Surges and spikes on data lines can fry your communications boards and garble data. This article describes the operation, installation and selection of what is probably the most common method of data line protection. Surge suppressors divert excess energy away from the port being protected into a ground connection. The operation of these devices relies on a high quality ground connection in order to safely shunt away unwanted energy.
Surge Suppressor Operation
Shunting harmful currents to ground before they reach the data port is the job of components such as Transient Voltage Suppressors (TVS, often referred to by the trade name Tranzorbs),
Metal Oxide Varistors (MOV) or gas discharge tubes. These devices all work by turning on at a set voltage. Once the clamp voltage has been exceeded, the devices provide a low impedance connection between terminals. These shunting devices are most often installed from each data line to the local earth ground, and should be selected to begin conducting current at a voltage as close as possible above the system's normal communications level. For RS-422 and RS-485 systems, the voltage rating selected is typically 5 – 7 volts, in RS-232 systems 12 - 15 volt devices are appropriate. These devices typically add some capacitive load to the data lines, similar to adding additional cable to the system. This should be considered when designing a system by reducing the total allowable line length. Several hundred feet is usually an adequate figure in RS-422 and RS-485 systems.
What does a surge look like?
While transients may not always conform to industry specifications, both the Institute of Electrical and Electronics Engineers (IEEE) and the International Electrotechnical Commission (IEC) have developed transient models for use in evaluating electrical and electronic equipment for immunity to surges. These models can offer some insight into the types of energy that must be controlled to prevent system damage.
Both IEC 1000-4-5: 1995 “Surge Immunity Test” and IEEE C62.41-1991 “IEEE Recommended Practice on Surge Voltages in Low-Voltage AC Power Circuits” define a “1.2/50µs - 8/20µs combination wave” surge which has a 1.2 µs voltage rise time with a 50 µs decay across an open circuit. The specified current waveform has an 8 µs rise time with 20 µs decay into a short circuit. Open circuit voltage levels from 1 to 6 kV are commonly used in both the positive and negative polarities, although under some circumstances voltages as high as 20 kV may be applied. Figures 1 and 2 illustrate the combination wave characteristics. In addition, IEEE C62.41 also specifies a 100 kHz “ring wave” test. The ring wave has a 0.5 µs rise time and a decaying oscillation at 100 kHz with source impedance of 12W as shown in Figure 3. Typical amplitudes for the 100 kHz ring wave also range from 1 – 6 kV.
Surge suppressors must be installed as close to the port to be protected as possible, and must have an extremely low impedance connection to the local earth ground of the unit being protected.