Imagine trying to conduct business without telephones or e-mail. For most people, it's a scary thought. Today's successful businesses depend on communication, and that means reliable communication networks. In order for these networks to operate continuously, they must be able to withstand transient overvoltages caused by inductive switching or lightning strikes. Typically, engineers achieve this goal by placing surge suppressors in the power lines at the main entrance and/or point of use. This increases the amount of protection, but it doesn't prevent overvoltages from traveling through I/O lines, network lines, and telephone lines and damaging equipment (see Fig. 1). That's why it's important to install surge suppressors in all paths that carry potentially disruptive transients.

Switching transients and lightning strikes generate most transient overvoltages. Switching transients occur when the flow of current in inductive motors, solenoids, relays, and transformers is interrupted during a power outage or restored after one. This generates large overvoltages that cause the most damage to equipment.

Lightning strikes are more catastrophic, but less frequent. The average strike produces more than 20,000A and can exceed 200,000A. Aside from direct strikes, lightning generates electromagnetic waves that can induce transient voltages in equipment up to 6 miles away from the point of impact. In addition, electrostatic discharge (ESD) can transmit transients of 3000V or more into equipment.

Equipment failure resulting from transient overvoltages can range from shortened operational life to sudden hard failure. While some pieces of equipment may survive the initial blows by exhibiting little or no operational damage, they can fail in six months' time as metallization creepage eventually shorts out the punch-through holes in microcircuit junctions. This is known as latent failure.

Soft failures occur when damaged equipment goes offline; needs resetting; locks up; loses or corrupts files; makes unmistakable output errors; or experiences errors in communication, reading, or measurement.

Hard failures are usually the direct result of lightning strikes or other overwhelming transient occurrences. Unlike latent or soft failures that can be impossible to see, hard failures typically leave equipment in a visibly damaged state — a charred mass of molten electronics, a blown lid cover, a cracked or burned component, or a vaporized circuit-board trace or wire. Sometimes, however, even hard-failure cases exhibit no signs of equipment or component damage. Instead, units suddenly become “out of service.”

Surge Suppression Guidelines

There are a number of devices connected to communication networks that need protection. These networks include DH485, DeviceNet, DH+, FOUNDATION Fieldbus, Remote I/O, and others. The following is a list of guidelines to use when installing surge suppressors for this purpose:

  • Connect the surge suppressor in series and as close as possible to the device it is meant to protect.
  • Make sure the surge suppressor is transparent to the network and does not present any loading or termination problems.
  • For some applications, such as the use of DeviceNet, surge-suppression systems should include suppressors for the control lines as well as the voltage lines.
  • For networks that require line termination resistors for the end devices, such as DH485, the surge suppressor should include line termination resistance. This gives end users the option of terminating the line right at the suppressor input.


Fig. 2 provides a more complete look at the types of surge suppressors needed to protect equipment associated with communication networks.

Protecting Network Lines

If communication network lines contain one or more of the following characteristics, they may exceed the limits of electronic equipment vulnerability and cause considerable damage to the overall system:

  • Overhanging (aerial) lines
  • Equipment fed from different power supplies
  • Equipment signal lines with different zero signal voltages
  • Signal lines with shields connected at both ends
  • Signal lines with high di/dt that pass through the neighborhood of lines


If you adequately protect network lines that exhibit any of these characteristics and follow the surge-suppression guidelines outlined in this article, you'll go a long way toward preventing transient overvoltages from damaging equipment and affecting the bottom line. More important, you'll be helping your company thrive by keeping the lines of communication open.

Tony Valles is director of marketing for Circuit Components Inc., located in Tempe, Ariz. You can reach him at avalles@cci-msc.com.