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Section 5: Uninterruptible Power Supply/Battery Back-up (UPS/BBU) System

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The purpose of this section is to provide guidelines for the installation of uninterruptible power supply/battery backup (UPS/BBU) systems at signalized intersections.

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An uninterruptible power supply/battery backup (UPS/BBU) system provides emergency power to connected equipment by supplying power from a separate source (batteries) when utility power is not available. The system may also function as a power conditioner and/or voltage regulation device. UPS/BBU systems consist of an enclosure or cabinet, the batteries, the power inverter/conditioner, a battery charger (usually integral to the inverter), and automatic and manual bypass switches.

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The Texas Department of Transportation (TxDOT) started installing LED signal lamps at signalized intersections around the mid 1990’s. Initially, TxDOT decided to utilize LED signal lamps because they were expected to last longer than incandescent lamps and would not require re-lamping on a yearly basis. Additionally, the LED lamps save in power consumption when compared to incandescent lamps. The installation of LED signal lamps, which use approximately 20 watts or less per indication as compared to incandescent light bulbs at 150 watts, made the installation of UPS/BBU systems more practicable.

Most signalized intersections using LED lamps operate at 400-600 watts, and in a power outage situation, can be powered by four 12-volt batteries for up to four to six hours.

TxDOT has installed UPS/BBU systems at signalized intersections on a limited basis since about 2002. The installation of UPS/BBU systems has become more common and can be found at locations on the state system that are maintained by local governments.

Initially, most of the intersections chosen were at locations that experienced reoccurring power issues. Power issues can damage signal equipment, impair video detection equipment performance, or cause traffic signals to go dark. Some examples of power issues include failures (blackouts), voltage sags and surges, brown outs (under voltages), and over voltages. Power failures require TxDOT maintenance and signal technicians to respond to emergency calls, place the traveling public in less orderly traffic control situations, and result in increased delays, auto emissions, driver frustration, and possibly crashes.

Installing UPS/BBU systems at locations where there have been power issues helps reduce down time and electrical damage to equipment. A signalized intersection that is equipped with UPS/BBU can continue to operate through short-term power losses. Maintaining the green, yellow, and red signal operation through power outages is very beneficial in reducing problems with congestion, substantial delay, and safety. UPS/BBU systems at intersections with preemption for emergency vehicles or railroad crossings are also extremely beneficial.

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Recommended Practice

UPS/BBU systems are not currently required at signalized intersections; however, the 2011 Texas Manual on Uniform Traffic Control Devices (TMUTCD), provides guidance that “Except for traffic control signals interconnected with light rail transit systems, traffic control signals with railroad preemption or coordinated with flashing-light signal systems should be provided with a back-up power supply.” Even though back-up power is not currently required, there are many locations that can benefit from the installation of battery backup systems.

The decision to install UPS/BBU systems is at the discretion of each district, but should be based on a study of conditions at the proposed intersection. In deciding whether to install a UPS/BBU system at an intersection, there should be an evaluation of the conditions at the intersection. Installation of UPS/BBU systems should be based on a priority established by the characteristics or conditions at the location. Locations where UPS/BBU systems are most beneficial include:

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  • intersections with unique geometry such as wide medians, conflicting left turns that require lead-lag operation, protected-only left-turn operation, or split phasing where right-of-way assignment is difficult for a four-way stop operation
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  • intersections over capacity with heavy directional traffic flow
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  • intersections with a history of signal malfunction due to power quality or reliability issues
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  • intersections on high volume roads (total volume of all approaches in excess of 20,000 ADT)
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  • intersections with rail preemption
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  • intersections with emergency preemption, or intersections near fire stations (within 1200 ft.)
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  • intersections with signal repair response time in excess of 30 minutes
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  • intersections with 1320 ft. proximity to another intersection with UPS/BBU
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  • intersections that are part of a coordinated system, or are part of a corridor that functions as a major arterial in an urbanized area
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  • intersections with high speed approaches.

Intersections that consistently experience trouble with utility line power, have railroad interconnect for preemption, or have a unique geometry, are high in priority. Intersections that rarely have utility power issues and/or have simple geometry would be considered low priority and would not be good candidates for a UPS/BBU system. A UPS Guidelines Worksheet is available from the Traffic Safety Division that can be used to develop intersection priority for installation of UPS/BBU systems. Not all intersections will have all the conditions listed, and in some cases, the engineer may consider the relative importance of the conditions at the intersection a higher priority than listed on the spreadsheet.

Documentation detailing the criteria for the installation that was considered should be maintained by district traffic operations for each location where a UPS/BBU system is installed.

Installation of UPS/BBU for new traffic signal locations should be identified during the initial design development process.

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Design Considerations

To be eligible for a UPS/BBU system, the intersection must be equipped with LED traffic signal indications.

Installation of UPS/BBU requires determining where to install the unit in the intersection. The UPS/BBU system will normally be provided with its own cabinet that will house the inverter, batteries, and auxiliary equipment. Cabinets can be provided for installation on the side of the signal cabinet or a separate base mount. In some cases, the inverter/charger and auxiliary equipment can be installed in the signal cabinet with the batteries located in a separate cabinet or ground box. However, this is not recommended, as signal cabinets do not have room for all the equipment and it can require wiring modification to the signal cabinet.

In determining whether to install the cabinet to the side of the signal cabinet or base mounted, the traffic signal cabinet base should be evaluated to ensure it is structurally adequate and there is adequate area for access by technicians. Additionally, there should be no obstruction to sidewalks and pedestrian walkways. Base mounted cabinets located in low areas that may be subject to storm water may need a cabinet extension to elevate the cabinet and prevent possible exposure to high water.

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Maintenance Considerations

UPS/BBU systems require maintenance. In order to ensure that UPS/BBU systems operate properly when needed, units should be checked periodically. Batteries should be inspected and replaced when needed. Additionally, all ancillary equipment in the cabinet (i.e. fans, lights, circuit breakers, charging circuits, temperature sensors) should also be checked periodically for proper operation.

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Removal of UPS/BBU

The decision to remove UPS/BBU at a specific location should be made by the district’s engineer responsible for traffic signals and district traffic operations.

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