CIRCUIT PROTECTION RUNS DEEP

Circuit protection is probably one of the most necessary but least appreciated aspects of modern telecom networks. It literally affects every part of the network and is a resident technology in almost every piece of network equipment.

The importance of circuit protection devices cannot be understated-these solutions bring all network equipment into compliance with strict industry standards for power induction, power contact and lightning resistance (Figure 1).

All of these network elements contain electrical fuses that accommodate the power conductance of the equipment. When fuses are blown, ongoing equipment performance-and possibly network reliability-are at risk. Carriers should have a strategy in place to protect the reliability of their fuses.

A proper circuit protection strategy reduces long-term maintenance needs and other costs, and minimizes system downtime. In addition, a number of standards originating in both the electrical and telecommunications industries need to be met to use circuit fuses.

These issues have prompted the need for circuit protection for some time. However, the industry demand for circuit protection is increasing as more factors emerge, according to Gary Wiseman, commercial marketing director of Raychem Corp.'s Electronics OEM Components division in Menlo Park, Calif.

The telecom industry's migration to digital networking has taken several years but is now nearly worldwide. The shift to digital networks triggers numerous benefits that affect network efficiency, performance, capacity and reliability. However, one side effect of this trend is the fact that distributed electronics are more sensitive to fuse outages.

Also, the migration to new network architectures and equipment means that different network elements are constantly being replaced or installed, brought on-line or taken off-line. This type of situation is conducive to fuse overloads and other potential problems.

Furthermore, an increasingly vital and viable international communications marketplace is emerging. Developing nations want to be part of the Internet-driven communications era as much as anyone else. New networks are being built, and old ones are being overhauled, creating a huge need for circuit protection where none existed before.

Standards-making bodies in the telecom and electrical industries are in the midst of revising and improving long-time standards for circuit protection (Table 1). "The FCC's requirements are getting stronger, the UL requirements are getting stronger," says Wiseman. In particular, UL 1950's third edition was recently released, and UL 1459 should be approved in April of next year. Although these standards originate from the electric utility industry, they have major implications for the new breed of data-intensive information technology equipment that is becoming increasingly prevalent in telecom networks.

In fact, Underwriters' Laboratories is attempting to upgrade all of its telecom-related standards by blending some of the past and once-distinct requirements for the computer and telephone industries, says Wiseman. This has caused some controversy in both industries because telecom traditionalists feel they are being forced to absorb computer industry rules, and computer industry natives are puzzled by the telecom guidelines.

As more revisions and new standards are completed over the next year, circuit protection devices will be forced to satisfy new criteria, creating a busy environment for upgrades and replacements.

In regard to telecom standards, Bellcore GR-1089 specifies that network equipment must meet stringent power induction, power contact and lightning resistance requirements.

The current state of the circuit protection market, and the anticipation of further vitality, is driving advancementsin circuit protection technology. Network infrastructures containing hundreds or thousands of pieces of equipment have provided a daunting landscape for circuit protection efforts over the years. Teams of technicians used to be deployed routinely to reset and replace fuses, and to keep circuits up and running. This was an expensive, time-consuming and monotonous task.

Luckily, since the mid-1980s, resettable fuses have become fairly standard throughout telecom networks. The fuses automatically reset themselves after current overloads and outages, thus saving the expense and time of calling in field technicians. Because blown fuses can also result in further equipment damage, having an automatic reset capability drastically reduces the risk of such damage.

However, for the last decade or more, fuses have continued to be an added-and therefore non-intrinsic-component in network equipment. This made circuit protection an afterthought-and perhaps even an unnecessary line item-in their infrastructure cost plans.

Yet now, in a highly competitive age when network reliability has become more critical than ever, carriers and their customers have expectations that network equipment will not go down under any circumstances-especially power surges or fluctuations.

This has spurred new technology developments in circuit protection. Such advancements include surface-mount devices, radial-leaded devices and crowbar resettable surge protectors.

The surface-mount protectors further reduce the cost of such devices and make them intrinsic to network equipment and customer premises equipment designs. "They are mostly for new equipment installations," says Wiseman. "They are being designed into equipment already and are better than ceramics or other types of fuses that typically would be used."

Radial-leaded devices offer lower resistance than other fuses, and this discourages the occurrence of catastrophic surges. The crowbar devices are thyristor surge protectors that automatically switch themselves into a low-impedance state when breakover voltage is exceeded, with surge current conducted away from sensitive telecom electronics, says Wiseman.

These technology advances will undoubtedly make things easier for telecom network equipment designers and manufacturers. They will improve the cost and long-term efficiency of network deployments. Meanwhile, however, Wiseman wonders if the increasingly complex and controversial standards environment will catch some equipment manufacturers and network planners by surprise.

As new network are be constructed rapidly, often with new types of equipment that haven't been widely deployed in traditional networks, meeting the correct circuit protection deployment practices and standards could again become something easily forgotten, says Wiseman.

In the year or so ahead, the telecom industry will need to become more educated about circuit protection and the details of applicable standards, he contends. Otherwise, occasional surges and power overloads that occur deep in the network may unfortunately bring reliability problems to the surface.