Because of the low-voltage operation of twisted pair and coaxial cables, effective surge protection is critical to avoid problems with electrical communications equipment.

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. Why are these devices so sensitive to surges and transients? When you consider that a single integrated circuit (IC) package can contain more than 100,000 memory bits and 5000 logic gates, it's easier to put the challenges of this type of surge protection into perspective.

What causes electrical surges? The most common source is a nearby lightning strike, which affects 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 a current flow through a nearby data-line to equalize the ground potential. Electrostatic discharge (ESD) is another form of an electrical surge. ESD occurs when two non-conducting materials rub together, causing electrons to transfer from one material to another. Although often overlooked, ESD can potentially be a harmful transient to fragile data equipment.

How long do electrical surges and transients last? Although the life span of such electrical phenomenon is short (a typical transient event can last from a few nanoseconds to several milliseconds), they can carry several thousand volts and at least a few hundred amps of current. This means these events can lockup your electronic equipment, cause loss of memory, and create problems in retrieving data.

Now that you understand the basics, how do you apply surge protection in electrical environments? Here are a few guidelines.

Protecting data communication equipment. Picture every piece of your equipment with an imaginary circle drawn around it. Transients can break through this circle in two ways: through the power line or through the communication line. In this case, voltage surges and electrical transients can travel through the AC power line and the DC communication line, damaging your equipment.

You can safeguard data communication equipment from surges and transients by using external transient voltage surge suppressors (TVSS). Ground all surge protectors and UPSs to a common earth/ground, as shown in. This avoids differences in ground potentials that may generate a current flow through a nearby data line to equalize the ground potential. To protect your equipment from incoming surges through the data-line, determine the electrical specifications of the equipment you want to protect. Generally, you can break down DC communication applications 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. A common example of a twisted-pair application is a telephone line. A single telephone line consists of two copper wires: one for transmitting electrical impulses; another for receiving them. Coaxial applications, on the other hand, 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.

Protecting twisted-pair applications. For a twisted pair, you must answer four essential questions before selecting the most effective surge protection solution.

- 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?

Why is it so important to know the nominal voltage of the twisted pair application? Without it, you cannot 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.

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 power handling capability, 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. illustrates an 8/20 msec 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.

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

- 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.

You must know the power rating to 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.

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 equipment is vulnerable to voltage surges and electrical 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.