By Tony Valles
Florida is famous for many things, including intense electrical storms that roll in during the summer months. Two years ago Bob Henderson, Chief Electrician of Tallahassee's dual waste water treatment plants, decided it was time to tame Mother Nature.
"In mid 1998 we did a switch-over to a new Rockwell plant monitoring and control system. We thought it would be a good time to install a reliable surge and lightning protection system as well," Henderson said.
The city of Tallahassee, Fla., operates the Thomas P. Smith and Lake Bradford treatment plants. These plants serve a population of approximately 200,000 and process a combined average of 20 million gallons of wastewater a day.
"When you run an operation of this size, you want a centralized point to monitor and control operations. You also want as much automation as possible," Henderson said.
The control center is connected to the field by over 300 lines, both fiber and copper. These cables interface the network processors with the server. They carry data about the network from the system's pressure, temperature, water level and flow meters, as well as other plant systems.
To protect the equipment against lightning-induced voltage surges, Tallahassee installed more than 50 multi-line surge suppressors supplied by Circuit Components, Inc. (CCI), of Tempe, Ariz. The suppressors are connected to an Allen Bradley/Rockwell RSView control system. This system includes AB SLC 5/04 and 5/05 processors with analog and digital I/O modules.
Henderson selected CCI's 20-line SPU-20LM devices which can handle surges up to 20,000 volts. All 20 lines per device are used and many of the lines are protected on both their transmitter and receiver ends. Power conditioning and power supply protection is provided by Sola devices, Henderson noted.
SPU Series surge suppressors can be used with either Allen Bradley I/O modules for control instrumentation or with other manufacturers' SCADA systems, instrumentation or PLC I/O modules. CCI's products are available for either 1-20 point or 21-40 point applications. They are supplied with terminal blocks on either side to accept individual wires for universal applications and are designed for easy installation on any standard DIN rail configuration.
Electrical Failures
In theory, solid-state semiconductors don't wear out, so the life of electronic equipment should be indefinite, said Sam Johnson, CCI Business and Technical Consultant.
The reality is something else again. Repeated electrical overstress, caused by lightning strikes or system switching transients, is the leading cause of semiconductor-based equipment failure, Johnson said.
"In fact, studies show that 75 percent to 90 percent of all reported failures can be attributed to electrical overstress."
Sources of this overstress (called transients) range from lightning activity to the normal operation of "noisy" equipment. For example, the everyday action of solenoids, relays, motors, generators, and inductive loads switching on and off, all generate transient voltages. This stress wears out electronic equipment.
Lightning poses the most immediate threat to solid state equipment. Lightning produces electromagnetic fields that induce harmful high voltage transients. These transients travel on the conductors of low voltage AC circuits, data communications lines, phone lines, and transmission cables. Equipment doesn't have to be in physical contact with a lightning strike. It is susceptible to voltage transients even when lightning discharges between clouds, never hitting the ground.
Surge suppressors
Surge suppressors provide protection by absorbing excess energy or diverting it to a ground. Sub-nanosecond response time (less than 1 billionth of a second) stops equipment failure. Once the surge is dissipated, the suppressor resets to ready itself for the next surge, spike or strike.
Low-end surge suppressors may help equipment that is exposed to minor surge voltages, but eventually the harmful effects of "let through" overvoltage caused by occasional surges of larger amplitude will shorten equipment life and lead to failure. The "let through" overvoltage is caused by the inability of a low-end suppressor to absorb or divert all of the surge energy present on the line, Johnson said.
To meet suppression requirements, protectors need to turn on rapidly and capture all of the energy present in a surge, Johnson said. Manufacturers of heavy industrial-grade suppressors build their devices to the specifications of organizations such as such as the IEC (International Electrotechnical Commission), the ANSI/IEEE (American National Standards Institute/the Institute of Electrical and Electronic Engineers), Underwriters' Laboratories, and Factory Mutual.
Tallahassee's experience
Despite intense lightning storms during the summer of 1999, (which caused occasional power outages in the City of Tallahassee), Henderson said there were no equipment failures or power outages at either treatment plant.
"I no longer have downtime associated with trouble shooting and replacing equipment due to lightning damage. It's really nice when your equipment works correctly," he said.
About the Author
Antonio Valles is Director of Marketing at Circuit Components Inc., Tempe AZ. He earned A B.S.E.E. from Instituto Tecnologico de Monterrey, Mexico, and an MBA from the Keller Graduate School of Management in Phoenix, AZ.
