Choosing the Incoming Device: MCCB or ACB?
MCCB vs ACB: Differences between moulded case and air circuit breakers, selection criteria and a comparison on selectivity and maintenance.
ContentsIn a low voltage switchboard the incoming device is the single point that feeds the entire installation, which puts it at the centre of both protection and operational decisions. In practice the choice lies between two families: moulded case circuit breakers and air circuit breakers. The MCCB vs ACB question is not only about current range; selectivity capability, maintenance strategy and the architecture of the board all follow from it.
The character of each family
A moulded case breaker holds a sealed mechanism inside a moulded housing; its contacts are not accessible, so maintenance means replacement. An air circuit breaker has its arc chambers and contacts inside an accessible frame, and it is a withdrawable, maintainable device. The second group is larger, heavier and more expensive, but it offers short time delay settings and a genuine short time withstand rating.
| Criterion | Moulded case (MCCB) | Air circuit breaker (ACB) |
|---|---|---|
| Typical rated current | 16 A to 1600 A | 630 A to 6300 A |
| Short time withstand (Icw) | Often undeclared or very short | Full Icw rating up to 1 s |
| Time delayed selectivity | Limited; solved by current limiting | Full time selectivity with short delay settings |
| Maintenance | Replaceable, no contact servicing | Contacts and arc chambers can be serviced |
| Withdrawable execution | Plug-in options available | Withdrawable on a truck as standard |
| Trip unit | Thermal magnetic or electronic | Electronic, including metering and communications |
| Volume and cost | Compact, economical | Large, high capital cost |
The deciding criterion: selectivity
Selectivity means a fault is cleared only by the nearest protective device while the upstream stage stays in service. Time delayed selectivity requires the incoming device to carry the fault current for a defined period, and that is exactly where the Icw rating becomes decisive. Because moulded case breakers usually have no declared Icw, selectivity has to be built through current limiting and energy coordination, and full time selectivity cannot be achieved. In plants where downtime is expensive that single fact justifies an air circuit breaker. The short circuit side of the subject is covered in our article on Icw and Ipk.
Practical selection rules
- Below 630 A and in simple radial distribution a moulded case breaker is both economical and sufficient
- Between 630 and 1600 A the decision follows from selectivity and maintenance strategy
- Above 1600 A, and wherever generators run in parallel, an air circuit breaker is practically mandatory
- If the load profile is harmonic rich, an electronic trip unit with true rms measurement is essential
- In critical plants a withdrawable incomer reduces maintenance downtime from hours to minutes
- The communication protocol of the trip unit must match the plant automation system
Key takeaways: MCCB vs ACB
- The character of each family — A moulded case breaker holds a sealed mechanism inside a moulded housing; its contacts are not accessible, so maintenance means replacement.
- The deciding criterion: selectivity — Selectivity means a fault is cleared only by the nearest protective device while the upstream stage stays in service.
- Standard source — IEC publication catalogue: iec.ch.
Frequently asked questions
Can both families be used in the same board?
They can and commonly are: an air circuit breaker as the incomer with moulded case breakers on the outgoing ways. That arrangement balances selectivity against cost. What matters is coordinating the protection curves together.
Is an electronic trip unit always necessary?
No. For simple, steady loads a thermal magnetic release is sufficient. But where selectivity settings, energy metering, remote monitoring or fault recording are wanted, the electronic unit is the right choice.
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- All technical articles
At Devpan we select the incoming device together with the prospective short circuit current, a selectivity study and the maintenance scenario, and we coordinate the protection curves with the outgoing ways in the documentation. Share your single line diagram and we can compare MCCB and ACB options on cost and continuity.
