This piece was originally published in the January 2018 issue of electroindustry.
Ronald Glaser, Senior Power Engineer, Citel Inc.
Traffic signs have been used as long as man has constructed roads. The Romans popularized mile markers as they interconnected a matrix of roadways across Europe. As traffic increased, so did the amount of information people needed.
The use of variable message signs (VMS) has significantly increased. Electronically controlled light-emitting diode (LED) or plasma-based VMS are highly visible regardless of weather conditions and provide not only directions and speed constraints but also the capability for moment-by-moment road condition updates.
Strategically placed VMS warn travelers about changing road speeds, traffic congestion, road conditions, accidents, and construction. Because these signs are digital and dynamic, state departments of transportation and local law enforcement can remotely change hundreds of signs in seconds.
No matter how helpful they are, the costs of the technology and maintenance are critical. Most states require that VMS have low maintenance and long life (50k+ hours or seven years). LEDs operate by means of microprocessors and digital circuits that create words, graphics, and moving images. The displays require electronic drivers that are controlled by computers that receive data from radio, cell, telephone, and Wi-Fi. The whole system operates on a power supply that converts all incoming ac voltage to dc voltage.
As the diagram shows, each VMS has at least three critical risk points related to outside induced voltage transient surges and incoming poor power quality. Without proper surge protection, a single cloud-to-cloud lightning strike can couple enough energy to knock out sensitive electronics. There is even more risk to the electronic equipment because it is surrounded by metal that can couple high energy directly into the system. Metal is found in environmentally protective enclosures, mounting equipment, conduit, and cabling that is either copper or aluminum.
Only by designing and installing surge protectors at high-risk areas in the VMS can sensitive electronic equipment survive to predicted life models and create safer highways.