Voice and data transmissions may not be the only things traveling on your communications infrastructure.
Electrical disturbances that enter a building through AC lines pose a threat to valuable electronic equipment. Although most electricians know how to protect those lines from high-voltage transients, some still fail to consider the underground and aerial data communications lines that can also transport high-magnitude transients into a facility through complex coupling mechanisms. The disruptive and destructive effects of power surges, spikes, and transient voltages on your telecommunications and computer networks include system lock-up, malfunctions, and erroneous output. In many cases, those effects can also include physical damage, such as burned PC boards or melted electronic components.
The logical remedy involves the implementation of surge protective devices (SPDs) for both your power and data lines. But before you set up SPDs to protect data communications lines, you need to understand the causes and effects of transients.
An SPD attenuates the magnitude of surges, but it doesn't necessarily reduce the surge to zero amplitude. In computer systems, it's only necessary to reduce transients to levels of about 150% to 300% of line voltage (400V to 500V peak) to prevent equipment damage. Thus, SPDs are adequate for attenuating the surge to a safe level. High-quality SPDs also have the ability to eliminate high-frequency noise.
Consider transient sources.
The first step you should take in formulating a viable SPD defense is to take a close look at coupling mechanisms. Transient voltages couple into data circuits by two mechanisms, one of which is inductive coupling. In this scenario, the data line forms part of an inductive loop. Surge currents that flow through metallic conductors produce a changing magnetic field, which induces a voltage in the loop formed by the data line. The greater the rate of change of surge current, the higher the voltage that current will induce in a data line loop. To lessen the effects of inductive coupling on data lines, you need to design the data line layout to comply with the rules of NEC Art. 300 and 800. For example:
Don't drape data lines over wireways. The NEC prohibits the use of conduits or other wireways to support external wiring of any kind.
Don't assume metal wireways will shield wiring from magnetic coupling.
Keep lines away from ballast transformers, lightning down-conductor areas, and building steel.
The second, and more destructive, of the two coupling mechanisms is ground potential difference. This occurs when a data line connecting two items of computer equipment is subjected to any difference in voltage that may exist between different, or unbonded, equipment grounds.
To avoid ground potential problems, use NEC Art. 250 as your minimum design reference. This will allow you to eliminate unbonded electrodes and design a system of bonding jumpers that tie into the main bonding jumper, which ties into the main grounding electrode conductor. This arrangement precludes the inadvertent development of multiple ground points, one effect of which is the creation of differences in line-to-ground voltage at various points in the distribution system. This results in the flow of undesired currents on low-voltage data lines.
Protective shield concept.
The protective shield approach to transient voltage surge suppression establishes a series of defined transient environments or zones, and requires you to adhere to the following two principles:
Keep the highest surge currents out of inner-building wiring. Just as locks prevent thieves from getting inside doors, the first protective shield must divert high-magnitude surge currents at the point where cables and communications lines enter a building. On AC systems, it's common to install a first line of defense at the service entrance panel. On incoming telephone lines, the telephone company may provide a basic level of protection in the form of a UL 497 primary protector at the main demarcation point.
Reduce transients to a level below equipment susceptibility and vulnerability at specific points by setting up additional protective zones within a facility for local control of transients. In some instances, control is mandatory for specific equipment or for a computer room.
It's common to drive separate ground rods for the data and power SPDs and not bond them together. However, this isn't a recommended practice because it sets up a potential difference between the two systems. Bond the AC and communications line surge suppressors to the same electrical ground.
Sometimes it's not practical or economical to set up a systematic approach in some sites. In these cases you can set up a local protective shield for individual equipment. By using a combined AC and data-line SPD, you can establish a local ground reference, or ground reference equalizer. This arrangement will precisely control the maximum transient voltages present between any line and local ground within the protective shield, eliminating the problem of potential differences.
Typical telecom SPD application.
Setting up a systematic defense against transients requires a network of strategically placed SPDs. The UPS usually has an integral power line conditioner that filters high-frequency noise with normal and common mode surge suppression. Low-voltage and data line surge protectors connect to modems and communications devices, forming a protective zone for CPUs and terminals. The SPDs are configurable to LANs, WANs, data lines, and CATV and CCTV cable networks. You can apply individual devices as needed, but they must share a common ground point. Products chosen for the network should meet the requirements of NEC Art. 800 and UL 497A — UL 497B for data — compliance standards for telecommunications protectors. When you're selecting data line protectors, make sure they're compatible with the frequency of the LAN — otherwise, your SPD will reduce or interfere with data transmission rates.
Troubleshooting an SPD installation.
If surge protectors are failing to do their jobs, it could be attributable to one of the following conditions:
Poor transient control level. When the selected SPD clamps the voltage at too high of a level, select a surge rating that matches the equipment needs. Your goal is to protect the equipment, not block surges.
Wiring errors. Problems with the wiring can lead to problems like voltage across the pigtail, which adds to the overall clamp level. To prevent this, avoid long leads and pigtails. Practice wiring methods per NEC Art. 300.
Poor bonding. Differences in potential can render protection from outside surges moot. Eliminate these differences by bonding per NEC Art. 250.
Unprotected lines. These can serve as a back door by which undesired currents can enter. Make sure you account for all the lines in your facility.
Protector circuitry exhibits high capacitance, resulting in signal interference. To prevent this, ensure your SPD data sheet agrees with your bandwidth requirements.
Taking a systematic approach to SPD installation can mitigate the effects of transients in data lines. By setting up a strong defense now, you can reduce downtime and decrease the need to replace equipment. An investment in a well thought out SPD strategy can save you money in the long run.
Wakeham is marketing director for Leviton's power quality division, Little Neck, N.Y.