Machine Design

Tackling the Dirty Dozen

Follow these guidelines to avoid the 12 most common mistakes of specifying circuit protection for electrical equipment.

Tackling the dirty dozen

Ken Cybart
Senior Applications Engineer
E-T-A Circuit Breakers
Mt Prospect, Ill.

Thermal circuit breakers, such as the ESS20 from E-T-A Circuit Breakers, Mt. Prospect, Ill., are DIN-rail mountable and protect 24-Vdc circuits.

Many engineers design electrical equipment with too little or too much circuit protection. Under-protected circuits leave equipment vulnerable to electrical surges. Overprotected circuits, on the other hand, add cost and can lead to nuisance tripping. Keeping the following tips in mind will help avoid these mistakes.

Specifying the wrong type of circuit breaker: This is the number one mistake. There are four basic types of breakers: thermal, magnetic, thermal magnetic, and high performance. Each has a different trip profile in relation to time and current and distinct mechanical qualities. And the range of current protection can be from 6 mA to 500 A, with voltages to 220 Vdc or 600 Vac.

Magnetic circuit breakers operate via a solenoid, and trip nearly instantly at the threshold current. They're good for PC-board applications and impulse disconnection in controls. Often, a magnetic circuit breaker is combined with a hydraulic delay to make it better tolerate current surges. Make sure the circuit breaker mounts horizontally to prevent gravity from influencing solenoid movement.

Thermal circuit breakers incorporate a heat-responsive bimetal strip or disk. They have a slower characteristic curve that discriminates between safe temporary surges and prolonged overloads. They operate like slow-blow fuses and take seconds to minutes to respond to moderate overcurrents. Main applications include machinery or vehicles with high-current in-rushes that accompany starting of electric motors, transformers, and solenoids.

Thermal-magnetic circuit breakers combine the benefits of thermal and magnetic types. High overcurrents make the solenoid trigger the release mechanism rapidly, typically in a few milliseconds, while the thermal mechanism responds to prolonged low-current overloads. They have a characteristic two-step trip profile that provides fast short-circuit protection while minimizing the risk of disrupted system operation.

High-performance circuit breakers provide high interrupting capacity, typically 6,000 A at 28 Vdc, and are reliable under adverse conditions. They're typically used in aerospace, defense, and other heavy-duty applications where vibration, mechanical shock, and other conditions are present.

Rating too high: Engineers are used to oversizing fuses as a way to avoid nuisance tripping. But there's no need to oversize a circuit breaker.

Unlike a fuse rating, a circuit breaker rating gives the maximum current the device will consistently maintain in ambient room temperature. So 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 tripping. Thermal-type breakers have a natural delay, and magnetic breakers can have added hydraulic delays. The trick to matching in-rush current is to make sure the breaker trip curve is slow enough to let the in-rush current flow without tripping the breaker.

Adequate spacing: It's important to maintain recommended minimum spacing requirements between nontemperature-compensated thermal circuit breakers. Just a 1-mm spacing is usually enough. Without this tiny thermal gap, the breakers can heat up and increase the sensitivity of the bimetal trip mechanism. If the breakers must touch, derate them to 80% of their normal amperage rating.

Circuit-breaker specification begins with the four basic types (thermal, magnetic, thermal magnetic, and high-performance) and understanding the application needs.

Overspecifying: When specifying circuit breakers, use established standards such as EN 60529/ IEC 529 as a measure. For example, a combination switch breaker installed in medical equipment might need a water-splash protection rating. But it probably doesn't need a rating for continuous immersion in water. Truly watertight and dusttight breakers are available, but they're expensive and usually unnecessary.

Selecting the correct actuation: There are many types of actuators including press-to-reset, push-pull, push-push, rocker, toggle, baton, and press to-reset with manual release. For instance, critical applications usually call for push-pull style actuators, because they're the most resistant to accidental actuation. Consider the location of the breaker, whether it needs illumination, and operator safety and convenience.

Double duty: Many circuit breakers are designed to be both a breaker and on/off switch. The advantages are fewer components, less consumption of panel space, reduced wiring, and increased reliability.

Terminals: Circuit breakers with plug-in style quick-connect terminals simplify installation and replacement. Screw-terminal connections are more secure and suited for high current and high vibration. Quick-connect terminals can be used for circuit breakers rated to 25A.

Fuse or circuit breaker?: Although fuses are inexpensive, the cost savings should be weighed against the low total cost of ownership of circuit breakers. Circuit breakers can be quickly reset, minimizing downtime. There's also no assurance that a replacement fuse will be of the proper rating. Replacing a fuse by a higher rated fuse can cause overheating and equipment failure.

Circuit breakers are also relatively stable over time. As fuses age, their trip characteristics change, leading to the likelihood of nuisance tripping and more downtime. Circuit breakers also offer the designer more options. For instance, an auxiliary contact can communicate an alarm condition to an LED indicator or process software. Remote trip is another option available with circuit breakers but not with fuses.

High-vibration environments: 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 these circuit breakers particularly vulnerable to vibration, which can close the armature prematurely. In contrast, a typical thermal circuit breaker is comprised of a thermal actuator and a mechanical latch. Thermal circuit breakers, therefore, work well despite high levels of shock and vibration. If a magnetic circuit breaker is best for the application, its vibration resistance can be improved by using a push-pull style actuator.

Failure to derate: As a rule of thumb, the circuit breaker should be rated for 100% of the load. But some applications require the breaker operate continuously in either high or low temperatures. It's important here to follow manufacturer guidelines for derating. For example, an application calling for 10-A protection needs a 12-A rated thermal circuit breaker when operated at 50°C.

Unnecessary derating: Thermal circuit breakers are sensitive to fluctuations in ambient temperature. They will trip at higher currents in the cold and at lower currents when it's hot.

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

Another misconception is that magnetic-hydraulic style 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 temperatures, reducing the time of the hydraulic delay.

Overspecifying interrupting capacity: Interrupting capacity is the maximum amperage a circuit breaker can safely interrupt. Manufacturers publish this specification along with the number of times the breaker will perform this feat. To comply with various standards, engineers must specify breakers with enough interrupting capacity.

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