It's only a circuit breaker. How hard could it be? A surprising number of engineers specify the wrong type of circuit breaker to protect their equipment. Engineers frequently overprotect or underprotect designs, resulting in increased costs or equipment failures that can lead to fire. Specifying a circuit breaker isn't difficult. Understanding how to avoid a few common mistakes when specifying circuit breakers will help you protect your design.
Few design engineers understand the load characteristics of their designs and what circuit protection they require. Many fail to consider inrush current, a temporary surge in current associated with motors, transformers, and large capacitors. To accommodate inrush current, designers must specify a thermal or hydraulic-magnetic type of circuit breaker, which has a delay to avoid nuisance tripping.
Designers also must evaluate the load characteristics during equipment startup, under normal operating conditions, and during equipment failure to identify current spikes that may occur. Putting a design on an oscilloscope will help engineers to fully understand the load characteristics and determine the exact magnitude and duration of the inrush current. The rating of the circuit breaker doesn't have to exceed the height of the spike. If the circuit breaker is a delay type, a temporary spike won't trip the breaker.
The general rule is to specify a circuit breaker rated at 100% of the continuous or average load. However, if a normal surge current lasts longer than one minute, specify a circuit breaker rated at 100% of the surge current.
If an oscilloscope isn't available, calculate the combined inrush of motors and other components at startup. This can be estimated using specifications such as the locked rotor current, published in the data sheets of component manufacturers. But various factors, including line loss, make this approach an estimate.
Many engineers are wary of nuisance tripping, so they specify circuit breakers rated much higher than needed. This is mainly due to the confusion between fuse and circuit-breaker current ratings. Generally, fuses interrupt a circuit when current equals the rating of the fuse. A circuit-breaker rating is the maximum current that it will carry continuously. A 10-A thermal circuit breaker will easily accommodate a temporary surge of 14 A.
Another common mistake is selecting the wrong circuit-breaker technology for an application. The four choices are thermal, thermal-magnetic, magnetic, and high performance. Each type offers different electrical characteristics and options.
Appropriate for equipment where high-surge currents accompany the start of motors, thermal circuit breakers use bimetal technology to discriminate between safe switch-on currents/transients and prolonged overloads. A typical thermal breaker must trip within one hour at 140% of its rating. A latching mechanism makes these devices highly tolerant of shock and vibration.
Used extensively in telecommunications, process control, and factory automation, thermal-magnetic circuit breakers protect critical systems while minimizing the risk of disrupting equipment operation. High overcurrents cause a solenoid to trigger the release mechanism rapidly, while the thermal mechanism responds to prolonged low-level overloads.
Magnetic circuit breakers use a solenoid and trip quickly once the threshold current is reached. A typical trip time is 10 ms or less. Often, the solenoid is combined with a hydraulic delay to make the breaker tolerant of current surges without nuisance tripping. These circuit breakers should be mounted vertically to avoid the effects of gravity on the solenoid, which can affect the trip time.
High-performance circuit breakers are designed for rugged environments like those found in aerospace, military, and construction applications. They have many of the same trip characteristics as thermal, thermal-magnetic, or magnetic circuit breakers but meet a higher level of environmental or electrical conditions. Some models tolerate vibration up to 15 g (70 to 2000 Hz) and shock up to 75 g (11 ms).
Some applications require the circuit breaker to operate continuously in high or low temperatures. In these cases, designers should follow the manufacturer's guidelines for temperature compensation. Thermal breakers react differently to overcurrents depending on the ambient temperature. They have longer delay in a cold environment, and lower trip currents when exposed to high temperatures.
A common mistake is to assume that derating is necessary for all environments that experience high rises in ambient temperature. Actually, the performance of a thermal circuit breaker tracks the performance needs of a system, assuming that it's exposed to the same ambient temperature. For example, motor windings need more protection from overheating at 90°C than the same windings require at 20°C.
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