Choosing a Backup Power Strategy for Emergency Lighting

Brian Kennedy, Senior Manager, Power Management
Emerson Network Power

In 2014, some 107,500 non-residential structure fires occurred across the United States. Statistics like that make you realize how critical it is to deploy and maintain proper life safety—and other emergency equipment—in commercial buildings of varying types.

Construction and operating codes have long been established to protect human life. National Fire Protection Association standards (e.g., 70, 99, 101, 110 and 111) describe safe building conditions, and these standards work in accordance with NEC 700/701 and UL 924. These are all in addition to the local codes that may apply to a particular building.

Liebert-eXM-UPSUL 924 is part of the Life Safety standards for emergency egress. It states that equipment shall automatically supply illumination or power or both to critical areas and equipment in the event of failure of the normal electrical supply. It requires that building lighting provide at least 90 minutes of battery powered illumination, even under extreme conditions, such as electrical surges and full loads, to ensure that building occupants are provided time to evacuate.

Additionally, Life Safety codes specify that emergency lighting shall engage automatically and immediately after a primary power failure. In applications with an appropriate utility backup, such as a generator, UL 924A may qualify and shorten the UPS/inverter battery time to 30 minutes or less.

The challenge for building developers and managers is ensuring buildings meet these requirements reliably and cost effectively. One issue emergency lighting system designers should evaluate is whether to use a centralized or a decentralized backup power approach within emergency lighting. The effective answer usually is tied to the size and type of the facility as well as the corresponding power load.

In a decentralized plan, backup power is supplied at the individual device or set of devices.  In a large facility, this would require a high quantity of smaller, isolated backup power devices. Lower initial costs in this case might be offset with the ongoing maintenance of so many devices.

Some facilities prefer a centralized UPS approach. This allows the facility to leverage economies of scale that come with larger UPS units.

Larger commercial facilities will generally require a centralized UPS/lighting inverter in the 10 – 200kW range, with most facilities falling in the 10-60kW range.  Depending on the electrical needs, a 208/277 Volt or 480 Volt output system will be used. Other factors to consider include:

  1. UL 924 or 924A Listed. The ability to leverage 924A is directly related to the ability to tap into an auxiliary power source such as a generator. Obviously this can reduce the amount of battery time needed to meet code. Watch that your fixtures and systems are compatible with the generator synchronization.
  2. Proven Reliability: It may be tempting to skimp on quality for a system that is used infrequently, but this is exactly why reliability is so important in an emergency lighting UPS/inverter. Even with regular testing, you may not find out your UPS didn’t withstand the last thunderstorm until the time when you need it the most. Leveraging a well-designed and engineered central system, with battery monitoring features can pay dividends in the elements of service, control and replacement.
  3. Compatibility of power loads to backup:  In addition to generator friendliness, some facilities may deploy different lighting technologies that should be evaluated. Sodium-type lighting is one example of an illumination source that cannot withstand any momentary electrical outage. A true online UPS will keep sodium lighting continually powered to avoid going dark and taking minutes to re-light.
  4. Proactive service and support: With today’s technology, service doesn’t just mean regular maintenance and fast issue response, although those are still important. The current generation of UPS systems enables 24×7 equipment monitoring that provides early detection of trends and operating anomalies that may lead to critical failures if not addressed. Additionally, data collected from the UPS can provide regular reports on system health.

Selecting the right backup power system for emergency lighting is more complicated than just finding a UL-compliant device. Deciding on a distributed versus centralized strategy, optimizing efficiency and choosing the right technologies can all factor into the cost and performance of the emergency lighting system.

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