No matter what application you're dealing with, today's data communications equipment is vulnerable to surges and transients.



What do programmable logic controllers (PLCs), multiplexers (MUXs), highway unit bulbs (HUBs), remote terminal units (RTUs), supervisory control and data acquisition (SCADA), and telemetry equipment have in common? They're all especially vulnerable to electrical surges. Because of their low operation voltages, an electrical surge as low as 20V can severely damage these components.

A nearby lightning strike is the most common source of electrical surges and transients, which can affect nearby data lines through induction. Industrial transients are also significant because they're man-made disturbances, caused by switching and commuting of electrical motors. The operation of such devices can cause abrupt shifts in the ground potential that can generate current flow through nearby data lines to equalize the ground potential.

Electrostatic discharge (ESD) is another form of an electrical surge. ESD occurs when two nonconducting materials rub together, causing electrons to transfer from one material to another. Although often overlooked, ESD can be harmful to electronic equipment.

Safeguarding Your System

Picture every piece of your equipment with an imaginary circle drawn around it. Transients can break through this circle in two ways: through the AC power line or through the communication line.

You can safeguard data communication equipment from surges and transients by using a transient voltage surge suppressor (TVSS). Be sure to ground all surge protectors and UPS equipment to a common earth/ground, as shown in Fig. 1, on page 22. This avoids differences in ground potentials that can generate current through nearby data lines equalizing to the ground potential.

To protect your equipment from transients and surges through the data line, first determine the electrical specifications of the equipment you want to protect.

Generally, you can break down DC communication applications techniques into two categories: twisted-pair and coaxial.

Twisted-pair applications make up the most common form of wiring in data communications. Both wires in the pair have the same impedance to ground, making it a balanced medium. This characteristic helps to lower the cable's susceptibility to noise from neighboring cables or external sources. Common applications for twisted-pair include high-speed data communications in networks (Cat. 5 cable) or plain old telephone service (POTS) lines.

Coaxial applications consist of a solid wire core surrounded by one or more foil or braided wire shields — each separated from the other by a plastic insulator. The inner core carries the signal, and the shield provides the ground conductor.

Twisted-Pair Applications

For a twisted pair, you must answer four questions before selecting the most effective surge protector.

  • What is the nominal voltage of the application?
  • What is the transmission speed of the data passing through the circuit?
  • What is the current rating of the application?
  • How many “twisted pairs” does the application incorporate?


It's important to know the nominal voltage of the twisted-pair application because without it, you can't assign a proper clamping voltage. According to Ohm's Law (V4IR), voltage is proportional to the current, keeping the series resistance constant. Once the voltage level reaches the surge protector's clamp voltage, the excess energy that could potentially damage your DC communication equipment diverts to a common earth/ground point. Typically, the clamping voltage of a surge protector should not be more than 1.4 times the application's nominal voltage (see Table, on page 24).

You must also determine the transmission speed of the data passing through the circuit. This information deals with the capacitance placed on your twisted-pair line by the surge protector. Especially important for high-speed data rate applications (including Cat. 5, 10Base-T, RS485, and T1/E1), capacitance can cause signal loss or be the source of signal reflections if not properly used in a specific data line.

Identifying the current rating of the twisted-pair application is just as significant as the transmission speed and clamping voltage. The current ratings of TVSS devices for data lines, such as RS485, 4mA to 20mA loops, and telemetry equipment, is no greater than 200mA. Applications with higher current ratings (e.g., 500mA, 1A, and 2A) will cause premature failure to a low-voltage surge protector.

While determining the last three parameters, keep a simpler question in mind: How many of these twisted pairs do you need to protect? It's important to recognize that every wire connected to your equipment provides a path for harmful transients to reach your sensitive equipment.

Once you've answered the four questions outlined above, it's time to investigate the TVSS's power handling capability, which is also known as peak pulse current or maximum discharge current. We define these terms as the maximum current that the surge protector can withstand for a given pulse duration.

Pulse duration is the length of time needed for the peak pulse current to reach a maximum value plus the length of time needed for the peak pulse current to reach 50% of its peak value.

Fig. 2 illustrates an 8/20 μs waveform, a good test for the power handling capabilities of most surge protectors. This waveform simulates real-life, lightning-related surges. This shows us how the TVSS must provide low-voltage clamping and divert the lightning surge or industrial transient away from the DC communication equipment without short-circuiting.

Coaxial Applications

Just as with twisted-pair applications, you must answer four key questions to determine your surge protection strategy for coaxial cables.

  • What is the frequency range of the application?
  • What is the power rating?
  • What is the connector type of the application?
  • Is an in-line or bulkhead mounting style preferred?


Coaxial surge protectors are made of either gas discharge tubes or quarter wavelength stubs. The later devices, which have no active components, act as filters. They short circuit any frequency that is not within the desired frequency of the application. In either case, you must know the frequency at which your coaxial equipment operates.

It's important to know the power rating so you can assign a proper clamping voltage. Standard gas discharge tube protectors are available to protect power ratings up to 50W, 400W, and 1000W. Just like the twisted-pair protector, once the voltage level reaches the surge protector's clamp voltage, the excess energy that may damage your DC communication equipment diverts to a common earth/ground point.

To connect the surge protector directly to your coaxial apparatus, choose a compatible connector type. Common connector types include N-type, BNC, TNC, SMA, and 7/16 DIN. (see Photo, on page 20).

Finally, you must consider the type of installation. Typical mounting styles are available in in-line and bulkhead types. The in-line protectors mount directly in series with the coaxial cable and an external ground screw (attached to the body of the surge protector) grounds your equipment. The advantage of using in-line protectors is they're easy to install and ideal for retrofit applications.

Bulkhead coaxial protectors differ from in-line types in the way they ground equipment. In this case, the device grounds through the chassis of the protector, and the excess energy discharges through the panel that it's mounted on. In general, bulkheads provide better electrical contacts for discharging excess energy from an electrical surge.

No matter what application you're dealing with, today's data communications equipment is vulnerable to surges and transients. If you don't protect your data circuits, you leave the door open for disaster. Although it's only a small component in the overall picture of a complex electrical system, a solid surge protection plan can save you time, money, and downtime in the long run.