Don't Fall Into Circuit Protection's Pitfalls
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 ICN 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.
NUISANCE TRIPPING Many engineers also specify too high a rating in an effort to avoid nuisance tripping. In-rush or transient currents cause such nuisance tripping. Most engineers are already concerned about this problem. Yet this concern often makes them specify a circuit breaker that is rated much higher than needed. This decision is usually driven by the confusion between fuses and circuit breakers. Engineers are used to oversizing fuses as a way to prevent nuisance tripping. However, there is no need to oversize a circuit breaker.
Unlike a fuse rating, a circuit-breaker rating tells the engineer the maximum current that will be consistently maintained by the circuit breaker in ambient room temperature. Thus, a 10-A circuit breaker will maintain a 10-A current without nuisance tripping. In fact, a typical 4-A circuit breaker with a slow trip profile will tolerate a temporary 10-A current surge without nuisance tripping.
Often, the in-rush currents associated with certain electrical components cause nuisance tripping. Big capacitors are an example of such a component. In these cases, the designer needs to specify a circuit breaker that has a delay. Thermal circuit breakers have a natural delay, while magnetic circuit breakers can have added hydraulic delays. Match the delay to the duration of the expected in-rush currents.
Another common mistake is the specification of the entirely wrong type of circuit breaker. This problem especially impacts base stations and other stationary installations. They need to be protected from vibration—especially in earthquake-prone regions.
A typical thermal circuit breaker comprises a thermal actuator and mechanical latch. They are highly tolerant of shock and vibration. These circuit breakers also tend to be less costly than other circuit-breaker technologies with equivalent ratings.
Typically, the trigger of a magnetic circuit breaker is a hinged metal armature. This armature closes in response to the magnetic field that is created by a coil carrying the load current. Due to this design, magnetic circuit breakers and magnetic-hydraulic circuit breakers are particularly vulnerable to vibration. It can cause the armature to close prematurely.
If a magnetic circuit breaker is the best type for the application, however, its vibration resistance can be improved. Just use a push-pull actuator. This type of actuator has a latching design. It also is the most resistant to accidental actuation.
Some newly designed, more sophisticated magnetic circuit breakers can provide excellent shock and vibration resistance. For example, the previously mentioned 8345 is rated for 10 g vibration from 57 to 2000 Hz and 100 g shock. This resistance beats the numbers offered by some traditional thermal breakers.
Thermal-magnetic circuit breakers combine the features and benefits of a thermal and magnetic circuit breaker. They offer a delay that avoids the nuisance tripping caused by normal in-rush current. Yet they also flaunt a fast response at short-circuit currents. These thermal-magnetic circuit breakers have a characteristic two-step trip profile (FIG. 2). This profile provides overload protection for expensive electrical systems. At the same time, it minimizes the risk of equipment failure due to short-circuit conditions.
PROVIDE SPACING In rack-mount systems, the recommended minimum spacing requirements must be maintained between non-temperature-compensated thermal circuit breakers. Only 1 mm of space is required between breakers. Without this separation, however, the heat generated from neighboring circuit breakers can increase the sensitivity of the bimetal trip mechanism. If the breakers must touch each other, derate them to 80% of their normal amperage rating.
Normally, thermal circuit breakers are not derated with temperature. After all, the equipment that they are protecting is exposed to the same temperature. As the equipment becomes hotter, a slightly reduced trip point is appropriate. But in some applications, circuit breakers must operate continuously at high or low temperatures. Under these conditions, the breaker's rating should be corrected in accordance with Table 2.
Power distribution is an important aspect of rack-mounted telecommunications systems. Increasingly, OEMs are outsourcing the manufacture of power-distribution units to companies with expertise in key components. Typically, circuit breakers make up 40% to 50% of the cost of a power-distribution system. Circuit-breaker manufacturers can integrate circuit breakers into a custom solution at a lower cost, however.
Circuit-breaker specifications can be confusing. In their efforts to make products as broadly focused as possible, some manufacturers inadvertently add to this confusion through their advertising. For the user, the best course is to thoroughly understand the application and the requirements set by standards organizations. Make sure that the selected device is appropriate for a particular application. Failing to do so can waste money and even leave the equipment with inadequate protection.