Editorís note: Guest editor with Tim Patel is Johnny Chang, Product Manager, Littelfuse, Inc.
Municipalities around the world are making the switch from high-intensity discharge lights to LED streetlights. However, installing them can be a very expensive proposition. Planners must be able to justify the expense by getting a payback on their investment within a reasonable period based on the LEDsí lower wattage demands, lower maintenance costs, and longer operating life. To guard against premature failures, particularly due to lightning-induced surges, high durability and reliability are critical.
Protecting outdoor LED lighting from lightning-induced surges requires diverting high voltage/current transient interference away from sensitive electronics in the lighting fixture. Various surge protective devices (SPDs) are used in outdoor LED lighting to suppress surge energy and minimize surge impact. LED lighting equipment manufacturers rely on a variety of surge-protective devices (SPDs), including carefully chosen fuses, metal oxide varistors (MOVs), and transient voltage suppression (TVS) diodes to meet important regulatory and safety standards related to overvoltage transients.
To prevent damage caused by surge energy, enhance reliability, minimize maintenance, and extend the useful life of an LED installation, a robust surge-suppression circuit is crucial. Figure 1 illustrates the various elements often incorporated into a streetlight surge-protection circuit.
Elements of an LED streetlight protection scheme.
Although some LED luminaires feature surge-protection devices embedded in the power supply unit, circuit-protection device manufacturers often recommend that the surge-protection circuit be kept separate from the luminaire power supply. In this way, luminaire manufacturers can easily market the same luminaire anywhere by attaching different surge-protection modules to meet differing surge-level requirements, based in part on lightning-strike frequency data.
MOVs are widely used in surge-protection circuits for their fast response times, high surge energy handling, compact size, and cost-effectiveness. However, after MOVs absorb a certain number of surge strikes, they will begin to degrade and can no longer provide the same protection as new ones. Having a separate surge-protection module allows for easy replacement when the original module reaches the end of its useful life.
MOV technology offers an affordable, effective way to suppress transients in power supplies and other applications such as the SPD modules often located in front of an LED driver. They are designed to clamp overvoltage transients within microseconds, but when built into SPD modules MOVs can be subject to temporary overvoltage conditions caused by loss of neutral or by faulty installation wiring. These conditions can severely stress an MOV and cause it to experience thermal runaway, resulting in smoke, overheating, and possibly fire. Robust SPD designs feature thermal disconnects to protect the MOVs from thermal runaway.
As mentioned previously, after exposure to a large surge or several small surges, MOVs tend to degrade steadily, which leads to increasing MOV leakage current. Even under normal conditions, this degradation will increase the MOVís temperature. A thermal disconnect (Figure 2) placed next to the MOV can be used to sense the increase in MOV temperature as it continues to deteriorate. When the MOV reaches the end of its operating life, the thermal disconnect will open the circuit, remove the degraded MOV from the circuit, and prevent its catastrophic failure.
A thermal disconnect can open a circuit and prevent a degraded MOV from failing catastrophically.
Once the thermal disconnect removes the MOV from the circuit, the SPD module no longer provides surge suppression. Therefore, itís important to supply proper indication so maintenance personnel will know the SPD is no longer working and that it requires replacement.
Ready to learn more about circuit protection for outdoor LED lighting installations and how it helps ensure higher return on investment? View and download the Focus on Fundamentals Course materials: Keeping the Lights On: Safeguard LED Lighting With Proper Circuit Protection.