Wayfinding and Rescue in Heavy Smoke or Blackouts:
Low Tech Markings Outperform Sophisticated Concepts
InterConsult Group ASA
Newly recognized earth-borne pigments have made a ten-fold improvement in luminance of photoluminescent (PL) wayfinding systems and removed doubts of deficiency. The debate on sufficiently visible or not has shifted to a discussion on how much the width of marking lines may be reduced and still conform.
Due to a comfortable margin for error new products tolerate minor design flaws, periodic loss of charging light, blackouts in excess of 10 hours or degradation caused by aging or dirt.
The superior reliability, low cost and unmistakable marking of earlier zinc sulphide PL systems has been retained.
The PL manufacturing industry concern is to establish an international standard of their own and to arrest public misconceptions of PL marking versus powered emergency lighting.
Photoluminescent systems are non-radioactive materials containing pigments that store energy from natural and artificial lights. When the lights are extinguished, the materials glow in the dark by releasing this stored energy.
The new PL (after-glowing) materials employ rare earth-borne pigments of heavy metals such as strontium, barium, europium, boron, arsenic, mercury and cadmium. Compared to conventional zinc sulphide PL the visible after-glow persist for 3 days rather than 5 hrs, and the 60 min luminance requirement is surpassed 10-20 times. 25-35mm strip widths may be as effective as conventional 60-100mm strips. The IMO and DIN standards is yet to be upgraded.
The new PL comes along with new product designs such as afterglowing handrails for use in utility service tunnels or complex industry structures. The PL material is moulded in special resins to withstand harsh environments. At least one manufacturer offer a 5 year warranty for offshore applications. They may be embedded on site to decks of concrete protected by thermosetting top layers against impact from vehicles or tools. Some lacks the dreaded PVC. They are UV-stable and may be used in sunlight.
Improvements Add Substantial Safety Margins to Underrated PL
PL marking has been a trusted concept to provide reliable wayfinding in industry for decades. At the start of the 90-ies researchers in Norway tested the performance of PL in smoky conditions against powered lighting concepts. It was proven that despite the relatively low afterglowing luminance PL was capable of outperforming their high luminance electrically powered counterparts in the most severe smoke density.
The key factor is the short viewing distance provided by linear also referred to as proximity,delineation or continuous marking. Strips of PL material identifies the escape route or rescue items located at floor, skirting boards, threads or handrails (low location lighting) ensuring that the viewing distance of evacuees never exceed roughly one meter when they are bending slightly forward. Within this viewing range luminance is no longer important provided a visible contrast only. Indeed, added luminance may cause glaring and diffusion effects in smoke making visibility worse.
There was but one difficult shortcoming of zinc sulphide PL: Luminance was just barely sufficient at extreme smoke densities where they left no margins for upright evacuees looking at floor markings or for the visually impaired. Conventional wisdom stated the more luminance the better and many therefore mistakenly preferred electrified lighting.
The high intensity PL materials offer luminance with a safety margin. These wayfinding marking systems are even less susceptible to design flaws, abuse or degradation over time than their predecessors. They require a longer activation time, but accept a low charging illuminance and the brighter and long-lasting luminance makes up for this deficiency.
A stronghold of PL marking is the underground power plant industry. Most of Norways 863 hydro power plants are equipped with PL linear marking for wayfinding and rescue operation in power outings or smoke. In Sweden an underground power plant caught fire and two survivors insisted they would have perished without sticking to the PL lines.
The new Oslo Gardermoen Airport is equipped with the novel PL in underground areas, train terminal and tunnels. Both linear marking and signs is made to perform better than 300/45 mcd/m2, surpassing conventional zinc sulphide PL by a factor of 20. Tunnels challenge PL systems as charging lights from passing trains and vehicles may be scarce.
Following the attack at the World Trade Center in New York the high rise towers are provided with PL linear marking. All lighting power supplies in stairwells broke down in 12 minutes. The evacuation in dense smoke took seven hours.
The many hazards at North Sea Oil Rigs require escape marking to be both effective and reliable. Disasters such as the Piper Alpha oil rig, the Herald of Free Enterprise ferry and the Scandinavian Star ferry pointed at the hazards of inadequate marking and power supplies. The new marking materials points to a definite solution. The PL manufacturing industry express great expectations of increase in both offshore and maritime applications.
When accidents involve fire, explosion, collapse, short-circuiting or ingress of water all electrical systems including emergency power supplies are at risk. Therefore, PL are increasingly used in road and train tunnels.
It is no wonder then, that PL systems have been tested and deemed adequate by FAA and the larger airlines to replace electroluminescent marking of the aisles in aircrafts. The heavy metal pigments fulfilled the requirement of 11 hours for longhaul airplanes and provided intense visible delineation to the end of test at 16 hours.
When All Fails
There is no neon glamour or fancy features in PL wayfinding. There is no flood light luminance, no self-checking, no high-tech bulb fault reporting features and no long lasting battery packs to boast of. But if all fails, PL perform. There is virtually nothing that may go wrong: You may even remove or obscure parts of the marking, or keep the escape route in total darkness for a whole working shift period without preventing PL to perform when called upon.
Electrically powered emergency lighting is a completely different concept that complement rather than substitute PL escape route marking. Emergency lighting ensure illumination when normal lighting fails, and support PL in large public spaces to mark exits from a distance, or in smoky conditions to help identify these exits at close range. Electrified low location lighting (EP) is an alternative in luminance. But like emergency lighting EP is prone to an array of failure modes by itself and suffer a bad record of reliability and cost. Point sources of light are easily obscured by smoke and a survey concluded their performance as a wayfinding system is very low only 8 % of evacuees in real fire catastrophes noted the presence of these.
Low location marking with PL provide reliable wayfinding safety in the worst of conditions. A Finnish study recommends linear PL as prime marking system in the most extreme of smoke.
Breathing Apparatus Required
Without breathing apparatus humans may not survive a minute of smoke densities exceeding roughly OD 1.5/m. At this density EP single light sources beyond 2 m is totally obscured. A PL handrail at 0.5 m viewing distance is useful at much denser smoke. But users must wear eye protection or breathing apparatus. In fact, eye irritation prevents effective use of visual aids from OD 0.2/m on. A 1993 study concludes directional tactile handrails may provide superior wayfinding performance in smoke more dense than OD 1.5/m with or without visual PL/EP strips embedded.
The Low Location Lighting Objective
The objective of wayfinding systems for evacuation is to guide all persons to a safe haven as orderly, fast and unambiguously as possible. A flaming fire create a smoke layer against the ceiling in the fire room and vicinity leaving better visibility at floor levels. The smoke layer obscure point sources of light very quickly. The idea of low location lighting evolved on this during the 80-ies. Others realized that away from the fire vicinity smoke become evenly dispersed due to cooling and that marking systems must be linear to be visible. At first one kept mixing the ideas of low location, lineation and point sources of light. This led to delineated spaced EP point sources of high luminance at floor level. But in fact, linear marking perform best at eye level in evenly dispersed smoke and does barely require any luminance at all. Therefore fully linear PL systems emerged and performed equally well or best in tests.
Safe Escape and Rescue in Smoke
Emergency lighting and signage do not perform as expected at fire disasters. Very often systems are found to be nonoperative, not providing useful wayguidance or both. There are two common denominators: the systems are traditionally not required to be designed for smoky conditions and they are prone to failure by lack of maintenance or complexity.
A decisive factor to evacuees is the time they become exposed to heavy smoke. This time is governed by the perceived wayguidance of the evacuees - less by the length of the smoke filled escape route, which is within 16 m only in 90% of fire disasters by statistics - therefore wayfinding systems for smoky conditions play an important role.
One need to design for wayfinding in smoke exceeding OD 1.5/m in some industry as well as some public structures, contrary to current proposed general limits of OD 0.4/m within some international standardization bodies.
Careful studying has indicated a level of smoke severity and time of exposure (irritant smoke at density 1.53 to 2.26/m, exposure time 3 to 4 min, escape path 25 to 30 m) as the limit to what normal people will endure without protection - no matter what emergency egress information system is being used.
At industry plants, underground installations and complex high risk structures such as nuclear power plants or offshore drilling platforms visibility may become crucial in severe smoke. In order to perform shutdowns, extinguish fires, closing valves, rescue work or other intervening operations at imminent disaster, or indeed when all things have failed, personnel may encounter very dense smoke, if ever so briefly.
Very high performance of guiding people in smoke may be provided by simple nonpowered wayfinding systems: a tactile safety hand rail system or photoluminescent marking or both. Decisive factors in designing optimum wayfinding systems for smoky conditions are continuous marking less than 1meter above floor level, simplicity, independence of electric power, long operational times, simple maintenance and low cost.
Viewing distance, not power or luminance, is the crucial factor at high smoke density. This relies not on obscuration by distance alone, but on the simple reason that visibility of any luminance must converge to zero at 100% obscuration.
A photoluminescent strip at 0.5 m distance to the eye is more visible than the most powerful luminaries at just 1.5 m.
The photoluminescent strip built into a directional handrail in test was 25 mm wide only, and tilted to an angle of only 30o to the view direction. Amazingly, this design performed well even at smoke densities between OD 1.4/m and 1.5/m, but is explained by the short viewing distance.
Without respiratory protection the time available in severe smoke is approximately 50 seconds at a mean travel speed of 0.35 m/s. This is relevant to the 16 m length of escape in smoke which accounts for 90% of incidents. Thus, given a good wayfinding system or a familiar route the life saving potential is very substantial. This calls for Class I wayfinding systems (OD >1.5/m) and escape hoods or breathing apparatus to prevent irritation of eyes and respiratory system.
Research recommend fast escape in upright or forward bent walking mode. Optimum system designs involve continuous marking at waist height, at skirting board or at the floor center line.
Metal Safe Sign International
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