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EETE NOV 2015

LED Lighting Fig. 1: Elements of an LED street light protection scheme won’t provide the same protection as new ones. 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. 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 as shown in 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. 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 requires replacement. Luminaire specifiers can choose from two main types of SPD module configurations based on their maintenance strategies: parallel- or series-connected surge protection subassemblies. Parallel Connection – The SPD module is connected in parallel with the load as shown in figure 3a. An SPD module that has reached end-of-life is disconnected from the power source while leaving the AC/DC power supply unit energized. The lighting remains operational, but the protection against the next surge to which the power supply unit and LED module are exposed is lost. In a parallel-connected SPD module, replacement indication can be added using a small LED that indicates the SPD module status to the maintenance technician. Options for a green LED indicating an online SPD module or a red LED indicating an offline SPD module are available. Or, rather than an LED indication at each lighting fixture, the need for SPD module replacement could be indicated remotely to a light management center with SPD module end-of-life indication wires connected to a networked smart lighting system. Series Connection – The SPD module is connected in series with the load as shown in figure 3b, where the end-of-life SPD module is disconnected from the power source, which turns the light off. The loss of power to the luminaire indicates the need for maintenance call and isolates the AC/DC power supply unit from future surge strikes. General preference for this configuration is growing rapidly because the luminaire investment remains protected while the SPD module is awaiting replacement. It’s far less expensive to replace a series-connected SPD module than the whole luminaire as in the case of a parallelconnected SPD module. Fig. 2: A thermal disconnect can open a circuit and prevent a degraded MOV from failing catastrophically. Fig. 3. An SPD module that’s parallel-connected to a luminaire (a) and an SPD module that’s seriesconnected to a luminaire (b). 38 Electronic Engineering Times Europe November 2015 www.electronics-eetimes.com


EETE NOV 2015
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