Advertisement

Blog

It Was a Dark & Stormy Night (Because Someone Neglected Circuit Protection)

Editor’s 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 LEDs’ 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.

Figure 1

Elements of an LED streetlight protection scheme.

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 MOV’s 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.

Figure 2

A thermal disconnect can open a circuit and prevent a degraded MOV from failing catastrophically.

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, it’s 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.

14 comments on “It Was a Dark & Stormy Night (Because Someone Neglected Circuit Protection)

  1. nasimson
    December 27, 2014

    This blog could have generated some interesting discussion. And that would have been much learning. It didnt because of holidays I guess. Eds, please see if you can feature this piece so we can attract some thoughts on this important topic impacting so much of urban modernization these days.

  2. Tim Patel
    December 27, 2014

    @nasimson — Thanks for your comments.  We sure do hope to spark some interesting discussion about circuit protection.  Are there any particular topics that you would like to discuss?

  3. uchiha
    December 29, 2014

    Interest in LED lighting is at an all-time high as industrial facilities increasingly zero in on energy efficiency, sustainability and cost containment as key metrics for capital investment decisions. Given the importance of these metrics to a facility's
    bottom line and long-term competitiveness, few question that LEDs will soon become the lighting technology of choice in the industrial sector.

  4. fasmicro
    December 29, 2014

    >> This blog could have generated some interesting discussion. And that would have been much learning

    They will get back to it as we run the monthly or quarterly Table of Contents. Circuit protection is a hot area and will continue to be.

  5. fasmicro
    December 29, 2014

    >> Interest in LED lighting is at an all-time high as industrial facilities increasingly zero in on energy efficiency, sustainability and cost containment as key metrics for capital investment decisions. 

    The fear is that let not the drop in crude oil price depress this interest in sustainability and efficiency. If the prices of gas goes very low, the cost analysis may not necessarily favor extra money for efficiency. Why worry if gas is very low that we can waste it!

  6. uchiha
    December 30, 2014

    Yes.. Maybe you are correct.no need to worry about that.

  7. nasimson
    December 30, 2014

    @Tim:

    > Are there any particular topics that you would like to discuss?

    An interesting learning can be around what are different approaches around circuit protection with special reference to LED lights? And whats their efficacy and cost to benefit ratio?

  8. Tim Patel
    December 30, 2014

    @nasimson:

    Great questions!

    There are various approaches for including circuit protection into LED lighting.  All are dependent on the specific application and intended environment for the LED lighting. 

    For example, indoor LED bulbs and lights typically are not subjected to the same levels of transient overcurrents and overvoltages as those installed in outdoor locations, so the level of protection required is normally not as high.  Typically, fast-acting fuses with low nominal operating currents are used in indoor LED lighting.  Where overvoltage protection is needed, semiconductor-based components like TVS diodes can provide an adequate level of overvoltage protection with fast response time.

    On the other hand, for outdoor LED lighting applications, a higher level of overvoltage and overcurrent protection is needed.  This is due to the occurrence of surges on AC power lines originating from lightning strikes, utility power factor correction, load switching, and other events.  For some outdoor LED lighting or even indoor industrial LED lighting installations, high-reliability in the lighting system is required, and thus more robust circuit protection, such as high-energy withstand surge protection modules and surge-resistant fuses, would be needed.

    The benefit of most overvoltage protective devices is that they typically draw very little leakage or “quiescent” current, and therefore there is very little loss in efficiency.  Overcurrent protective devices, on the other hand, do normally dissipate some finite amount of power during normal operation.  However, the benefit in terms of protecting against disastrous short-circuit or overload conditions (which could potentially lead to explosions and/or fire) is most often worth the small cost to add such protection into circuits.

  9. SunitaT
    December 31, 2014

    @uchiha: I think if LED's are coupled with smart lighting solutions then the energy efficiency and the cost to benefit ratio graph would increase substantially towards profit. I've seen LED's being used in street lighting and they give a far clearer road view and much more energy savings than traditional CFLs or high watt light bulbs.

  10. uchiha
    January 8, 2015

     @sunitaT0 : Yes of course your argument is correct. It'll increase the profit but the problem is how far it is possible?

  11. Scott Elder
    January 25, 2015

    Hi Tim,

    I've been studying the problem of retrofitting street lighting (i.e. high pressure sodium) with LED systems.  While the lifetime and efficiency benefits of LEDs can't be argued, I wonder about the cost benefits when power line issues are added into the problem.  It seems the inductive ballasts and relative simplicity of existing technologies (HPS) solve these issues quite well.  Certainly the robustness of HPS has been substantially tested over the past several decades.  Can you share your experience with this issue in regards to cities that have adopted LED street lighting?  What are their experiences with power line robustness?

  12. Tim Patel
    January 26, 2015

    Hi Scott,

    You certainly raise a good point about the robustness of traditional lighting technologies (e.g., HPS) when it comes to dealing with power quality or transient issues on the power line.

    Cities like Seattle and Las Vegas have had success in retrofitting street lighting over the last few years.  However, some studies performed by various LED lighting consortiums show that power supply / driver component failures is the most prominent source of failure in an LED luminaire.  Many of these failures are attributed to power line issues such as surges, low power quality, temporary overvoltages, etc.

    In some regions of the world, power quality is a major concern, and LED lighting designers are having to design added protection into drivers or luminaires (from a systems level) to mitigate the effects of these issues.  Although the cost of adding this protection may appear to detract from the overall benefit of retrofitting existing lighting, over the expected longer period of lifetime for LED luminaires, the cost is still expected to be recovered.

  13. Scott Elder
    January 26, 2015

    Thanks Tim.

    “Many of these failures are attributed to power line issues such as surges, low power quality, temporary overvoltages, etc.”

    So do the SPDs and MOV-type elements set the actual (read: in the field) lifetimes for off-line LED products?  In other words, if you were to order the list of failed elements, LEDs would not be at the top, but what would be the top two?  Passives, SPD, MOVs,… ?

  14. Tim Patel
    January 26, 2015

    Hi Scott,

    Most of the failed components are passives and semiconductor components (e.g., bridge rectifier diodes).  These would be in unprotected luminaire designs — i.e., those without overvoltage protection provided in the driver or those without SPDs installed.  MOV-based SPDs normally do not reach their end-of-life or go offline within the expected lifetime of the luminaire unless they were subjected to an abnormally high number of surges.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.