Safety questions from 1974 concerning tungsten halogen lamps could provide a useful history lesson vis a vis the adoption of solid-state lighting today, according to a recent column in LD+A by contributing writer Jerry Plank.
Tungsten halogen lamps were, and are, fantastic due to consistent and higher color temperatures throughout life; compact size; longer lamp life; and the fact that no support ballasts or power supplies were needed. Unfortunately, the same qualities that made tungsten halogen great set the scene for problems back in the mid ’70s. Let’s look at some of the problems of tungsten halogen lamps and draw parallels to what we see today with SSL lighting products.
Fire Hazard. The propagation of flame is always a concern when lighting products are mounted on or adjacent to combustible materials. The UL/ANSI standards for lighting use 90 deg C as the benchmark for the limit to prevent fires. Prior to the introduction of tungsten halogen lamps, many incandescent products were test-exempt provided the fixture met certain size restrictions. Now that the tungsten halogen lamp was more compact, the fixtures became smaller while the lamp source became hotter to maintain the halogen regenerative effect. Further, as the tungsten halogen lamp was often thought to be a hybrid incandescent source, there was some confusion regarding whether the lamp required a thermal test at all.
The safety standard at that time, UL57, The Standard for Electric Lighting Fixtures, did not address the new tungsten halogen lamp source with any clarity, leaving too many products to fail in the field. The failed fixtures followed a trend where either the building was scorched or burned, the lamp metal foil used to transmit power to the filament became too hot and shortened the lamp life, or glassware exploded from the intense heat from the lamp.
Today, we’ve seen a familiar pattern in that standards for SSL products have not been vetted fully. As many SSL products operate at Class 2 levels, the appropriate standards overlook critical temperature limitations, such that the energy transmitted in the light beam is not considered during an investigation. As LEDs become more powerful, fixtures are becoming smaller and smaller, and we will see escalating temperature profiles that will soon be capable of igniting combustible materials.
Electrical Shock Hazard. Tungsten halogen lamps are available in many base types and sizes and operate at line and low voltage. The base that is used with the line-voltage tubular types is a recessed single contact; the electrical contact is located on either end of the lamp such that during relamping it is possible to contact the live end. The current safety requirements require provisions to prevent contact to the live end by an interlock switch that de-energizes the fixture when the glassware is removed, or by having a rejection feature such that the hot or live end needs to be installed first removing the potential shock hazard. Early designs of double-ended tungsten halogen lamps without these safety features were used in the field for over 10 years, and a marking on the fixture required the user to de-energize the unit to reduce the likelihood of receiving an electrical shock.
Since today’s LEDs are normally low-voltage sources we’re apt to dismiss the likelihood of an electrical shock happening with a Class 2 circuit. Class 2 limits require a circuit to have limited voltage and energy such that the voltage cannot exceed 60 volts DC (30 volts RMS). The voltage limitation of 60 volts DC (30 volts RMS) equates to the impedance of the human body under dry conditions, meaning that the voltage needs to break through your skin which cannot happen until the voltage exceeds 60 volts. Product safety testing is predicated on the fact that any Listed product meets the minimum level of safety but does not suggest that the user is protected for every fault or possibility.
Casualty Hazard. Tungsten halogen lamps operate at higher temperatures than incandescent lamps; at end of life there is the possibility that the lamp envelope can rupture, since higher pressures are involved. The safety standards and specifically UL57 did not address the violent end-of-life issue with tungsten halogen lamps in the beginning, although lamp manufacturers did recommend that adequate screening techniques be used in the field. Case in point is a series of tungsten halogen surface fixtures that were installed in the World Trade Center; the open fixtures provided little containment of the glass particles when a few of the lamps failed violently. A solution was designed comprising stainless steel screens. The main point here is that safety standards need to be developed for new technologies and often we can’t predict all the possible fault scenarios without field feedback.
Looking forward, there will be much more we need to consider regarding SSL product safety. For example, LED sources will get to the point where they’re so bright that we need to be concerned with the effect on the human eye to prevent retinal damage. There is talk of photobiological hazards with LEDs, but no hard data to suggest at what energy level or wavelength we should be concerned. This chapter won’t be written until more research is done, but stay tuned as casualty hazards always seem to bite hard when they appear.