Frequently asked questions
What are the various sources normally used in public lighting?
What are the principles of vision in Public Lighting?
What are the various factors to be considered before installing the Street Lights?
Are there any innovative options for this issue?
How are lighting installations classified?
How is the cabling and power supply determined for the cases classified above?
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What are the various sources normally used in public lighting?
The choice of source for public lighting is guided by the following principles:
• Luminous Flux
• Economy (determined by lumens, wattage watt and life)
• Dimension of the light sources
• Color characteristics
The sources normally used are:
• Incandescent lamps
• Mixed Incandescent & high-pressure mercury vapour lamps
• High-pressure mercury vapour lamps with clear or fluorescent bulbs
• Tubular fluorescent bulbs
• Sodium vapour lamps
• Mercury-halide lamps
• High-pressure sodium vapour lamps
What are the principles of vision in Public Lighting?
As per the document laid down by the Superintending Engineer Bangalore circle (North), the principles of vision in Public Lighting are:
• Requirements of drivers
• Visual field of the drivers
• Visibility
• Glare and visual comfort
Although the intention is to service both the drivers as well as the pedestrians, in practice the requirements are more stringent for the needs of the drivers.
What are the various factors to be considered before installing the Street Lights?
The design, spacing and column heights are governed by the road width and the classification of roads. Normally the thumb rule in setting up streetlights is to provide for one every thirty meters. Thus, per kilometer there would be 33 street lights
The methodologies of setting up streetlights at different places are as follows:
• For a highway the requirement is that the streetlights facilitate smooth movement of all types of vehicles. Therefore the poles are set up in the medians with double faced (twin) fitting, and Sodium Vapour lamps are used (250 watts x 2) that emits an illumination of 30 LUX. The advantages of using a SVL are that it does not heat up as soon as the others do; therefore they do not burnout and hence are long lasting. The consumption of power is around 16 Kilo Watts in a Kilometer of lighting. The transformers for these poles come with electronic switches auto ON / OFF facility, thereby also saving on the power.
• In residential areas the BDA normally has to provide for the electricity connections to all the households, therefore street lighting on its own is not a separate issue. It is clubbed with the entire package. This is subsequently handed over to the corporation. While separate poles are installed for streetlights on a highway, the same is not so for a residential area, since the poles that carry the overhead electric wires are used for supporting street lighting as well. With the pole already existing the only requirement is to provide for a protruding angle to house the tube lights. As per the present practices there is a 100 KVA transformer installed for every 30 sites in a layout. Each transformer comes with an electronic automatic ON / OFF switch and a meter to keep track of the consumption of power.
• In the case of large public areas, ex: Railway stations, Bus stops, Airports, Markets, busy intersec-tions etc, the preferred mode of lighting should be high mast lamps (Flood lights). The pole sizes vary anywhere between 12 to 35 meters, and are provided with different numbers of luminaries depending on the requirement of that area. Each luminary consumes around one KW of power. A high mast lamp can illuminate an area of up to three acres. The drawbacks of using high masts are the costs involved and the difficult maintenance. The various costs are mentioned in the annexure.
Are there any innovative options for this issue?
The most common solution considered when exploring the options available for replacing the traditional method of power supply is the use of solar power. Solar power provides a sustainable solution in the long run, there is much Research & Development still required. Currently its usage poses several drawbacks:
• High numbers of batteries needed for streetlights.
• Maintenance is time consuming.
• Usage of solar powered lighting during monsoons has been observed to hamper proper functioning, especially where there is a high consumption of power. The process of the energy absorbing electrodes in the solar power mechanism is affected even by cloudy weather. Solar powered lighting can instead be installed in places where the power consumption is low. For instance in places of traffic lighting, where every junction can have a unit installed independently. This uses Light Emitting Electrodes (LED), and has low power consumption.
• Fungus growth due to the moisture content in the air results in the cells getting blocked, thereby hampering its capacity of absorbing sunlight.
• Limited function time of 10-12 hours. This can result in a problem if there is more requirement than the usual capacity. This capacity comes down even further with the passage of time.
Alternative options are installing a special line on the grid only for the streetlights or using back up battery power. This would ensure street lighting even where there is load shedding. A cost/benefit analysis is essential since it requires retro-cabling which is laborious. Also, to have a backup requires high maintenance, as many batteries are needed.
How are lighting installations classified?
The classification of lighting installations in public thoroughfares is based on the volume speed and composition of the traffic using them. The system of lighting, from the engineering and economy points of view, should take into account all relevant factors such as the presence of factories, places of public recreation, character of street (whether a shopping area or a ring road in non-built up area), the existence of lumps, bends, long straight stretches and over-hanging trees.
The general conclusion from the above table is:
Note 1
In-group A, the level and uniformity of illumination shall be as high as possible
Note 2
In group B lighting, greater tolerances on uniformity and glare are admitted, which may be justified by the character of the roads and by the presence of facades
Note 3
Mounting heights less than 7.5 meters are undesirable except in special cases, such as lighting of residential roads or roads bordered by trees.
Legend:
1 Lights in residential areas
2 Lights on traffic medians
3 High mast lights in town and city centres
How is the cabling and power supply determined for the cases classified above?
Underground cables are to be laid for power supply to the street lamps. For roads under groups A, B and D separate mains are to be laid for group control of lamps on these roads. The street lighting cables should be terminated in separate junction boxes or street lighting pillars. These pillars are provided with electrically operated contractors of suitable current rating.
The cable circuits for each section of the roads should be so designed as to prevent important section of the roads from being completely dark in case of a fault in the underground cables. For roads under groups A1, A2 and D, two independent circuits are to light each stretch of road. The junction boxes with suitable size contractors with independent fuses are to be provided for each phase.
The supply to the streetlights may either be through overhead wires or underground cables. The power supply to lamps on roads under groups A, B and D should preferably be through underground cables laid specially for street lighting purposes. In case overhead wires are employed these should be allocated specifically for street lighting purpose.