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12 Most common mistakes when specifying circuit protection


It’s only a circuit breaker.  Yet there is enough complexity and confusion when it comes to specifying circuit protection that many engineers are designing equipment with too little or too much protection.  Under protected circuits leave equipment vulnerable to damaging electrical surges.  Over protected circuits add cost and can lead to nuisance tripping.  Like Goldilocks and the Three Bears, the goal is to specify circuit protection that is ‘just right’.

As a global manufacturer of circuit breakers for more than 50 years, E-T-A has selected the 12 most common pitfalls for this article.

 1. Specifying the wrong circuit breaker type for the application

The number one mistake made is specifying the wrong circuit breaker technology for the application.  There are four choices of circuit breaker technology: thermal, magnetic, thermal-magnetic and high performance.  Each has a different trip profile in relation to time and current, and each has distinct mechanical characteristics.

Magnetic circuit breakers

Magnetic circuit breakers operate via a solenoid, and trip nearly instantly as soon as the threshold current has been reached.  This type is appropriate for printed circuit board applications and impulse disconnection in control applications.

Thermal circuit breakers

Thermal circuit breakers incorporate a heat-responsive bimetal strip or disk.  This type has a slower characteristic curve that discriminates between safe temporary surges and prolonged overloads.

Thermal-magentic circuit breakers

Thermal-magnetic circuit breakers combine the benefits of a thermal and magnetic circuit breaker: a delay that avoids nuisance tripping caused by normal inrush current, and fast response at high currents.

High performance circuit breakers

Where reliable operation under adverse conditions is required, high performance circuit breakers provide high interrupting capacity and excellent environmental specifications. Typically these circuit breakers are specifically designed for aerospace, defense and similar heavy-duty applications where extreme vibration, mechanical shock, and other conditions are present, and where circuit breaker performance is absolutely critical.

2. Specifying too high a rating in an effort to avoid nuisance tripping caused by in-rush or transient currents

Most engineers are concerned about nuisance tripping, as they should be, by they often specify a breaker rated much higher than they should.   Part of the reason is 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 you the maximum current that the circuit breaker will consistently maintain in ambient room temperature.  Thus, a 10A circuit breaker will maintain a 10A current without nuisance tripping.  In fact, a typical 4A circuit breaker with a slow trip profile will tolerate a temporary 10A current surge without nuisance tripping.

3. Failure to provide spacing in design

It is important to maintain recommended minimum spacing requirements between non-temperature-compensated thermal circuit breakers.  A mere 1 mm spacing between breakers is all that is required.  Without this tiny thermal gap, the circuit breakers can heat up and increase the sensitivity of the bimetal trip mechanism.  If the breakers must touch each other, derate them to 80% of their normal amperage rating.

4. Over specifying or ambiguously specifying the degree of protection

Terms such as drip-proof, ignition protection, water splash protection and dust proof are in common usage but may be misleading unless standard definitions are applied.  When specifying, use the established standards as a measure, such as EN 60529/IEC 529, which defines the degree of protection of Electrical Equipment.  Using these standards, decide which protection is correct for the application.

5. Selecting the correct actuation

Circuit breakers are reset manually by means of an actuator.  There are many types of circuit breaker actuators, including press-to-reset, push-pull, push-push, rocker, toggle, baton and press-to-reset with manual release.  The actuator type is more than a cosmetic consideration.  For example, critical applications usually call for push-pull style actuators, because they are the most resistant to accidental actuation.  The type of actuator you select will be determined by the location of the circuit breaker, the need for illumination, the need for human operator safety or convenience and the consequences of accidental engagement.

 6. Failure to consider using circuit breakers as on/off switches

Many circuit breakers are designed to be both a breaker and on/off switch.  The advantages of a combination device are a reduction in components, less consumption of panel space, reduced wiring and increased protection over ordinary switches.

7. Specifying the wrong type of terminal

Circuit breakers with plug-in style quick connect terminals simplify installation and replacement (they must also be soldered).  Screw terminal connections are more secure and suited for high current and high-vibration environments.  Quick connect terminals may be used for circuit breakers rated up to 25A.

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.

 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.

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.

 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.

Summary

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.