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37 Electrical System Requirements Current Guide The current AASHTO Roadway Lighting Design Guide (AASHTO 2018) provides some rec- ommendations on the approach to electrical design for lighting systems. It contains general discussions of grounding, voltage drop, control, and overcurrent protection. Additional Considerations for LED Sources The operating characteristics of solid-state devices are very different from those of older road- way lighting systems. The equipment has different starting characteristics, is more prone to damage from overvoltage surges and spikes, and has operating ranges different from those of older technologies. Starting Characteristics and Surge Suppression LED luminaires have a significant inrush current upon starting. Depending on the product used, inrush current can be more than 100 times the input current for the luminaire. The dura- tion, however, is very shortâless than 160 Âµs in many products. Although an inrush current of such short duration may pass through time delay circuit breakers, instantaneous fusing can sometimes be an issue. Depending on circuit lengths and conductor sizes, circuit impedance can help in dealing with the high inrush current. When LED luminaires are being installed, evaluation of the inrush currents and tripping characteristics of all of the overcurrent devices is suggested to determine whether there is a coordination issue with the components and whether modifications are necessary. Solid-state components are also susceptible to failure from short- term surges and spikes in the distribution system. Depending on the expected frequency of surges at the electrical service or within the electrical distribution system, installation of a higher-rated surge pro- tection device, such as the ANSI C136.2 20Kv/10kA arrestor, for surge protection in the luminaires is suggested. It may also be beneficial to install a surge protection device in the pole handhole to reduce the amplitude of the surge that reaches the luminaire. In addition, installation of a transient voltage surge suppression device in the service panel would further protect the SSL electronics and prolong life. Such a device must be approved to UL 96A, âStandard C H A P T E R 7 Inrush currents are high with solid-state devices but usually of very short duration. Instantaneous overcurrent protection devices may create some issues. Consider the impact of inrush currents during LED lighting systems design. Voltage surges and spikes can affect LED sources more than HID sources. Consider surge suppression devices in design.
38 Solid-State Roadway Lighting Design for Installation Requirements for Lightning Protection Systemsâ (see UL 1449, âStandard for Surge Protective Devices,â 4th ed.) or IEEE C62.45-2002, âIEEE Recommended Practice on Surge Testing for Equipment Connected to Low-Voltage (1000 V and less) AC Power Circuitsâ and have a surge capacity per mode of at least 50 kA with a short-circuit current rating of 200 kA. The surge protection device must be installed within 150 mm (6 in.) of the breakers within the panel, and wiring must run in straight paths with minimal bends. Susceptibility to Faults LED drivers are generally designed to operate either in the 100 V to 277 V range or the 347 V to 480 V range. Because they are solid-state devices, they are not tolerant to short-term over- voltages. In early installations, there have been some instances of phase to neutral faults that have damaged the LED drivers. This happens when services are 480/277 V three phase or 480/240 V single phase when the luminaires are operated at 277 V or 240 V, respectively. These luminaire voltages are derived by connecting the fixture to the phase and neutral conductor of these circuits. If, however, a fault occurs between the neutral conductor and another phase conductor, the luminaire will experience 480 V applied to the driver, which will result in damage that requires replacement. Other devices and driver ranges are being developed that may also address these issues. Although protective devices are being considered to resolve this problem, designs and installations should be aware of this risk and try to address it at each phase of installation and operation. Faults can also be an issue for ungrounded systems, and the electronic and internal surge protection in the fixture can be susceptible to ground faults. When an ungrounded system is being used, consultation with the fixture manufacturer is suggested. Voltage Drop Requirements The current AASHTO guide recommends that the maximum voltage drop on a circuit not exceed 5% from the electrical source to the farthest lighting pole. The National Electrical Code also contains an informational note, which is not a mandatory requirement of the code, to limit voltage drop on branch circuits to 3% and no more than 5% on both the feeder and branch circuits. LED drivers, however, will operate on any voltage within the driver range. This means that a circuit could be designed with a much greater voltage drop and the luminaire would still function as designed; however, where the voltage is decreased at the fixture, the current will rise, which may have an impact on breaker sizing. Equipment and local requirements for electrical systems vary. Therefore, evaluation of higher voltage drops would be required during the design process to determine whether there are any operating issues and to consider advantages and disadvantages. Power Factor When connected to an alternating current supply, electrical equipment containing reactive elements (such as capacitors and inductors) will draw current that is not in phase with the applied voltage. A purely resistive load will draw current that is exactly in phase with the line voltage at an ideal power factor of 1.0 (where the power factor is a ratio of power used to power delivered). Where reactive elements are included in a circuit, the load draws the additional reactive current. This current is rarely used by the end user, and it is difficult to measure and therefore difficult to collect revenue from. Essentially, this reactive power will cause the delivered power to be larger than the power required by the end device. This can cause the utilitiesâ infra- structure to operate at or above capacity unnecessarily. For example, if the utility were to supply
Electrical System Requirements 39 a load of 50 W with a power factor of 0.5 (50%), it would have to generate and transmit 100 VA of apparent power. Conversely, if the utility were to supply a load of 50 W with a power factor of 1.0 (100%) it must generate and transmit 50 W of apparent power. To compensate for such increased generation and transmission costs, utilities will often apply surcharges to a customerâs bill until their power factor has been corrected. Typically, utilities will now apply surcharges for a power factor lower than 90%. A power factor as a percentage may be affected by dimming; however, as the load (watts) is reduced, so are the overall system impacts. The power factor should be reviewed and discussed with the local power utility. Control System Requirements In general, to make a luminaire control capable of being control ready, the proper 5- or 7-pin receptacle as well as a dimming driver should be provided in the luminaire. This will allow a wireless network node to be installed on the luminaire to control and monitor the fixture. The one exception to this would be if the powerline carrier system being used required a controller placed within the luminaire and an isolated electrical distribution system for carrying the imposed signal on the power conductors. To obtain the most flexibility from a long-life LED system, the LED luminaires should be either controlled or controls-ready. Key Issues for Electrical System Requirements â¢ Assess inrush starting currents during the design process by considering all expected over- current devices. â¢ Consider overvoltages for solid-state components. â¢ Assess the allowable voltage drop against local requirements and benefitâcost. â¢ Consider an adaptive control system to control and monitor the electrical conditions of the lighting system. â¢ Consider adding a surge protection device in the pole handhole. â¢ Consider additional transient voltage surge suppression at the electrical service panels feeding the lighting system.