Traffic light cabling is a unique sector of the industry, and cablers need to take a different approach to the installation and connection of these systems.

We often take operational traffic lights for granted. But when they stop working, all hell can break loose on the roads – particularly in peak hour traffic.

Given the necessity for such a product to work reliably and efficiently, what exactly do cablers need to know when faced with the task of working with traffic light cabling?

After all, there are many ways to detect vehicles. For traffic control, inductive loop technology is often seen to be the most reliable system.

How it works

If you look at any traffic signalling location throughout Australia, you will come across the same type of system. According to the document The Basics of Loop Vehicle Detection from US-based company Marsh Products, an inductive loop vehicle detector system consists of three components: a vehicle detector loop cable, vehicle detector feeder cable and the vehicle detector.

“When installing or repairing an inductive loop system, the smallest detail can mean the difference between reliable detection and an intermittent detection of vehicles. Therefore, attention to detail when installing or troubleshooting an inductive loop vehicle detection system is absolutely critical,” the document says.

In this case, the vehicle detector loop cable is buried in the traffic lane. The loop is made from a continuous run of sheathed cable that enters and exits from the same point. The two ends of the loop cable are connected to the loop feeder cable, which in turn connects to the vehicle detector. The detector powers the loop causing a magnetic field in the loop area, and the loop resonates at a constant frequency that the detector monitors.

“A base frequency is established when there is no vehicle over the loop. When a large metal object, such as a vehicle, moves over the loop, the resonate frequency increases. This increase in frequency is sensed and, depending on the design of the detector, forces a normally open relay to close.

“The relay will remain closed until the vehicle leaves the loop and the frequency returns to the base level. The relay can trigger any number of devices such as an audio intercom system, a gate, a traffic light, etc.”

When it comes to troubleshooting, most detectors provide LEDs that will indicate a problem with the loop, such as a short or an open.

“It is possible for a problem to occur that will cause the error indicating LED to stay on and yet the installation is ok, but simply needs a reset. Lighting can cause such a problem. Electrical storms can cause havoc with equipment, especially vehicle detectors because the loop is outside.

“If problems persist, check the connections to the extension cable and to the loop lead-in wires. Bad connections are a very common problem with inductive loops.”

There are also three types of cable used in all traffic signal installations: multicore power cables, available in various core numbers; vehicle detector feeder cable; and vehicle detector loop cable. These cables are used to detect vehicles and activate the signalling system. This cable is connected to the feeder cable using jointing methods that will keep the connection protected from environmental elements, as well as wear and tear.

Choosing the right cable

Increased pressures on suppliers and installation companies to lower their costs can have a direct impact on the quality of the job. In order to ensure that traffic signalling system installations are completed to a certain standard, State Road Departments certify a number of electrical contractors for traffic signalling system installs.

Madison Technologies is a supplier of high quality traffic-specific cables and associated networking technologies that assist in the creation of advanced and intelligent traffic systems. According to the company’s Victorian State Manager Brad Liddle, “each State Road Authority have their own particular standards and approved cables that they specify for approved electrical contractors to install.”

“Garland cables, for example, have recently been certified for traffic signalling installations by VIC Roads. This means that VIC Roads certified electrical contractors are approved to use Garland multicore signalling power cables, Garland feeder cables, and Garland loop detector cables in any traffic signalling installation throughout Victoria,” he says.

“In maintaining a certain standard of both contractor and cable, VIC Roads ensure that the each install is performed consistently and effectively.”

Brad also explains that “due to the critical nature of traffic light networks and systems and the often harsh environments they are installed into, the reliability and quality of the cable is critical.”

“Traffic cables approved for installation need to be hardwearing to cope with specific environmental factors. Both the cables and networking technology used need to be rugged and reliable to withstand the environmental conditions and extreme temperatures seen in underground and outdoor installations.”

On the surface, a traffic signalling system can appear relatively simple; however, there is no question that under the bitumen, there is a complex signalling system and associated cabling working hard to keep these critical systems running.