Selecting the right circuit breaker is critical to ensuring electrical system safety and efficiency. Key factors such as load capacity, fault levels, and system discrimination all play a role in determining the best fit for an application. This guide explores the differences between Air Circuit Breakers (ACBs) and Moulded Case Circuit Breakers (MCCBs), providing a clear comparison to help inform your decision-making process.
Understanding ACBs and MCCBs
Both ACBs and MCCBs serve the fundamental purpose of protecting electrical circuits by interrupting excessive current flow. They share core components, including a contact system for arc quenching, an operational mechanism, and protection and indication systems. However, their design and applications differ significantly, making it essential to select the right type based on system requirements.
Load Considerations
The rated current (In, A) of a circuit breaker must exceed the expected circuit load. MCCBs are generally available in ratings up to 1600A, for special applications this can be higher and are an excellent choice for compact, space-constrained applications. However, for higher power requirements 1600 A and above, MCCBs may become less cost-effective. ACBs, while larger, provide a more economical and practical solution for high-power installations, ensuring robust protection and reliable performance.
Fault Limitation and Interruption Times
MCCBs are designed to limit fault current by interrupting the circuit before the peak of the current waveform. High-performance MCCBs, such as ABB’s SACE Tmax XT, can interrupt faults in as little as five milliseconds in a 50 Hz system. This rapid response helps minimise fault energy and protects sensitive downstream equipment from damage.
ACBs, in contrast, are engineered to withstand fault currents rather than limit them. A typical ACB interrupts a short circuit within 25 to 40 milliseconds, allowing one to two cycles of fault current to pass before disconnection. This means that components protected by ACBs such as transformers and busbars, must be rated to withstand short-term fault currents.
Fault Level and Breaking Capacity
Circuit breakers must be capable of safely interrupting the maximum potential short-circuit current at their installation point. Selecting a breaker with an adequate breaking capacity is crucial, as overspending on an unnecessarily high rating can increase costs without added benefit.
Fault current levels are influenced by several factors, including:
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- The available power from the transmission network
- Transformer characteristics
- Impedance of conductors in the distribution system
A comprehensive fault level study, incorporating transformer and conductor impedance data, helps ensure the right balance between performance and cost efficiency.
Discrimination and Selectivity
The main incoming breaker in a low-voltage system protects essential components such as transformer windings, conductors between the transformer and switchboard, and the switchboard’s main busbars. However, if a fault occurs in a downstream circuit, an improperly selected incoming breaker could trip unnecessarily, leading to a total system outage.
To avoid this, proper discrimination (selectivity) must be maintained. The goal is to ensure the incoming breaker remains operational while allowing the downstream breaker to clear the fault first. One effective way to achieve this is by selecting an incoming breaker with sufficient withstand capability for at least one second, alongside short-circuit protection settings that incorporate a time delay for selective tripping.
Why ACBs Are Preferred for Incoming Protection
ACBs are commonly used for incoming protection in low-voltage distribution systems due to their superior withstand capabilities. Many ACBs, including ABB’s SACE Emax 2 models, can withstand fault currents up to their breaking capacity (Icw, (1s) = Ics). When correctly selected based on fault level studies, ACBs provide reliable discrimination without requiring extensive additional calculations, assuming relay settings are configured correctly.
Choosing between ACBs and MCCBs requires a thorough assessment of system demands. MCCBs offer rapid fault limitation, making them ideal for protecting sensitive downstream equipment. ACBs, on the other hand, excel in handling high fault currents, making them the go-to choice for main system protection. By evaluating load capacity, fault levels, and discrimination requirements, you can ensure an optimal and cost-effective circuit breaker selection for your electrical infrastructure.
Ready to make the right choice for your electrical system? Get in touch with us today on 1300 556 601 or [email protected] to discuss your project requirements or to learn more about our range of high-performance ACBs and MCCBs.