7.3 Uninterruptible Power Supply and Battery Backup Systems
Key Takeaways
- Caltrans TEES Chapter 4 requires a battery backup system to provide a minimum of two hours of full run-time operation for an LED-only intersection.
- A qualifying system delivers at least 1000 watts continuous output at 25 degrees Celsius with at least 80 percent inverter efficiency, producing 120 VAC pure sine wave output at 60 Hz.
- Battery backup DC system voltage is either 24 or 48 volts, assembled from individually replaceable 12-volt deep-cycle sealed AGM or VRLA batteries.
- A manual bypass switch is required so the inverter can be removed and replaced without taking power away from the intersection.
- The uninterruptible power supply back-feeds the cabinet after the utility service is opened, so it must be isolated as a separate lockout step before any energized work.
7.3 Uninterruptible Power Supply and Battery Backup Systems
When utility power fails at a signalized intersection, the default outcome is a dark signal. Most state laws treat a dark signal as an all-way stop, but drivers do not reliably behave that way, and the crash risk at a high-volume intersection during an outage is substantial. A battery backup system (BBS), also called an uninterruptible power supply (UPS) in this application, keeps the intersection in normal color operation on stored energy.
The practice question bank routed to this exam carries a dedicated UPS category, and the technology arrived in the field after many working technicians were trained — which is precisely why it is examined.
System Architecture
| Component | Function |
|---|---|
| Inverter / charger | Charges the battery string from utility power and inverts DC to 120 VAC when utility power fails |
| Power transfer relay | Switches the cabinet load between utility and inverter |
| Battery string | Stored energy, assembled from individually replaceable 12 V batteries |
| Manual bypass switch | Non-electronic switch that connects the cabinet directly to utility power, allowing the inverter to be removed without dropping the intersection |
| Battery cable harness | Interconnects the batteries and connects the string to the inverter |
| Contact closures | Dry contacts reporting "on battery," "low battery," and "timer expired" to the controller or central system |
| Battery cabinet or shelf | Houses the batteries, often in a separate enclosure beside or above the controller cabinet |
Performance Requirements Agencies Specify
The Caltrans Transportation Electrical Equipment Specifications (TEES) battery backup chapter is the most widely referenced standard, and other agencies write to the same shape.
| Parameter | Typical Requirement |
|---|---|
| Full run-time | Minimum 2 hours of full operation for an LED-only intersection |
| Continuous output capacity | Minimum 1000 W at +25 °C |
| Inverter efficiency | 80 percent minimum in backup mode |
| Output voltage | 120 VAC ± 5 VAC, pure sine wave, 3 percent THD or less |
| Output frequency | 60 Hz ± 0.05 Hz |
| DC system voltage | 24 VDC or 48 VDC |
| Transfer time | Agency specifications commonly require transfer within roughly 40 to 150 milliseconds, fast enough not to disturb the controller or the monitor |
| Surge protection | Compliant with IEEE/ANSI C62.41 |
| Instrumentation | Front-panel display, event counter, and hour meter recording total time on battery |
The phrase "LED-only intersection" is doing real work in that first requirement. The two-hour run time assumes LED modules drawing tens of watts per indication. An intersection that still carries incandescent lamps presents several times the load and will exhaust the same battery string in a fraction of the time. Converting an intersection to LED is a prerequisite for meaningful battery backup, not an unrelated project.
Extended Flash Operation
Many specifications add a second tier: after the full-color run time is exhausted, the system drops the intersection to flashing operation at a much lower load — often specified as an additional period at roughly 300 watts. This stretches useful protection well past the full-color window while signalling to drivers that the intersection is operating abnormally.
Batteries
Chemistry and Construction
Specifications call for deep-cycle, sealed, valve-regulated lead-acid (VRLA) batteries using absorbed glass mat (AGM) construction, in 12 V units that are commercially available off the shelf and individually replaceable. AGM is chosen because it is sealed and requires no watering, tolerates the temperature range inside a roadside enclosure, and can be mounted in a cabinet without free electrolyte.
A 24 VDC system uses two 12 V batteries in series; a 48 VDC system uses four. Larger installations add parallel strings for capacity.
Service Life and Temperature
Battery service life in a roadside enclosure is dominated by temperature. Lead-acid chemistry degrades faster the hotter it runs, and a battery cabinet on a sunny corner in August is a hostile environment. Agencies typically plan on a replacement cycle measured in a small number of years and treat battery replacement as a scheduled preventive maintenance item rather than a failure response — because a battery bank that has quietly lost capacity provides zero protection and gives no warning until the outage arrives.
The practical maintenance items:
- Record the installation date on every battery at the time it is installed.
- Run the system's self-test on the maintenance schedule and record the result, including the run time achieved.
- Check terminal torque and inspect for corrosion, swelling, or case deformation. A swollen case is a battery at end of life and is removed immediately.
- Read the hour meter and event counter. A unit that has logged many hours on battery is telling you the intersection has an unreliable service, which is a separate problem worth reporting.
- Replace batteries as a matched set. Mixing an old battery into a new string drags the whole string down to the weakest unit's capacity.
The Bypass Switch and Technician Safety
Why the Bypass Switch Exists
The manual, non-electronic bypass switch connects the cabinet load directly to utility power, bypassing the inverter entirely. It allows an inverter to be removed, replaced, or serviced without taking the intersection dark. A technician who pulls an inverter without first placing the system in bypass drops the intersection.
The Back-Feed Hazard
This is the single most important safety fact about battery backup, and it belongs in every lockout procedure:
Opening the utility service disconnect does not de-energize a cabinet that has a UPS. The UPS does exactly what it was installed to do and re-energizes the cabinet from the battery string within milliseconds.
Isolating the UPS is a separate lockout step: open the UPS output breaker or select maintenance bypass with the utility side isolated, then verify the signal bus is dead with a proved meter. Technicians have been injured at intersections they were certain they had killed.
The Battery String Itself
A 48 V battery string is not a shock hazard in the usual sense, but it can deliver hundreds of amperes into a short circuit. A dropped wrench across two terminals vaporizes metal and can rupture a case. Standard practice: remove rings and watches, use insulated tools, disconnect the string at the main battery disconnect before working on individual batteries, and never lay a tool on top of a battery.
Field Diagnosis
| Symptom | Likely Cause |
|---|---|
| Intersection goes dark during a brief outage despite a UPS being present | Batteries at end of life; bypass switch left in bypass; inverter failed |
| UPS runs far less than its rated time | Degraded battery capacity, or the intersection is not LED-only and the load exceeds the design |
| Controller resets at the moment of transfer | Transfer time out of specification, or a failing transfer relay |
| Frequent "on battery" events in the counter | Unstable utility service — report it to the utility rather than replacing the UPS |
| Battery cabinet warm and batteries swollen | Overcharging or thermal runaway; remove from service immediately |
A widely referenced battery backup specification requires a minimum of two hours of full run-time operation. What qualifier is attached to that requirement?
What is the purpose of the manual bypass switch in a traffic signal battery backup system?
A 48-volt battery backup system uses individually replaceable 12-volt batteries. How many batteries are in a single series string?
A technician reviews the battery backup event counter and finds a large number of logged transfers to battery over the past year, with the intersection otherwise operating normally. What does this indicate?