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6 it is the intent of ITE to further refine these specifications by current standards requirements and designs to ensure that harmonizing the language and content of these specifications only the most fault-tolerant approaches are used. This tech- with that of the new ITE 2005 VTCSH-LED. nical review should consider all aspects of the electrical designs of LEDs with a focus on potential failure modes TECHNICAL ISSUES (including failing on and off). Many technical issues have been overcome since the initial Another technical issue is the harmony between the ITE implementation of LED traffic signals. The 2005 ITE speci- standard for LEDs and the NEMA standard for the signal fication addresses many of the early problems with LEDs. monitor portion of the MMU in TS 2 or the Conflict Monitor The following discusses the issues that have been addressed, (CM) in TS 1. The MMU or CM specifies operation down to as well as some issues that are still outstanding. 60 10 volts, while LEDs are only required to operate at 80 volts. This creates an inconsistency in the expectation of Traffic signal safety monitors continually look for potential two devices that should operate in harmony. problems in the operation of the traffic signal. The monitor- ing logic is designed around the field indication circuit and A load switch is a solid state device, containing triacs, electrical characteristics of a simple incandescent lamp. which is designed to use the traffic signal controller's low An incandescent circuit is simply a filament that is connected voltage DC outputs to switch on high voltage AC outputs to across the hot and neutral leads of the field wiring to that the signal heads. The load switch requires a minimum current particular indication. When power is applied, the filament flow to trigger and hold the output in the "on" state. In some quickly heats up and the lamp emits light. When power is cases, extremely low wattage LED modules may not draw removed, the filament quickly cools and the lamp no longer the necessary load current to satisfy the trigger current or emits light. If the lamp fails, the circuit is open. It is very sim- hold current requirements of the load switch, resulting in ple and very predictable. An LED module is a system of trans- flickering of the signal indication and/or permanent damage formers, electronic circuitry, and light-emitting diodes. Unlike to the load switch. This issue is a practical constraint on the a filament, the electrical characteristics of an LED module are usage of low wattage LEDs. Current NEMA load switch component- and design-dependent. The slow voltage decay specifications provide an operating range of 0.1 to 10 amps, can be interpreted by the signal monitor as two conflicting effectively requiring a 12 W minimum load to operate. This indications being energized simultaneously causing the mon- is problematic in some applications such as single arrow itor to place the signal into conflict flash. This condition has application for left turns where the load could be as little as been addressed and corrected in the new specification. 5 W. This has lead to the practice by some of installing a load resistor in the output circuit, which can defeat the conflict A far worse condition than a signal monitor falsely plac- monitoring process. This is an example of the lack of har- ing a signal in conflict flash is a scenario that would war- mony between load switches that were designed for incan- rant a conflict flash condition that is ignored or missed. descent lamps that draw higher currents and LEDs whose There have been reports of malfunctioning LED modules purpose is to reduce load. While there are technical solutions that no longer emit light, which continue to present an such as designing a new load switch, there are issues of cost electrical load to the conflict monitor, giving the monitor and interchangeability that make an interim solution difficult the appearance of normal operation. This potential prob- in practice. lem has been identified and addressed in the new ITE 2005 VTCSH-LED specification by requiring a failed state Transient voltage protection is also an issue in areas with impedance circuit that will sense a problem and effectively lightning strikes. Although the 2005 ITE VTCSH-LED spec- appear as an open circuit to the signal monitor similar to an ification makes reference to the NEMA TS 2-2003 voltage incandescent lamp. However, it must be realized that the surge protection, the environment of the LED signal head only true test is observation of the presence or absence of electronics is significantly different from that with the road- adequate light output, which can only be inferred from LED side cabinet. These differences bring into question, as noted electronics. Although the failure mode issue is addressed in the standard, the effectiveness of the current specification. by the 2005 VTCSH-LED, it is not clear whether the Some users, most notably Texas DOT, have had concerns requirements are sufficient to provide the most practically with LEDs and lightning strikes. reliable systems. Since LED modules are extremely energy efficient, they A different conflict occurs when the LED emits light when do not generate waste heat and do not melt snow and ice from not energized. This can occur as a result of the design of the their lenses like their incandescent counterparts. This can be a LED electronics. Some designs may discharge stored energy problem and may require some additional maintenance work in a means that could allow the energy to be stored and dis- to clear the faces after a driving snow storm. Some agencies charged in an unintended manner. This issue, which is highly have had success using products that are designed to repel rain technical in nature, suggests an independent assessment of from automobile windshields to mitigate this effect.