of continuous operation.

Circuit breakers used within the Level 1 EPSS shall be exercised annually with the EPS in the “off” position. Breakers rated above 600 volts shall be tested every six months. These breakers include the main and feeder breakers between the EPS and the transfer switch load terminals and should also be tested under simulated overload conditions every two years.

Author Bio: Mario Vecchiarello is a senior vice president and technical delivery manager at CDM Smith. He is a member of the Consulting-Specifying Engineer editorial advisory board. Jeff Donaldson is a senior electrical engineer and project manager at CDM Smith. He has more than 10 years of experience working in the power electrical engineering field providing design engineering and construction observation of electrical systems for municipal, industrial and private clients. Tyler Roschen is an electrical engineer at CDM Smith, where he is focused on electrical power system design. He has six years of industry experience in electrical power systems and construction services.