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Page 4 & 5 - Dirty Dozen

8. Specifying a fuse when a circuit breaker would be better

Although fuses provide inexpensive circuit protection, the cost savings should be weighed against the low total cost of ownership of circuit breakers.  Foremost, circuit breakers can be quickly reset, enabling the circuit to be restored with a minimum of downtime.  In addition, there is no assurance that a replacement fuse will be of the proper rating.  If a fuse is replaced by a higher rated fuse, overheating and catastrophic equipment failure may occur.

Circuit breaker performance is relatively stable over time, but as fuses age, their trip characteristics change.  This may lead to nuisance tripping and increased downtime.

Circuit breakers offer designers more options than do fuses.  An auxiliary contact may be added that can communicate an alarm condition to an LED indicator or process software.  In addition, a circuit breaker can be combined with a switch, saving space and adding overload protection.  Remote trip is another option available with circuit breakers but not with fuses.

Furthermore, unlike fuses, circuit breakers have a variety of types and trip profiles, and therefore can be more precisely matched to loads and environment.

Finally, fuses cannot be tested without destroying them.  How can you be sure the fuse you specify will open is there is an overload?


9. Specifying the wrong type of circuit breaker for a high vibration environment

Typically, the trigger of a magnetic circuit breaker is a hinged metal armature that closes in response to the movement of a magnetic coil.  This design makes magnetic circuit breakers (and magnetic-hydraulic circuit breakers) particularly vulnerable to vibration, which can cause the armature to close prematurely.

In contrast, a typical thermal circuit breaker is comprised of a thermal actuator and a mechanical latch and are therefore highly tolerant of shock and vibration.

If a magnetic circuit breaker is the best type for the application, its vibration resistance can be improved by using a push-pull style actuator.  This type of actuator has a latching design.

10. Failure to derate

As a rule of thumb, the circuit breaker should be rated for 100% of the load. However, some applications require a circuit breaker to operate continuously in either high or low temperatures. In these cases, follow the manufacturer's guidelines for derating. For example, an application calling for 10A protection requires a 12A rated thermal circuit breaker when it is operated at 50° C.


11. Derating when it is not necessary

The performance of a thermal circuit breaker is sensitive to fluctuations in ambient temperature. It will trip at higher amperage in a cold environment, and it will trip at a lower amperage in a hot environment.

One common mistake is to assume that derating is necessary for thermal circuit breakers in environments that experience rises in ambient temperature. Actually, the performance of a thermal circuit breaker tracks the performance needs of the system, assuming it is exposed to the same heat source. For example, motor windings need more protection from overheating at 90° C than the same windings need at 20° C. A cold motor requires more in-rush current to get started, and therefore a longer delay is advantageous on a cold day.

Another misconception is that magnetic-hydraulic style circuit breakers are immune to performance changes in rising ambient temperatures.  On the contrary, these circuit breakers contain a dashpot with a liquid core that becomes more fluid at higher temperature, reducing the time of the hydraulic delay.


12. Over specifying interrupting capacity

Interrupting capacity is the maximum amperage a circuit breaker can safely interrupt.  Circuit breaker manufacturers publish this specification along with the number of times the circuit breaker will perform this feat.  For example, E-T-A publishes two types of interrupting capacity specifications.  One is called Icn, or normal Interrupting Capacity.  Icn is the highest current the circuit breaker can interrupt (three times minimum, per IEC934/ EN60934 PC2).  Icn gives a rough idea of circuit breaker quality.  The other specification is UL 1077 interrupting capacity.  UL1077 interrupting capacity is the maximum current a circuit breaker can safely interrupt at least one time without causing a fire hazard.

To comply with various standards, engineers must specify circuit breakers with adequate interrupting capacity.  Unfortunately, applying the appropriate standard may be confusing.  For example, UL 489 requires interrupting capacity from 5000A and above.  While perfectly appropriate for main power distribution applications, this standard has been perpetuated in other industries, where the short circuit rating, governed by circuit resistance, is much lower.  the UL 1077 standard for supplementary protectors covers the short circuit test and lists the current at which the breaker was tested.

Although certain devices such as UL 489 molded case circuit breakers have higher interrupting capacities, they may not be well suited for lower current applications where precise overload protection and adequate short circuit protection is better provided by a UL 1077 supplementary protector.

The telecom industry is particularly prone to over specifying interrupting capacity because some vendors of circuit breakers for DC telecom equipment also market the same circuit breakers for AC power distribution.  Although the potential supply of current seems high in telecom applications, the realistic amount of current available is actually far less, due to line loss.  In most telecom applications, a circuit breaker with 2000A interrupting capacity is more than adequate.


If you keep these tips in mind, it is easy to specify the right measure of circuit protection at the lowest cost.  Start the selection process by working to truly understand your load.  Then decide which type of circuit breaker is suited to your application.  Avoid the common mistakes, and you will be rewarded with a reliable design.