The basics of emergency illumination

07/22/2013


Generator systems

The second common emergency power source for emergency egress lighting is an on-site generator. The emergency power supply (EPS) and EPSS per NFPA 110 have various classifications and types stipulated in Chapter 4. This article is limited to Class 1.5 type 10, Level 1 EPSS.

Essentially, the generator is required to start and be on-line within 10 sec, and run for 2 hours. This is fairly standard requirement for most model codes; however, the run times vary in length based on the building type and use. The egress lighting levels and classification requirements are stipulated in NFPA 101. The requirement for automatic start and transfer is reiterated in both NFPA 70 and NFPA 99. The life safety circuitry must be segregated completely from the normal power sources. This was further clarified in the 2008 NEC (NFPA 70-700.9. (B) (c.)), where legally required systems (egress lighting) and optional standby power cannot originate the same vertical switchboard section, panelboard enclosure, or individual disconnect enclosure as emergency circuits. In the past, a generator could supply a common switchboard without the internal division for the automatic transfer switch (ATS) loads that supply life safety and standby power circuits. The loads and their respective ATSs supplied by the generator other than life safety must have load shed capability, to not endanger the life safety loads should the generator become overloaded, or have an alarm condition.

Figure 5: This computer-generated output of the expected photometric results is based on the proper spacing of the designated emergency lighting fixtures for a medical college. Courtesy: Affiliated Engineers Inc.

The required functional testing of the generators and their associated ATSs must comply with the requirements of NFPA 110 Chapter 8. The diesel generators are to be tested once a month for 30 minutes and loaded to at least 30% of their nameplate rating. Under certain codes the generator shall be annually loaded to 30% for 3 hours and then 75% for the last hour of the test. Obviously, all of the tests are to be witnessed and documented as are the associated automatic transfer switches transition modes. Figure 6 depicts a typical emergency generator with multiple automatic transfer switches.

Figure 6: The one-line diagram depicts a typical emergency generator with multiple automatic transfer switches that serve designated EPSS loads. Courtesy: Affiliated Engineers Inc.

The total solution

A sound understanding of the defined means of egress is critical for proper implementation of emergency lighting systems. Though the various codes and standards appear convoluted at times, there is consistency among their interpretation and cross-references. Once the applicable codes have been determined, one can navigate through the minutia and derive a viable lighting solution that will be suitable to the AHJ.

The key parameters of emergency lighting for means of egress are 1.0 fc average, 0.1 fc minimum, a uniformity ratio of 40:1, and 90 minutes of continuous illumination. If the building is occupied, it will likely require emergency egress lighting. All egress areas require more than one lamp that serves that area.

The most common sources of emergency lighting are a battery-powered packaged unit, a UL 924 lighting inverter, a self contained battery-powered packaged unit (bug eye), and a generator. Size, use, and classification of a given building are used to determine the most cost-effective solution for an emergency lighting system.

For certain building types, the emergency illumination system will be a hybrid system that will incorporate both emergency generators and battery-powered sources. Monthly testing and annual functional testing are not precluded for hybrid systems. Consequently, good documentation for each system will be required for the AHJ review and certification. The responsibility for record keeping ultimately is the building owner’s.


Thom Flickinger is a senior electrical engineer at Affiliated Engineers Inc. He has more than 30 years of design and project management experience in power distribution, information, security systems, distributed power systems, construction administration, and software development, serving the healthcare, research, higher education, mission critical, cleanroom, infrastructure, museum, and expo center markets.


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