The need for communication line surge protection: Surge protection has been overlooked for a long time but cannot be ignored anymore. Data and telecom equipment must be protected by a surge suppressor that provides a fast response time and a precise clamp

Data equipment is extremely vulnerable to voltage surges and transients. Remote terminal units (RTUs), system control and data acquisition (SCADA) systems, demand recording meters and other telemetry equipment are particularly sensitive to transients, surges and overvoltages.

A single integrated-circuit (IC) package can contain more than 100,000 memory bits and more than 5,000 logic gates. The high sensitivity of the chips used in these packages makes them prone to fast degradation because of overvoltages. Their small size has considerably lowered their failure threshold. Private branch exchanges (PBXs), modems and telemetry equipment run on sensitive, low-voltage lines. Transistor-transistor logic (TTL) drivers for RS-232 circuits will fail when exposed to impulses ranging from 40V to 100V, in the same manner that complementary metal-oxide semiconductor (CMOS) chips will fail between 17V and 30V.

Tracking the transients In general, most ICs fail at transient energies_below 100mJ. Chips that do not fail immediately will prematurely age and degrade. The effects of transients can be observed on ICs as thin, white lines between the input resistors and the input transistor contacts. Consequences may be severe_burnt line cards, lockups, memory loss, data retrieval problems, altered data and garbling. All of these baneful effects are often overlooked by users, who will first think of a hardware problem or a power-related problem when troubleshooting a malfunction.

Power lines are generally well-recognized as a source of transients; however, too often the “back door” of communication lines is forgotten. Any equipment has an imaginary “circle” around it that can be broken in two ways: through the power line or through the communication line (telephone lines, leased lines, coaxial lines, RS-232 lines, RS-422 and RS-423 lines, and 4mA-20mA current-loop cables).

Sources of surges are numerous, but the most-common and most well-known source is a lightning strike, which will affect, by induction, all nearby data lines. Industrial transients are also significant. They are man-made disturbances, mainly caused by start-up, switching and disturbances generated within the commutating systems of electric motors. Data lines will also suffer from magnetic and electrostatic inductions from nearby power lines.

Telco primary protectors, such as carbon blocks and gas tubes, inadequately protect data lines. The firing voltage of these devices is too high; therefore, they let through too many unwanted surges. Also, their response time is too slow to catch a fast-rising surge before it enters the equipment. These devices have been designed to protect the operators, not the equipment.

Data and telecom equipment must be protected by a surge suppressor that provides a fast response time and a precise clamping voltage with respect to the data equipment being protected.

We have designed fast-acting, low-clamping, high-discharge hybrid circuitry to divert to ground all damaging transient voltages while being transplanted on line. This hybrid circuitry comprises a three-element gas tube on the input stage and a solid-state diode on the output stage. Gas tubes in our designs provide high-discharge capacity (.5,000A).

In spite of the possible existence of a primary protector, a gas tube in the secondary data protector is still needed. Once the surge passes the primary protector without being stopped, it will be clamped by the diode first, and then 99% of the energy will be handled by the gas tube. If there were no gas tubes in the secondary protector, the diode would “fry,” and the user would be regularly subject to downtime for troubleshooting and replacement.

Diodes in our designs provide a response time of less than 1ns. As mentioned above, the diode itself will dissipate only 1% of the total energy. Therefore, resistors are used in series to prevent the diode from clamping the primary protection, which would cause the diode to take all of the follow-on current.

Locating protection The placement of data circuit surge protectors is as important as their components. Every building entrance must be protected, particularly in a campus environment. In addition to entrance protection, terminals and other equipment should be protected if they are more than 500 feet from the entrance. In such a case, the ground for the protector should be the same ground used for the equipment.

Protection of data equipment must be thought of in terms of total protection: power lines and communication lines.