Emergency egress and illumination systems
Understanding the basic characteristics of emergency egress and illumination systems provides engineers with the knowledge to optimize lifecycle and emergency evacuation performance.
- Outline the key characteristics of emergency illumination systems.
- Compare the key characteristics of electrically powered, self-luminous and photoluminescent exit signs.
- Summarize the basic principles of photoluminescence employed in egress devices.
During a fire, emergency egress and illumination systems can make the difference between life and death. Effective design, specification, and lifecycle management can optimize the lifespan effectiveness of these life safety systems and enhance their performance during evacuation. A basic understanding of their various performance characteristics is warranted.
The proliferation of complexity in life safety systems requires a high level of technical literacy to ensure they can be relied upon for the duration of the building's lifecycle. Similarly, as aging building stock is retrofitted, engineers must exercise good judgment when assessing existing building conditions and reconciling various existing and new construction requirements related to emergency egress and illumination systems.
Engineers are constantly challenged to effectively integrate these complexities. In addition to the increasing complexity of life safety systems, the continual development of new codes and standards warrants careful consideration on the part of engineers to assess the ability of facility operators to effectively manage the specified systems. It is not uncommon for design and consulting engineers to have little to no understanding of the issues facility managers inherit when they assume responsibility for the long-term care of these fire life safety systems.
Codes and standards
UL is the key organization that validates products for emergency egress signage and illumination equipment in jurisdictions that adopt the International Building Code (IBC) and NFPA 101: Life Safety Code. The technical standard for these systems is UL 924, Emergency Lighting and Power Equipment. UL 924 applies to emergency lighting and power equipment for use in unclassified locations and intended for connection to branch circuits of 600 V or less.
This equipment is intended to automatically supply illumination or power or both to critical areas and equipment in the event of failure of the normal power supply in accordance with the NFPA 70: National Electrical Code (NEC), NFPA 101, NFPA 1: Fire Code, the IBC, and the International Fire Code (IFC).
Examples of equipment addressed in UL 924 includes:
- Exit signs
- Emergency lighting
- Unit equipment
- Central-station battery banks
- utomatic battery charging and control equipment
- Automatic load-control relays
- Derangement signal equipment.
Where the path of egress travel is not immediately visible to occupants, readily visible exit signs are required to clearly indicate the direction of egress travel to exits and within exits. Exit signs to identify exits are required to be readily visible in the direction of egress travel and, with a few exceptions, are required to be placed in corridors or exit passageways such that no point exceeds 100 ft from and exit sign or the sign's listed viewing distance, whichever is less. Note that some jurisdictions require different mounting heights or colors for the exit signs. In addition, local code may require the word "exit" to be a specific size.
Electrically powered, self-luminous, and photoluminescent exit signs are classified as internally illuminated exit signs and are required to be illuminated at all times. They are required to be listed and labeled in accordance with UL 924 and installed in accordance with the manufacturer's instructions and IBC Chapter 27.
Electrically powered exit signs and unit equipment are required to be fed from the same circuit that provides normal power to lighting in the area served and connected ahead of any local switches. This requirement is intended to ensure that these emergency systems operate in the affected area upon loss of power. NFPA 70 permits a separate branch circuit for unit equipment if it originates from the same panelboard as that of the normal lighting circuits, is provided with a lock-on feature, and the location is in a separate and uninterrupted area supplied by a minimum of three normal lighting circuits that are not part of a multiwire branch circuit.
The means of egress including the exit discharge is required to be illuminated at all times the building is occupied. The minimum duration for emergency egress illumination is 90 minutes.
The following locations require emergency illumination:
- Aisles Corridors
- Exit passageways
- Stair enclosures
- Exterior landings
- Exterior egress components at other than the level of exit discharge in buildings required to have two or more exits.
The following sources of power are suitable for emergency illumination:
- Onsite generator
- Storage batteries
- Unit equipment.
Although 90 minutes is the required minimum duration for emergency egress illumination, self-contained emergency lights are available with durations in excess of 90 minutes. These units may be a desirable specification option in buildings where the potential exists for evacuation times to exceed 90 minutes.
The IBC requires a minimum of 1 fc (11 lux) at the floor level (means-of-egress illumination in IBC 1006). Emergency egress illumination is required to be provided for a minimum duration of 90 minutes. Illumination levels are permitted to decline to 0.6 fc (6 lux) at the end of the emergency lighting time duration. A maximum-to-minimum illumination uniformity ratio of 40:1 shall not be exceeded.
Reliability provisions in the NEC require emergency lighting systems to be designed and installed so that the failure of any individual lighting element, such as the burning out of a lamp, cannot leave a space in total darkness.
Self-luminous tritium exit signs
Photoluminescence is the spontaneous emission of light from a material under optical excitation. Self-luminous and photoluminescent exit signs rely on the principle of photoluminescence. When light of sufficient energy is incident on a material, photons are absorbed and electronic excitations are created. When these excitations relax, electrons return to their ground state.
Self-luminous tritium exit signs require no external electrical input or external light source for charging. Tritium is a radioactive isotope of hydrogen that decays by emitting an electron called a beta particle. Tritium signs rely on the process of radioluminescence, whereby emitted electrons impact a phosphor layer within the exit sign and release photons in the visible spectrum.
Although beta particle radiation can be easily shielded by thin material such as paper, tritium can enter the body through the skin or open wounds. The main hazard associated with tritium is internal exposure from inhalation or ingestion.
Tritium exit signs should always be handled by qualified personnel who are knowledgeable about the unique characteristics and requirements of these devices. Although ingestion of a tritium dose capable of causing significant harm is unlikely, damaged exit signs should not be handled with bare hands and areas where tritium exit signs are stored should be ventilated.
The half-life of tritium is 12.3 years. As the excitation intensity decreases due to decay of the tritium source, the density of photoexcited electrons will decrease. Excitation intensity influences the quality of emitted light. Accordingly, tritium exit signs are labeled with a rating life that indicates the expiration date of the sign.
These signs are typically rated for a 10-year luminous life and must be replaced and disposed of in accordance with requirements of the U.S. Nuclear Regulatory Commission (NRC) or Agreement State applied to the generally licensed device.
Although self-luminous exit signs with ratings in excess of 10 years are available, in practice they are rarely specified. Accordingly, the specification of self-luminous exit signs has inherent deferred capital-improvement costs associated with disposal of the expired signs and their replacement with new equipment.
Tritium exit signs should be labeled and dated to indicate the expiration of the rating. They can be identified by labeling on the side, edge, or back of the sign that includes a magenta and yellow radioactive symbol and "CAUTION: RADIOACTIVE MATERIAL"; an NRC sticker, and the manufacturer's NRC license and contact information.
To dispose of a sign properly, a general licensee must transfer the sign to a specific licensee-such as a manufacturer, distributor, licensed radioactive waste broker, or licensed low-level radioactive waste disposal facility. Within 30 days of disposing of a sign, the general licensee must file a report to the NRC or Agreement State.
Notably, tritium exit signs are prohibited for use by the U.S. Department of Defense, some municipalities, and numerous college campuses.