MCB, MCCB or ACB: Choosing the Right Circuit Breaker
Miniature, moulded case and air circuit breakers each have a place in an electrical installation. This guide explains the difference and how to pick between them.
Every circuit needs protection against overload and short circuit. The device doing that job is usually a circuit breaker, and the three families you will meet most often are the MCB, the MCCB and the ACB. They overlap a little, but each is built for a different range of current and duty.
MCB: miniature circuit breaker
MCBs protect final circuits: lighting, sockets, small motors and control circuits. They are compact, mount on a DIN rail and are usually rated from a few amps up to about 125 A. Most have a fixed trip curve (B, C or D), so you choose the curve to suit the load.
- B curve: resistive loads and long cable runs.
- C curve: general purpose and moderate inrush, such as small motors and transformers.
- D curve: high inrush loads.
MCCB: moulded case circuit breaker
MCCBs sit higher in the system, protecting sub-distribution boards, feeders and larger motors. They cover roughly 100 A to 1600 A, and many have adjustable thermal and magnetic trip settings so you can tune protection to the load. Electronic trip units add features such as earth fault protection and metering.
ACB: air circuit breaker
ACBs are used at the incomer of a main switchboard, on large transformers and generators. They handle thousands of amps, offer high short-circuit withstand, and are designed to be maintained and serviced. They are larger and cost more, but they give the highest level of control and selectivity.
How to choose
Work through these points for each circuit.
- Rated current: the breaker must carry the load current continuously, and protect the cable.
- Breaking capacity: it must be equal to or higher than the prospective short-circuit current at that point of the installation. This is the most important safety check.
- Number of poles: 1, 2, 3 or 4 poles, depending on the supply and neutral arrangement.
- Discrimination: upstream and downstream breakers should be coordinated so that only the breaker nearest the fault trips.
- Accessories: shunt trips, auxiliary contacts and motor operators can be added to many models.
Buy genuine and check the standard
Make sure the breaker is tested to IEC 60947 or IEC 60898 as appropriate, and that it comes from an authorised supply chain. A fake breaker may look identical and still fail to trip when it matters.
Tell us the load, the supply and the fault level, and we will help you find a suitable breaker from our stock.