Lighting : LIGHTING Aug-Sep 2018
32 LIGHTING MAGAZINE | August/September 2018 August/September 2018 | LIGHTING MAGAZINE 33 are really the only practical option. LED emitters can be UV only, UV/visible or visible only. Emitters rich in UV are more suited to rear surface activation through air, thus avoiding unnecessary exposure to UV. LEDs emitting only visible light allow more options when selecting the PL matrix material and the reflective layer surface: the matrix only needs transparency in the 400-575nm range and the reflective backings likewise only need reflectivity in the 400-575nm range. Whether visible or UV, the emission spectra of the LEDs can be selected to most efficiently charge the PL particles, depending on the system layout. LUMINANCE PERFORMANCE OF HYBRID PL EXIT SIGNS The Australian and New Zealand building codes  require charging light of at least 100lux on the sign for PL exit signage and a minimum luminance (at the end of the evacuation period) of 30mcd/m2. A 1W ‘white’ LED light source delivering 100lm/W will produce an average illuminance of 1000lx on a 420mm x 230mm sign face (the size required in the Australian and New Zealand building codes for a 24m viewing distance pictographic PL exit sign). Therefore, hybrid signs can have significantly greater charging illuminance than standard PL signs that rely on room lighting. If LEDs are selected that charge PL particles more efficiently than conventional room lighting, the two factors together mean that hybrid signs can easily be designed to provide a luminance greater than 30mcd/m2 at the end of the evacuation period. See Figure 5 for an sample test report. [Note that in New Zealand, the evacuation period for most buildings is 30 minutes, while in Australia the Deemed-to-Satisfy (“DtS”) provisions specify a 90 minute evacuation period.] EXPECTED ‘LIFETIME’ OF A HYBRID PL EXIT SIGN Lifetime prediction is fraught with difficulties. Given that electronic assemblies can be suitably modeled based on a constant failure rate , a more meaningful concept is Annualized Failure Rate (AFR) – the estimated probability of the product failing in any twelve-month period. Most electronic components are supplied with statistical failure data and these can be combined for the complete assembly to produce an expected AFR. Good design of the complete hybrid sign may result in a lower AFR than those of current-technology battery- back-up exit signs. An important aspect is the simplicity of electrical control: there are no battery charging or monitoring circuits that can fail, whereas the extra circuitry needed in a battery-back-up sign impacts on its overall reliability. It is generally accepted that LEDs have long lives and low to moderate failure rates. LED ‘life’ is often used in the description of a battery-back-up sign. While LM-80 data extrapolated using TM-21 methodology  is part of the equation, it is an insufficient basis on which to claim product life. The TM-21 value is purely a projection of when the LED’s luminance will drop to a pre-determined level – it doesn’t consider that the LED (or another part of the electrical/electronic circuitry) may fail prior to this point. A basic high-quality battery-back- up exit sign typically has an AFR for the electrical system (excluding the battery) of around 2%, meaning that within 5 years, 10% of signs can be expected to have failed due to electronic malfunction. Signs with more complex circuitry, such as remote monitoring, have higher AFRs and budget-level signs can have even higher AFRs. In comparison, typical high-quality hybrid signs can be expected to have AFRs of around 0.5-1% (2.5% to 5% failure within 5 years). Hybrid signs that are designed to have short re-charge times, eg, 5 minutes or less, can be connected to a power supply that is timer controlled, potentially further increasing the life of the sign. OPERATIONAL RELIABILITY Hybrid signs intrinsically have a high level of fail-safeness: z they can readily be made to surpass the local code requirements. Figure 5 shows a hybrid sign that does not drop below 30mcd/m2 until after 3-4 hours of discharge time and remains usefully visible (above 5mcd/m2)  for many hours thereafter. z There is little difference in performance between PL material charged with 700lx and PL charged with 1000lx , so degradation of the light source output over time would have negligible impact on the long-term PL performance. z If the light source should fail, a hybrid sign can still charge from ambient light. z Following a power failure, the PL material in a hybrid sign will rapidly recharge, reaching a code-compliant level typically within 10 minutes. Nonetheless, hybrid signs do have electrical components, therefore, regular inspection is warranted to ensure that the light source is still operating. CODE COMPLIANCE The key requirements to show that a hybrid sign meets the BCA DtS requirements for PL exit signs (Spec E4.8) and the NZBC Acceptable Solution (F8/AS1 4.5 .4) are 100lx on the sign, and 30mcd/m2 at the end of the evacuation period. As indicated above, a well-designed hybrid sign can easily meet those requirements. APPLICATION Clearly, hybrid signs can have advantages over and can be used instead of, battery-back-up signs. However, in Australia, many battery- back-up exit signs are dual-purpose, meaning they can be used to provide emergency lighting as well as exit signage. While in most emergency lighting designs the light from the exit signs is not needed to provide compliance with EP4.1 (via EV4.1) of the BCA, the majority of designers routinely follow the DtS provisions which reference AS 2293.1 . AS2293.1 requires an emergency luminaire within 2m of a door requiring an exit sign. If the emergency lighting design hasn’t included a separate emergency luminaire within 2m of the door, a dual-purpose exit sign will meet the DtS provisions, whereas a current- technology hybrid sign will not. Therefore, in many Australian design situations, hybrid signs can only be included as part of a performance- based design (eg using EV4.1). The ABCB recognizes the potential of performance-based design to reduce unnecessary building costs and is intent on facilitating design practitioners to use performance solutions much more frequently than current practice. CONCLUSIONS Hybrid signs can take a variety of forms and utilise a variety of components. A well-designed hybrid sign can easily surpass the building code performance requirements for PL exit signs. Such hybrid signs can be expected to be significantly more reliable, and last significantly longer than conventional battery-back-up exit signs, resulting in safer buildings with greatly reduced exit signage operating costs, and better environmental credentials. The New Zealand design environment is well suited to utilise hybrid signs as an alternative to battery-back-up signs in locations where standard PL signs are unsuitable. In Australia, designers and code officials will need to become more comfortable using performance-based solutions to realise the potential benefits offered by hybrid signs. REFERENCES  AS/NZS 2293.2:1995 Emergency evacuation lighting for buildings. Part 2: Inspection and maintenance  National Construction Code 2016, Building Code of Australia, Specification E4.8 3(a) and 3(b)  New Zealand Building Code, Acceptable Solution F8/AS1 4.5.4  https://en.wikipedia.org/wiki/Failure_rate, and https://en.wikipedia.org/wiki/Annualized_failure_rate  ANSI/IES LM-80 -15 Approved Method: Measuring Luminous Flux and Color Maintenance of LED Packages, Array and Modules; IES TM-21-11 Projecting Long Term Lumen Maintenance of LED Light Sources  ‘Usefully visible’: the 5mcd/m2 value is based on the minimum luminance specified in ASTM E2072-14 “Standard Specification for Photoluminescent Safety Markings” Section 5; ISO/FDIS 16069:2017(E), “Graphical symbols – Safety signs — Safety way guidance systems”, Section 7.22; NFPA 101 Life Safety Code Section 7.2 .2.5 .5.10 Materials; and International Fire Code/International Building Code Para. 1025.4 Self-Luminous and Photoluminescent Materials.  Luminance tests conducted in accordance with ASTM E2073-10, but with charging luminance 500lx and 1000lx show the luminance difference at 90 minutes to be less than 10%.  AS2293.1 – 2005 Emergency escape lighting and exit signs for buildings. Part 1: System design, installation and operation. Figure 5. Luminance properties of a Hybrid photoluminescent exit sign. Figure 3. Hybrid PL schematics: internal light source (left) internal light source directed at reflective layer (right). Figure 4. Hybrid PL schematic: side mounted light source.
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