Often, little things make a big difference in a successful design. When it comes to telecommunications equipment, the incorrect specification of a circuit breaker can lead to unnecessary system shutdowns, superfluous costs, and the under-protection of expensive network systems. Sure, it's only a circuit breaker. By avoiding the most common specifying pitfalls, however, engineers can guarantee that their designs will be reliable and adequately protected.

One common error, for example, is the overspecifying of interrupting capacity. Interrupting capacity is the maximum amperage that a circuit breaker can safely interrupt. Some circuit-breaker manufacturers publish two types of interrupting-capacity specifications. The first, called I_CN or Normal Interrupting Capacity, is the highest current that a circuit breaker can interrupt repeatedly (three times minimum per IEC934/EN60934 PC2). Certain UL standards, such as UL 489, use this approach when defining interrupting capacity. During UL 489 testing, the device must survive short-circuit testing. It also must work during future overload conditions.

A second way of defining interrupting capacity is used in both the UL 1077 standard and international standards. UL 1077 (or IEC934/EN60934 PC1) specifies the maximum current that a circuit breaker must safely interrupt—just one time—without causing a fire hazard.

To comply with various standards, engineers must specify circuit breakers with adequate interrupting capacity. Unfor-tunately, applying the appropriate standard may be confusing. For example, UL 489 covers molded-case circuit breakers for branch circuit protection. It requires a minimum interrupting capacity of 5000 A. This standard is perfectly appropriate for main power-distribution applications. Yet UL 489 also has been adopted in applications in which the specification of 5000-A interrupting capacity can be detrimental to equipment.

In many cases, the short-circuit current governed by circuit resistance is much lower. Depending on the current rating, the UL 1077 standard for supplementary protectors requires lower short-circuit interrupting capacities up to 5000 A. Devices built to this standard can provide precise overload protection and adequate short-circuit protection at a lower cost than a UL 489-listed breaker.

When comparing devices listed to UL 489 and UL 1077, the biggest difference is that UL 1077 addresses supplemental protectors. These protectors are not intended as overcurrent protection on branch circuits. They are made for installation inside equipment. As such, they must be used in conjunction with UL 489 branch-protected circuits. UL 1077 supplementary protection is approved for use as part of a product that carries its own approval by a recognized testing agency.

Some UL 1077 devices also may carry international (IEC) ratings. These ratings can add to the user's confusion. Before specifying, be sure to know or look up the ratings and approvals for each device.

The telecom industry is particularly prone to overspecifying interrupting capacity because some circuit-breaker vendors for DC telecom equipment market the same circuit breakers for AC power distribution. (Low-voltage DC currents are far more sensitive to resistance than AC currents.) In certain AC applications, the maximum current available in a short circuit may exceed 50 kA. Due to line loss and lower source voltage, the available short-circuit currents in telecom applications are far less. In most telecom applications, a circuit breaker with 2000-A interrupting capacity is more than adequate.

Recently, a new UL outline of investigation was created to address this concern. It was specifically geared toward designers of communications equipment. Dubbed UL 489A, this new outline is essentially a subset of UL 489. It is similar to UL 489, except that it covers "single-pole or multi-pole DC circuit breakers intended as branch-circuit overcurrent and short-circuit protection in communication equipment." UL 489A retains the requirement that the device must remain operational after short-circuit testing and breaking 200% overload.

Table 1 shows the differences between UL 489, UL 1077, and UL 489A. One recently introduced circuit breaker, E-T-A's 8345, meets the requirements of all of them (FIG. 1). It has an interrupting capacity of 10,000 A at 80 VDC and 5000 A at 240/415 VAC.