UPS: Standby Power for Egress Lighting
In the first part of this series , I introduced the basics of UPS technology. This second installment gets more specific, covering lighting inverters used in commercial buildings for egress lighting.
The National Electrical Code , Article 700, indicates that all egress lighting shall be backed up within 10 seconds after a power outage. NFPA 101, Life Safety Code , further mandates a minimum of 1 foot candle (fc), or 10 lux, and that no point shall be less than 0.1 fc (1 lux) measured along the egress path at floor level. These illumination levels are permitted to decline to not less than 60% of the initial emergency illumination at the end of a 90-min. battery period.
Additionally, the International Building Code (IBC) requires extra lighting to be backed up by a second power source. Per 2003 IBC, Section 1006.3 (3), exterior egress areas at levels other than the level of exit discharge must now have the egress lighting on an approved standby power source. Furthermore, Section 1006.3 (5) indicates that the portion of the exterior exit discharge immediately adjacent to the exit discharge doorway in buildings requiring two or more exits is also required to have the lighting on an approved standby power source.
Note that exit discharge is the portion of the means of egress system between the termination of the exit and a public way. Depending on the location of the building on the property, the public right of way can be a significant distance from the exit of the building.
Egress lighting inside commercial buildings can be achieved through the use of battery backup ballasts in each fixture when fluorescent lamps are used. These backup ballasts can be specified with various amounts of lumen output, typically in the range from 300 to 1,400 lumens. The lighting system designer must ensure that the correct lumen output is specified for a given application. In addition, fluorescent and incandescent lighting can be backed up by a standby generator to achieve the code-required light levels within the NEC-prescribed 10 seconds. If utility power fails, the generator will typically sense the failure and start the engine and transfer to emergency power within seven to eight seconds.
When high intensity discharge (HID) lighting is used, if utility power is interrupted, even for a very short time, the light arc will extinguish. Because the arc extinguishes every half cycle of a 60-Hz power source, there must be enough voltage to reignite the arc every half cycle. Once the lamp is extinguished, lamp cool-down and restrike time can take up to 15 minutes. Even the best systems take at least one minute.
Outside and In
Furthermore, egress lighting issues for areas outside the building can become much more complicated if an emergency standby generator is not part of the electrical distribution system. Unlike fluorescent fixtures, metal halides and other HID fixtures cannot be equipped with individually mounted battery backup devices. There are solutions, but they can be expensive and complicated. One of these involves a second fluorescent system, in addition to the HID lighting system, with internal battery backup.
Another solution is a "fast transfer" centralized lighting inverter system that produces 90 minutes of battery power, as required by the NEC. To be code-compliant, a lighting inverter must also be able to fully recharge its batteries within a 72-hr. period. If an interruptible lighting inverter is utilized, (slower than 4 to 8 milliseconds) it must be noted that an arc maintenance device must also be provided in the metal halide or HID fixture.
A centralized lighting inverter can take up some significant electrical room space and potentially require major architectural and mechanical provisions and must be discussed and planned with the design team, architect and owner prior to design and installation. The majority of commercial building projects use either of two types of lighting inverter technology: interruptible systems or fast transfer systems. Interruptible inverter systems will typically transfer within a range of 50 milliseconds (ms) to one second and can be utilized for fluorescent and incandescent light sources. Fast transfer lighting inverters will typically transfer within 2 ms to 4 ms and must be used with metal halides or other HID light sources to prevent the arc from extinguishing. It must be noted that if the lighting inverter utilizes lead acid batteries, once 50 gallons of electrolytes are stored in a non-sprinkled building, or 100 gallons in a sprinkled building, Uniform Fire Code , Chapter 52, which requires spill control, room separation and ventilation, applies.
Inverters Up Close
Both inverter types—interruptible and fast transfer—are essentially standby UPS. Under normal operating conditions, the power flows from the utility power supply to the lighting sources. When the utility power source fails, the transfer switch changes position and the lighting fixtures draw power from the inverter battery system.
Lighting inverters will vary in transfer time, voltage regulation and power conditioning. An interruptible power supply (IPS) utilizes a mechanical transfer device and can take from 50 ms up to 1 sec. to transfer. The fast transfer inverter utilizes electronics to transfer the system and can achieve the 2 ms to 4 ms time frame. Most of the fast transfer systems use a ferroresonant transfer design or a pulse width modulation (PWM) for the inverter. PWM is more efficient than the ferroresonant transformer inverter.
When determining the appropriate lighting inverter for your project, make sure that the system is UL 924 listed or CSA 860. UL 925 listed systems must meet performance testing requirements to ensure that they can provide a specific amount of illumination for 90 min. during a utility power outage. Lighting inverter systems that produce a square or step waveform can overheat lighting ballasts and should not be utilized for lighting system applications.
One method of providing emergency egress lighting is through the use of unit equipment or fluorescent lighting with integral battery backup ballasts. Most lighting fixtures with integral battery and inverters specifically designed for emergency lighting tasks fall into one of these two categories. These systems include either separate unit equipment (also known as "bug eyes") that is typically off during normal power conditions and on when the utility power fails, or integral battery ballasts in fluorescent fixtures that take over and provide power to the lighting fixture, usually at reduced lumen output, during a utility power outage. If bug eyes are used, they must be on the same electrical circuit as the normal power in the same room or area. If these systems use different electrical circuits, a tripped breaker for the normal lighting would not trigger the unit equipment to come on and the entire area could be subject to complete darkness.
Bug eyes are available in a variety of configurations including wall-mounted, ceiling-mounted, recessed, low-profile, industrial, commercial, institutional and hazardous-location. This equipment can come with built-in test equipment or measures that periodically exercise the fixture to ensure correct operation and code compliance. If the lights fail during a test, a trouble indicator light on the unit equipment will activate. Wire guards and vandal-resistant plastic construction may be required in areas that are subject to abuse or heavy wear.
When specifying unit equipment, swivel-stem connected luminaries or other means that allow for easy manipulation of the light head are essential to most effectively light output. This flexibility can give the owner more options during commissioning and aiming of the lighting to provide the best possible light levels and uniformity ratios during a utility power outage and during an emergency situation.
Another type of unit equipment combines the battery-powered light heads with a full-sized exit sign. This combination is typically specified with two light heads and an LED exit sign for corridors and stairways. Some local codes may require three lighting heads. This combination fixture can often be very cost competitive, because depending on the length and configuration of a corridor, additional emergency lighting units may not be required to comply with the code-required light levels.
The typical LED consumes five or less watts of power. In addition, there is a solid-state electroluminescent exit sign available that consumes abouter lighting distribution than LED exit signs for improved exit sign recognition.
A centralized lighting inverter can provide for a significant savings in maintenance over individual unit equipment or battery packs integral with lighting fixtures.
The electrical engineer or electrical distribution system designer needs to be aware of all the electrical, building and fire codes (existing and new) that affect the design of the electrical distribution and lighting systems. It is important to know which codes and any special circumstances that are applicable in your permitting area or jurisdiction. In addition, the engineer or lighting system designer should be aware of the potential means and methods of complying with these codes in the most cost-effective manner. It is also important to analyze these options based on life-cycle costs as well as initial system costs. The lighting inverter option has some life cycle benefits from the standpoint of maintenance and energy efficiency.
Definitions and Conversions
Lumens/Sq. Ft × 1 = foot candles
Lumens/Sq. Ft × 10.76 = lumens/Sq. Meter
Foot candles × 10.76 = lumens/Sq. Meter
Foot candles × 10.76 = lux
Help From HID Devices
There is a recently developed technology, an HID arc maintenance device , that will help engineers comply with fire and electrical codes. This device is an auxiliary off-line inverter that can maintain the lamp arc in metal-halide lamps to eliminate the restrike period or the need for quartz restrike lamps. The combination of an NEC 700.12 compliant emergency generator or lighting inverter and the HID arc maintenance device should meet the IBC provisions required for emergency egress illumination on rooftop parking decks. If the arc maintenance device is used without a standby generator or lighting system inverter, it can help to ride through utility line disturbances that could otherwise extinguish the lamp arc and subject the building to light loss and a cool down and restrike period.
In addition, if high pressure sodium (HPS) fixtures are utilized, dual arc tube ballasts can be implemented to remove the cool down time from the equation. Total restrike times can be greatly reduced . The ten second requirement is still applicable. if the high pressure sodium light levels do not reach those egress lighting levels stated by the NFPA 101 code within this time frame indicated in the National Electrical Code, additional measure will have to be designed into the lighting system.