Robust circuit protection is needed to minimise costs and maximise return on investment of LED street lighting solutions.
LED street lights produce whiter, cooler light when compared with the yellowish light cast by high-pressure sodium lamps. They reduce the potential for collisions by enhancing drivers’ depth of field and peripheral vision. However, installing LED fixtures requires a substantial initial investment. Planners must justify the expense by getting a payback on their investment within a reasonable period of time, based on LEDs’ lower wattage demands, lower maintenance costs and longer operating life.
Financial analysis for any municipal project is critical to evaluating the project’s feasibility. This is especially true for projects that receive the level of attention LED lighting projects do.
Fig. 1: LED street light circuit protection scheme.
Around the world, various groups have created tools to aid in this financial analysis. One example is the street and parking facility lighting retrofit financial analysis tool. This was developed by a partnership between the United States Department of Energy’s (DOE’s), Municipal Solid-state Street Lighting Consortium (MSSLC), the Clinton Climate Initiative (CCI), C40, and the Federal Energy Management Programme (FEMP).
At least one-third of the total savings attributable to switching to LED street lights result from the extended lifetimes these lighting fixtures offer. To ensure their long-term cost-effectiveness, it is essential to take advantage of their potential for reducing maintenance costs. Protecting outdoor LED lighting from the damaging effects of lightning-induced surges requires diverting high voltage/current transient interference away from sensitive electronics in the luminaire fixture, specifically the LED drivers.
To suppress surge energy and to minimise surge impact, various surge protective devices (SPDs) are incorporated into the outdoor LED lighting during the design and testing phases. LED lighting equipment manufacturers rely on a variety of SPDs, including carefully chosen metal oxide varistors (MOVs), fuses and transient voltage suppression (TVS) diodes to meet important safety standards and regulatory requirements related to overvoltage transients.
Fig. 1 illustrates the various circuit protection elements typically incorporated into a street light surge protection circuit, including overcurrent protection via fuses and thermal protection within the SPD.
Fig. 2: A thermal disconnect can open a circuit and prevent a degraded MOV from failing catastrophically.
While some LED luminaire designs feature surge protection devices embedded within the power supply unit, circuit protection device manufacturers will frequently recommend that the surge protection circuit be kept separate from the luminaire power supply. By doing so, LED luminaire manufacturers can easily market the same luminaire fixtures anywhere by using different surge protection modules to meet differing surge level requirements, based in part on regional lightning strike frequency data.
Because of their compact size, high surge energy handling, fast response times, and cost-effectiveness, MOVs are widely used in surge protection circuits for LED luminaires.
However, after MOVs absorb a certain number of surge strikes, they will start to degrade and can no longer provide the same protection as new ones. Using separate surge protection modules in the design allows for easy replacement when the original SPD module reach the end of their useful life.
MOV technology offers an effective and affordable way to suppress transients in numerous applications, such as power supplies and the SPD modules often located in front of an LED driver. While they are designed to clamp overvoltage transients within microseconds, when they are built into SPD modules, MOVs can be subject to temporary overvoltage conditions caused by faulty installation wiring or by loss of neutral. These conditions can severely stress an MOV, causing it to experience thermal runaway. This can result in overheating, smoke, and possibly fire. Robust SPD designs feature thermal disconnects to protect the MOVs from thermal runaway.
MOVs tend to degrade steadily after exposure to a large surge or several small surges, leading to increasing MOV leakage current. This degradation will increase the MOV’s temperature, even under normal conditions. A thermal fuse element (see Fig. 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.
Once the thermal disconnect removes the MOV from the circuit, the SPD module can no longer provide surge suppression. It is therefore important to provide visual indication so that maintenance personnel will know the SPD is no longer functioning and requires replacement.
When to use parallel or series-connected SPD modules
LED luminaire specifiers have a choice of two main types of SPD module configuration based on their maintenance strategies: parallel or series-connected surge protection sub-assemblies.
Fig. 3: Example of an SPD module using either parallel (a) or series (b) connection to a luminaire.
Outdoor LED lighting surge protection modules
Modules such as the LSP thermally protected varistor SPD from Littelfuse are designed specifically for outdoor and commercial LED lighting applications. A built-in thermal disconnect function provides additional protection from catastrophic failures and fire hazards, even under the extreme circumstances of MOV end-of-life or sustained overvoltage conditions. Modules such as the LSP05 and LSP10 are replaceable and the latter has a special indication function which turns the light off when it is activated.
Contact Jeva Narian, Arrow Altech Distribution, Tel 011 923-9713, jnarian@arrow.altech.co.za
