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40 Chapter 7 â Electrical System Requirements Current Guide The current AASHTO guide provides some recommendations 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 Solid state devices have very different operating characteristics than older roadway lighting systems. The equipment has different starting characteristics, is more prone to damage from over-voltage surges and spikes and has different operating ranges than 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 over 100 times input current for the luminaire. The duration, however, is very short and is less than 160 microseconds in many products. Although such a short duration inrush current may pass through time delay circuit breakers, instantaneous fusing can sometimes be an issue. Also, depending on circuit lengths and conductor sizes, circuit impedance can help in dealing with the high inrush current. When installing LED luminaires, evaluate the inrush currents and the tripping characteristics of all of the overcurrent devices to determine whether there is a coordination issue with the components and if 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, consider installing a higher rated surge protection device like the ANSI C136.2 20Kv/10kA arrestor for surge protection in the luminaires. It may beneficial to also 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 (TVSS) device in the service panel would further protect the SSL electronics and prolong life. Such a device must be approved to: UL 1449 4th Edition, UL96A, IEEE C62.45-2002 Standards for Surge Protective Devices and have a surge capacity per mode of at least 50kA with a short-circuit current rating of 200kA. The surge protection device must be installed within 150 millimeters (6 inches) of the breakers within the panel and wiring must run in straight paths with minimal bends. Inrush currents are high with solid state devices but usually of very short duration. Instantaneous overcurrent protection devices may create some issues. Consider inrush current impacts during LED lighting systems design. Voltage surges and spikes can affect LED sources more than HID sources. Consider surge suppression devices in design.
41 Susceptibility to Faults LED drivers are generally designed to operate either in the 100V to 277V range or the 347V to 480V 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 480/277V three phase or 480/240V single phase when the luminaires are operated at 277V or 240V. 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 480V applied to the driver, which will result in damage requiring 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. Where using an ungrounded system, consult with the fixture manufacturer. 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 contain 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 circuit. 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 impact breaker sizing. Equipment and local requirements for electrical systems vary. Therefore, evaluation of higher voltage drops would be required during design to determine if there are any operating issues and to consider advantages and disadvantages. Power Factor When connected to an AC 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 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. Rarely is this current is 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 infrastructure to operate at or above capacity unnecessarily. For example, if the utility were to supply a load of 50 watts with a power factor of 0.5 (50%), it must generate and transmit 100VA of apparent power. Conversely, if the utility were to supply a load of 50 watts with a power factor of 1.0 (100%) it must generate and transmit 50W 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 impacted by dimming however as the load (watts) is reduced so are the overall system impacts. Power factor should be reviewed and discussed with the local power utility.
42 Control System Requirements In general, to make a luminaire control capable of 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 a powerline carrier system was being used that would require a controller placed within the luminaire and an isolated electrical distribution system for carrying the imposed signal on the power conductors. To provide the most flexibility from a long-life LED system, LED luminaires should be either controlled or controls ready. Key Issues â¢ Assess inrush starting currents during design considering all expected overcurrent devices. â¢ Consider over-voltages for solid state components. â¢ Assess allowable voltage drop against local requirements and benefit/cost. â¢ Consider an adaptive control system to control and monitor electrical conditions of the lighting system. â¢ Consider adding a surge protection device in the pole handhole. â¢ Consider additional TVSS at the electrical service panels feeding the lighting system.