01 Introduction
Urban road lighting not only provides functional illumination for traffic flow throughout a city but is also closely linked to ensuring traffic safety and improving traffic efficiency. Therefore, urban road lighting design must consider not only functional evaluation indicators such as average illumination, uniformity, and glare control, but also energy conservation and appropriate control methods.
"City Road Lighting" refers to a municipal road in a city in Sichuan Province, a local hub. The following briefly analyzes some key understandings of urban road lighting design, drawing on this project.
02 Key Points to Consider in Urban Road Lighting Design
Articles 3.1.1 and 3.1.2 of the CJJ 45-2006 "Urban Road Lighting Design Standard" clearly define the categories of road lighting: motor vehicle road lighting and pedestrian road lighting. Motor vehicle road lighting is categorized into three levels: expressways, main roads, secondary roads, and branch roads. Evaluation indicators for road lighting are divided based on motor vehicle road lighting and pedestrian road lighting. Evaluation indicators for motor vehicle road lighting include average road surface brightness or average illuminance, total and longitudinal illuminance uniformity, glare control, ambient ratio, and light inducibility. Pedestrian road lighting should be evaluated based on average road surface illuminance, minimum road surface illuminance, and vertical illuminance.
Road lighting ensures that drivers can see the road clearly at night, avoid excessive fatigue, and ensure driving safety. The road lighting designed for this project adheres to regulations and has an average illuminance of 1.5 to 2 cd/m². The standard average illuminance for trunk roads is 20/30 lx (lower limit 20 lx, upper limit 30 lx), with a design value of 29 lx. Glare control requirements: At the driver's viewing angle, the luminous intensity at 80° and 90° elevation angles must not exceed 30 cd/1000 lm and 10 cd/1000 lm, respectively.
To help drivers quickly identify road directions from a distance, urban road lighting design must ensure guidance. Guidance can be categorized as visual and optical. Visual guidance refers to the use of road guidance aids to help drivers identify their current location and the direction of the road ahead. Guidance aids include the center line, curbs, road markings, and emergency barriers. Optical guidance refers to the use of changes in the arrangement of lamps and poles, the appearance of the lamps, and the color of the light to indicate changes in road direction or approaching specific locations such as intersections. Therefore, the selection and placement of lamps are crucial in road lighting design. Streetlight placement requires consideration of spacing, mounting height, and lamp brightness. A reasonable placement enhances lighting effectiveness. Conventional lighting fixture arrangements include single-sided, staggered, symmetrical, centrally symmetrical, and horizontally suspended. This design utilizes a symmetrical arrangement, as shown in Figure 1.
The height of the lamp poles is determined by the lifting height of the lighting maintenance vehicle; poles between 8 and 12 meters are generally used. Light pole spacing is generally 30m to 40m. The lighting type, layout, installation height, and spacing of the lamps must meet the requirements of Table 5.1.2 of CJJ 45-2006. Therefore, this design uses 400W high-pressure sodium lamps for the main road lighting, installed at a height of 12m, with a spacing of 40m between lamps. Auxiliary sidewalk lighting uses 150W high-pressure sodium lamps, installed at a height of 8m. The lamps adopt a semi-cutoff pattern. The lamp efficiency must not be less than 70%. The lamp cantilever length must not exceed 1/4 of the installation height, and the lamp elevation angle must not exceed 15°.
Urban road lighting design should comprehensively consider various factors, including light source, lamp selection, lamp layout, and road environment, ensuring flexible design and a distinctive design. Several design considerations are important: First, lamps should be positioned away from bus stops, zebra crossings, and other areas. Second, the design illumination value should be enhanced at intersections. The lighting standards for intersections should be designed according to Table 3.4.1 of CJJ 45-2006. Intersections are prone to traffic accidents. Improving intersection lighting will make it easier for drivers to identify intersection conditions. This design adheres to Table 3.4.1 of CJJ 45-2006, maintaining a minimum illumination of 30 lx and a maximum illumination of 50 lx. Two 15-meter-tall center-pole lights equipped with three 400W high-pressure sodium lamps will be installed at each diagonally opposite corner of the intersection. Furthermore, specific considerations should be taken into account when designing lighting for curved sections of the road: Curved sections with a turning radius of 1000 meters or more can be illuminated as straight sections. For curved sections with a turning radius of less than 1000 meters, luminaires should be arranged along the outside of the curve, and the spacing between luminaires should be reduced, ideally to 50% to 70% of the spacing between luminaires in straight sections. The smaller the radius, the smaller the spacing. Overhangs should also be shortened accordingly. When lighting fixtures on wider roads require dual-sided placement, a symmetrical arrangement is recommended. Lighting fixtures at curves should not be installed on the extension of straight sections. Doing so could mislead drivers into thinking the road extends forward, leading to traffic accidents. Finally, when designing lighting for sloped roads, the symmetrical light distribution plane of the installed lighting fixtures, parallel to the road axis, should be perpendicular to the road surface. On convex vertical curves, the spacing between fixtures should be reduced, and light-cutting fixtures should be used.
03 Urban Road Lighting Power Supply and Distribution Design
Urban road lighting should be powered by dedicated streetlight transformers. High voltage power supply generally uses a 10kV line, while low voltage typically uses 380/220V. Dual power supplies should be used for lighting in urban areas such as transportation hubs, important roads, and crowded squares. Each power supply should be capable of withstanding 100% load. The load factor for road lighting transformers also differs from general project design. Regulations require that the load factor for distribution transformers should not exceed 70%.
Power supply should preferably be provided by buried cable lines. If overhead lines are used, overhead insulated distribution lines are preferred. Manhole covers and handhole covers for road lighting power supply lines, access doors on lighting poles, and outdoor distribution boxes for street lights should all be equipped with locking anti-theft devices that require special tools to open. The voltage deviation of terminal lighting fixtures should be maintained between -10% and +5%. Reactive power compensation should be used for gas discharge lamps, and single-lamp compensation should be used for street lights. The power factor after compensation should be no less than 0.85. The lamp efficiency should not be less than 70%. Gas discharge lamps should be protected by fuses. The fuse configuration should comply with the following regulations: 4A for 150W and below; 6A for 250W; 10A for 400W; and 15A for 1000W.
04 Street Light Control System
Street light control should utilize an intelligent controller and centralized remote control system that combines light control and time control. Light on and off times should be appropriately set based on local geographic location and seasonal variations, and should be adjusted based on ambient brightness. Time-sharing lighting should be implemented based on nighttime pedestrian and vehicle traffic. The natural light illumination level for road lighting should be 15 lx when on, 30 lx for expressways and main roads, and 20 lx for secondary and branch roads. When using a centralized remote control system for road lighting, the telecontrol terminal should have both automatic on/off control and manual control functions in the event of a communication interruption. When designing a streetlight control system, the system should be integrated with the local streetlight management office based on the specific project circumstances. The streetlight lighting system should be integrated into the local streetlight monitoring and management system, and system compatibility and scalability should be considered.
05 Grounding and Safety
The grounding type for the roadlight distribution system should be either a TN-S or TT system. The system grounding resistance should not exceed 4 Ω. Repeated grounding should be established at appropriate locations at the branches, ends, and intermediate points of the distribution line to form a network. The repeated grounding resistance should not exceed 10 Ω. This design project uses the TT system, with a system grounding resistance not exceeding 4 Ω. Starting with the first lamp, conceal a 40×4 hot-dip galvanized flat steel bar along the entire length of the landscape lighting circuit, reliably connecting it to the metal housing and grounding electrode. A pair of grounding electrodes should be installed at each end and in the middle. Each grounding electrode should be a 2.5m long vertical grounding electrode made of 50×50×5 galvanized angle steel. The top of the grounding electrode should be 0.8m above the ground. All conductive parts of electrical equipment and lighting fixtures should be connected to a grounding electrode that is electrically independent of the power system ground. A residual current circuit breaker with a rated operating current of 300mA should be installed on the power supply line to the distribution box to prevent electrical fires.
06 Electrical Energy Saving
In response to the national call for energy conservation and emission reduction, electrical energy conservation measures should be fully considered in road lighting design. Here is a brief introduction to some common design practices. First, the transformer should be located as close as possible to the load center to minimize line length and losses. Energy-saving transformers should be used, and an automatic electrostatic capacitor compensation device should be installed on the low-voltage side for centralized compensation. Secondly, road lighting design should determine the lighting power density (IPD) value according to CJJ 45-2006. High-efficiency lamps should be selected. Conventional lamps should have an efficiency of 70%. Floodlights should have an efficiency of 65%. Since the power factor of gas discharge lamps is generally between 0.4 and 0.6, they should be equipped with high-quality, energy-saving inductive or electronic ballasts that meet national energy efficiency standards. Local capacitor compensation should be used to increase the power factor to above 0.85. Furthermore, energy-saving control technology should be employed. Time-sharing lighting control should be implemented based on vehicle and pedestrian traffic. Lamp maintenance is also a means of energy conservation. Regular maintenance can effectively improve luminous flux utilization, ensure road lighting illumination, and conserve energy.
07 Conclusion
The quality of urban road lighting is crucial to energy efficiency and, more importantly, to human safety. Therefore, when designing urban road lighting, it is important to consider the actual project situation and strive to ensure safety, reliability, aesthetics, energy efficiency, and environmental protection while meeting functional requirements.
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