When adapting an AC circuit breaker for use in a DC system, several key differences must be taken into account. Although the basic principle of circuit protection remains similar, there are important distinctions in how overload and short-circuit protection functions, as well as in the design of accessories and arc suppression mechanisms.
Overload and Short Circuit Protection
1. Overload Long Delay Protection: In AC circuit breakers, thermal type (bimetallic) trip units rely on the I²R effect to detect overloads. Since the RMS value of AC current is equivalent to the average value of DC, these units can typically be used without modification for low-current applications. However, for high-current specifications, if a current transformer is used on the secondary side, it cannot be employed in a DC system because transformers are not suitable for DC operation.
If the long-delay trip unit is fully electromagnetic (such as a hydraulic or oil-bath type), its time-current characteristics must be adjusted. The minimum operating current should be increased by 110% to 140%. As a result, AC full electromagnetic trip units are generally not compatible with DC systems unless they are specifically redesigned for that purpose.
2. Short-Circuit Protection: The short-circuit protection in thermal-magnetic AC circuit breakers relies on a magnetic system. When used in a filtered rectifier (DC) circuit, the original AC current value must be multiplied by a factor of 1.3. This applies to both thermal-magnetic and all-electromagnetic short-circuit protection systems.
Circuit Breaker Accessories
Accessories such as shunt trip units, undervoltage release coils, and electric operating mechanisms require careful consideration. Shunt and undervoltage releases are voltage coils, so if the voltage rating matches, they can be used in both AC and DC systems without modification. However, auxiliary contacts and alarm switches are usually compatible with both types. The electric operating mechanism, on the other hand, needs to be redesigned for DC applications.
Other Considerations
One major difference between AC and DC circuits is the absence of zero-crossing in DC. This means that when breaking a DC fault current—even a small one—the arc does not naturally extinguish. To manage this, DC circuit breakers often require multi-pole designs (two or three poles) so that each pole shares the arc energy, improving arc extinction efficiency.
In addition to traditional thermal-magnetic trip units, modern electronic trip units are widely used. These offer better stability under varying ambient temperatures and provide more precise control over trip characteristics.
Note: Leakage protection devices must be highly sensitive. It's crucial to choose a reputable brand like Siemens, which is known for its precision and speed—often tripping within 0.1 seconds. This quick response helps prevent serious harm by cutting off power before the current reaches dangerous levels. Using a lower-quality or outdated device could delay the trip time, potentially leading to life-threatening situations. Always prioritize safety and choose reliable equipment for your electrical systems.
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