Choosing LV Circuit Breakers: MCB, MCCB and ACB

In an LV system, nearly every circuit begins at a circuit-breaker. The breaker carries the load current day after day, opens the circuit when the load runs above its setting for too long, and must clear a short circuit within milliseconds without being destroyed. LV breakers come in three broad families: miniature circuit-breakers (MCBs), moulded-case circuit-breakers (MCCBs) and air circuit-breakers (ACBs). Their current ranges overlap, so picking the right one is less about the amperes on the label and more about the breaker’s position in the system, the fault current it has to handle, and how it must coordinate with the devices above and below it.

This article answers the question: what distinguishes MCBs, MCCBs and ACBs, which ratings and protection functions matter when choosing between them, and how is a breaker specified so that it suits both its circuit and the LV assembly it sits in?

What every LV circuit-breaker has to do

Whatever its size, an LV circuit-breaker performs the same four functions:

1. Carry the circuit’s design current continuously without overheating.
2. Protect against overload. It trips on a sustained current above its setting, with a time delay that gets shorter as the overload grows. This protects the cable and connected equipment from heating damage.
3. Protect against short circuit. It trips quickly on fault currents and interrupts them safely, up to the breaker’s breaking capacity.
4. Switch and isolate. It allows the circuit to be switched on and off and, where the device is declared suitable for isolation, to be isolated for safe work.

The three families meet these duties in different ways, with different construction, adjustability and fault-handling capability.

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The three breaker families

Miniature circuit-breakers (MCBs)

MCBs are the small modular breakers, usually rail-mounted, used in final distribution boards. Most MCBs are made to IEC 60898-1, which covers AC circuit-breakers for household and similar installations with a rated voltage up to 440 V (between phases), a rated current up to 125 A and a rated short-circuit capacity up to 25 000 A. They are designed to be operated by uninstructed people and to need no maintenance.

Key features:

• Fixed tripping characteristics. The overload and instantaneous trip levels are set at the factory and cannot be adjusted by the user. IEC 60898-1 defines three instantaneous tripping types: B (3 to 5 × In), C (5 to 10 × In) and D (10 to 20 × In). Type B suits resistive and long-cable circuits, type C suits general circuits with moderate inrush, and type D suits high-inrush loads such as transformers or some motors.
• Short-circuit ratings. IEC 60898-1 assigns a rated short-circuit capacity (Icn), with standard values such as 6 000 A and 10 000 A, and a service short-circuit breaking capacity (Ics), which the standard links to Icn by fixed ratios.
• Residual current variants. An MCB combined with residual current protection is an RCBO, covered by IEC 61009-1 rather than IEC 60898-1.

Some MCB-style devices are also tested to IEC 60947-2 for industrial use. In that case the IEC 60947-2 terms (Icu, Ics) apply instead of Icn. Check which standard the datasheet quotes.

Moulded-case circuit-breakers (MCCBs)

An MCCB has its contacts, arc chutes, mechanism and trip unit inside a moulded insulating case. MCCBs cover the middle of the LV current range: sub-main feeders, sub-distribution board incomers, motor feeders and the outgoing ways of main switchboards. Frame sizes typically run from tens of amperes up to around 1 600 A, but ranges vary by manufacturer and should be read from the product data.

Key features:

• Trip units can be thermal-magnetic (fixed or adjustable) or electronic, with adjustable protection functions and, in some ranges, metering and communication.
• Breaking capacity is stated as Icu and Ics to IEC 60947-2. Most ranges offer several breaking-capacity versions in the same frame, so the specification should name the level required.
• Mounting can be fixed, plug-in or withdrawable, depending on the range.

Air circuit-breakers (ACBs)

An ACB is a larger, open-construction breaker whose contacts interrupt the arc in air, assisted by arc chutes. ACBs are used where currents are highest and where continuity matters most: main incomers, bus-section and bus-coupler breakers, generator breakers and large outgoing feeders. Typical ratings start at several hundred amperes and extend to the largest LV currents. As with MCCBs, the actual range depends on the manufacturer.

Key features:

• Stored-energy mechanism for fast closing, often motor-charged, which allows remote operation and automatic transfer schemes.
• Electronic trip units with adjustable long-time, short-time, instantaneous and earth-fault protection, usually with metering, event logs and communication.
• High short-time withstand current (Icw). An ACB can typically stay closed through a fault for a stated time while a downstream breaker clears it. This is the basis of time-graded selectivity at the top of the system.
• Withdrawable execution and maintainability. Contacts and arc chutes can be inspected and serviced, and the breaker can be racked out for isolation and testing.

A note on terminology

The 2024 edition of IEC 60947-2 (Edition 6.0) removed the previous classification of circuit-breakers by interrupting medium and by design. “MCCB” and “ACB” therefore describe construction styles recognised by industry, not categories defined by the standard. The breaker’s actual capability comes from its declared ratings and utilization category, not from its family name. Two breakers of the same nominal current but from different families can behave very differently under fault conditions.

Reading a breaker’s device ratings

A circuit-breaker datasheet contains many values. The ones that drive selection are:

Rating	Symbol	What it tells you
Rated operational voltage	Ue	The system voltage at which the breaker’s performance is declared. Breaking capacity depends on voltage, so read the value at your system voltage
Rated insulation voltage / rated impulse withstand voltage	Ui / Uimp	Insulation level, related to overvoltage category and installation conditions
Rated current	In	The current the breaker carries continuously at its reference temperature. For an adjustable trip unit, the setting (Ir) can be set lower than In
Frame size	—	The physical frame, which may accept trip units of several ratings
Rated ultimate short-circuit breaking capacity	Icu	The highest prospective fault current the breaker can interrupt under the standard test sequence. After this duty it is not required to carry its rated current continuously
Rated service short-circuit breaking capacity	Ics	The fault current the breaker can interrupt and then remain in service. Assigned by the manufacturer as one of the standard percentages of Icu (25, 50, 75 or 100%), so it can be lower than Icu; read it at the relevant voltage
Rated short-circuit making capacity	Icm	The peak current the breaker can close onto. Relevant when closing onto a fault
Rated short-time withstand current	Icw	The current the breaker can carry for a stated time (for example 0.5 s or 1 s) without tripping. Declared for utilization category B breakers
Utilization category	A or B	A: no intentional short-time delay for selectivity under short-circuit conditions, and no Icw declared. B: has an intentional short-time delay and a declared Icw
Rated short-circuit capacity (MCBs, IEC 60898-1)	Icn	The MCB counterpart to Icu, with Ics linked to it by fixed ratios
Number of poles, neutral protection	—	Set by the earthing system and installation rules

Two points cause frequent confusion:

• Icu or Ics? Icu is the absolute limit of interruption. Ics shows whether the breaker can keep working after clearing a fault. Ics is a value the manufacturer declares, and it is not always 100% of Icu: it can differ between ranges, breaking-capacity versions and voltages. For main incomers and critical feeders, where a breaker must remain in service after a fault, specifications often require Ics equal to 100% of Icu. For final circuits, an Icu-based selection may be acceptable. This is a project and installation-rule decision, so state it explicitly.
• Breaking capacity is voltage-dependent. The same breaker can have a different Icu at 400 V and at 690 V. Always read the value at the actual Ue.

Protection functions and trip units

The trip unit determines how the breaker responds to overcurrent. The response is drawn as a time-current curve: time to trip on the vertical axis, current on the horizontal axis, both on logarithmic scales.

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Thermal-magnetic trip units

• A thermal element (bimetal) responds to sustained overload. Its trip time falls as current rises, an inverse-time characteristic.
• A magnetic element trips instantly above a set multiple of the rated current.
• MCBs and many smaller MCCBs use this principle. In MCCBs the thermal and magnetic settings may be fixed or adjustable within limits.

Electronic trip units

Current transformers inside the breaker feed a processor that applies adjustable protection functions:

• L — Long-time (overload): current setting Ir and a time band. This replaces the thermal element.
• S — Short-time: pick-up Isd and time delay tsd. This lets the breaker wait while a downstream device clears a fault and is the key to time-graded selectivity. It is only useful if the breaker can withstand the fault current for that delay, which is why Icw matters.
• I — Instantaneous: pick-up Ii, with no intentional delay. On a category B breaker used for selectivity, it may be set high or switched off up to the breaker’s withstand limit.
• G — Earth fault: pick-up Ig and delay tg. Used where earth-fault currents may be too low to operate the phase protection, depending on the earthing system.

Electronic units also make it easier to match the breaker to the cable exactly (Ir instead of a fixed In), and many add metering, alarms and communication.

Overload settings and the cable

The overload setting must protect the cable. In IEC 60364-4-43 terms, the design current Ib, the device setting In (or Ir) and the cable’s current-carrying capacity Iz must satisfy Ib ≤ In ≤ Iz. The current that ensures effective operation of the device must also not exceed 1.45 × Iz. The conventional tripping currents defined in IEC 60898-1 and IEC 60947-2 are set so that standard devices meet this second condition. Check the values for the product and standard concerned.

Breaking capacity: matching the fault level

A breaker must be able to interrupt the prospective short-circuit current at the point where it is installed. That current is highest at the main switchboard, set mainly by the transformer rating and impedance, the MV source and any motor or generator contribution. It falls with distance as cable and busbar impedance add up downstream (see LV Distribution Explained).

Practical consequences:

• An ACB or MCCB on the main busbar needs a much higher breaking capacity than an MCB in a sub-board fed by a long cable.
• Breaking capacity should be selected from a short-circuit calculation (IEC 60909-0 method or equivalent) for the actual system, not from a rule of thumb.
• Where a downstream breaker’s own rating is below the fault level, it may still be acceptable if a tested cascading combination with the upstream device exists (see below).
• Where the system can be supplied from more than one source (transformer, generator, or both in parallel), the fault level should be checked for each supply condition. A generator usually gives a much lower fault current than a transformer, which affects both breaking capacity and whether protection operates fast enough. See How Generators, UPS and ATS Work Together.

Coordination: selectivity and cascading

Selectivity (discrimination)

Selectivity means that, for a fault anywhere in the system, only the breaker immediately upstream of the fault opens. Everything else stays in service. Selectivity can be:

• Total: guaranteed up to the maximum prospective fault current at the downstream device.
• Partial: guaranteed only up to a stated limit current. Above that limit, both breakers may trip.

It is achieved by:

• Current grading: the upstream instantaneous pick-up is set above the maximum fault current at the downstream device. This works where cable impedance makes the downstream fault level much lower.
• Time grading: the upstream breaker has a short-time delay, so it waits for the downstream breaker. This requires a category B upstream breaker with an adequate Icw for the delay.
• Energy-based selectivity: current-limiting downstream breakers let through so little energy that the upstream breaker does not trip. This is declared only through manufacturers’ tested tables.
• Zone-selective interlocking (ZSI): communicating trip units tell each other where the fault is, so the breaker nearest the fault trips without delay and upstream breakers hold.

Selectivity between two specific devices is confirmed from the manufacturer’s selectivity tables for that pair. Overlaying catalogue curves is a useful first check but not proof, especially in the short-circuit region.

Cascading (back-up protection)

In cascading, an upstream current-limiting breaker helps a downstream breaker interrupt a fault current higher than the downstream breaker’s own breaking capacity. It allows smaller, lower-cost breakers in sub-boards close to the main switchboard. The combination must be tested and declared by the manufacturer, normally for breakers from the same manufacturer’s ranges.

Cascading and selectivity often pull in opposite directions. When the upstream breaker helps to clear the fault it may also trip, so selectivity is lost for faults in that current range. Decide deliberately, circuit by circuit, which matters more. Life-safety and critical-process circuits usually favour selectivity.

Device ratings vs LV assembly ratings (IEC 61439)

This distinction causes more specification errors than any other in LV equipment.

• Device ratings come from the breaker standard (IEC 60947-2 or IEC 60898-1): In, Icu, Ics, Icw, Icm and the trip characteristics. They are verified by type tests on the breaker under standard conditions, usually in free air.
• Assembly ratings come from IEC 61439-1 and the relevant product part (IEC 61439-2 for power switchgear and controlgear assemblies, IEC 61439-3 for distribution boards operated by ordinary persons). They include the rated current of the assembly (InA), the rated current of each circuit (Inc), the rated diversity factor (RDF), and the assembly’s short-circuit ratings: rated short-time withstand current (Icw) with its duration, rated peak withstand current (Ipk), or rated conditional short-circuit current (Icc) with the specified protective device. The assembly manufacturer assigns them and confirms them by design verification.

What this means in practice:

1. A breaker’s In is not the circuit’s rated current inside a board. In an enclosure, the air temperature around the breaker is higher than in free air and neighbouring circuits add heat. The assembly manufacturer declares the circuit’s Inc, which may be lower than the breaker’s In. A 1 600 A ACB does not by itself make a 1 600 A circuit.
2. The same symbol, a different object. “Icw” appears in both standards. For a breaker, it is the current the device can carry for a stated time. For an assembly, it is the current that the busbars, connections and supports can withstand for a stated time. Both must be adequate for the selectivity scheme and the fault level.
3. A high-Icu breaker does not make the switchboard suitable for that fault level, and a well-rated switchboard does not make its breakers suitable either.
4. Conditional ratings depend on the named device. An assembly’s Icc is valid only with the short-circuit protective device it was verified with. Substituting a different breaker can invalidate it.

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Choosing by position in the system

Position	Typical family	What usually decides the choice
Main incomer from transformer	ACB (MCCB at lower ratings)	Transformer rated current; highest fault level; Icw for time-graded selectivity; withdrawable for maintenance; often motorised for transfer or interlocking schemes
Bus-section / bus-coupler, generator incomer	ACB	Interlocking and transfer logic; fault level from each source; protection settings suited to generator fault current
Large outgoing feeders (main switchboard)	ACB or MCCB	Rated current; fault level at the main busbar; selectivity with the incomer and with downstream boards
Sub-main feeders, sub-board incomers	MCCB	Feeder design current; fault level; selectivity or cascading with upstream and downstream devices
Motor feeders	MCCB (motor-protection or instantaneous-only types) or MCB with suitable characteristic	Motor starting current; coordination with contactor and overload relay (IEC 60947-4-1); see LV Distribution Explained for MCC functional units
Final circuits (lighting, sockets, small power)	MCB or RCBO	Circuit current; tripping type (B, C, D) for the load’s inrush; fault level at the board; residual current protection where required

These are typical patterns, not rules. Current ranges overlap, and the final choice depends on fault level, coordination requirements, space, maintainability and cost.

Illustrative worked example: choosing breakers along one path

Scenario (illustrative, not a design): the system from the worked example in LV Distribution Explained: a 1 000 kVA, 11/0.4 kV transformer with 6% impedance feeding a main LV switchboard, a sub-distribution board on a 250 A feeder, and a lighting final circuit.

Assumptions: the simplified transformer-only fault estimate from LV Distribution Explained: From Transformer to Final Circuit, about 24.1 kA at the main busbar. That figure assumes an infinite MV source and ignores motor contribution and the voltage factor, so the project short-circuit study may give a higher value. The fault level at the sub-board is not calculated here. In practice it comes from the study and depends on cable size and length. Selectivity and cascading are not assumed between any devices without manufacturer data.

Step 1: Main incomer

• Transformer rated secondary current ≈ 1 443 A, so a breaker with In (or Ir) of 1 600 A is the natural choice. At this size, that usually means an ACB.
• Breaking capacity must exceed the study value at the main busbar, not just 24.1 kA. A breaker with Icu only marginally above the simplified estimate leaves no margin for motor contribution or impedance tolerance.
• Because the incomer must stay in service after a fault and hold during downstream faults, specify Ics = 100% Icu (or as the project requires) and confirm that the offered breaker declares it at the operating voltage, together with utilization category B and an Icw with a duration that covers the short-time delay planned in the selectivity study.

Step 2: Outgoing feeder to the sub-board

• Design current Ib = 200 A, so a 250 A MCCB with the cable’s corrected Iz ≥ 250 A satisfies Ib ≤ In ≤ Iz.
• The MCCB sits on the main busbar, so its breaking capacity must also match the main-busbar fault level.
• An electronic trip unit lets the overload setting match the cable and the short-time settings grade with the incomer.

Step 3: Sub-board incomer and final circuits

• The fault level at the sub-board, from the study, sets the required MCB breaking capacity. If it exceeds the MCB’s own Icn (or Icu), the MCBs can be used only in a tested cascading combination with the upstream MCCB, and the effect on selectivity must be accepted knowingly.
• For an LED lighting circuit, a type B or C MCB is typical. The choice depends on the drivers’ inrush current, which limits how many drivers one MCB can supply (see Specifying LED Luminaires).

Step 4: Check the assemblies separately

• The main switchboard’s IEC 61439 short-circuit rating (Icw/Ipk or Icc) must be at least equal to the study value at the main busbar.
• The circuit ratings Inc declared by the assembly manufacturer, not the breaker labels, confirm that the 1 600 A incomer and 250 A feeder can carry their currents inside the enclosure at the expected room temperature.

Limitations: no product data are used. All selectivity and cascading statements require the manufacturer’s tables for the actual breakers chosen.

Practical specification checklist

When requesting quotations for LV circuit-breakers, loose or within an assembly, state for each breaker or breaker type:

• Standard: IEC 60947-2 (MCCB/ACB) or IEC 60898-1 (MCB), and the edition if the project requires one.
• System data: Ue, frequency, earthing system, number of poles and whether the neutral is protected.
• Current: rated current In, and the required setting range for adjustable trip units.
• Short-circuit: required Icu and Ics (or Icn for MCBs) at Ue, based on the short-circuit study for that location. For selectivity-critical breakers, add utilization category B and Icw with duration.
• Trip unit: type (thermal-magnetic or electronic), protection functions required (L, S, I, G), metering, communication protocol, ZSI if used.
• Coordination: required selectivity (total or partial, up to which current) and any planned cascading, with the manufacturer’s tables to be provided as evidence.
• Execution: fixed, plug-in or withdrawable; manual or motor operation; shunt and undervoltage releases; auxiliary and position contacts; mechanical or electrical interlocks.
• Installation conditions: ambient temperature in the switchroom, altitude and any special environment, so that derating can be checked.
• For breakers within assemblies: the assembly ratings (InA, Inc, RDF, Icw/Ipk or Icc) specified separately under IEC 61439, plus the expected design verification and routine verification evidence.

AdepaTrade sources LV circuit-breakers and LV assemblies to project specifications from Türkiye-based and international manufacturers. It does not carry out installation. On request, AdepaTrade can arrange on-site engineering supervision and commissioning support under a separate scope.

Common mistakes

• Choosing a breaker by rated current alone, without checking breaking capacity against the prospective fault current at its location.
• Comparing Icu values from different voltages, or comparing one product’s Icu with another’s Ics.
• Assuming that an ACB or high-Icu MCCB makes the switchboard suitable for the same fault level, or treating the breaker’s In as the rated current of the circuit inside the assembly.
• Assuming selectivity because the upstream breaker is larger, without tested selectivity tables.
• Relying on cascading with devices from different manufacturers or ranges that have no declared combination.
• Specifying category A breakers at the top of a system that relies on time-graded selectivity.
• Choosing an MCB tripping type without regard to load inrush (nuisance tripping) or loop impedance (fault disconnection time).
• Forgetting that a generator supply gives much lower fault currents, so instantaneous settings chosen for the transformer supply may not operate when running on the generator.

Conclusion

MCBs, MCCBs and ACBs are three answers to the same task, sized for different places in the LV system. MCBs, to IEC 60898-1, give simple fixed protection for final circuits. MCCBs, to IEC 60947-2, cover feeders and sub-boards with fixed or adjustable trip units. ACBs, also to IEC 60947-2, handle the highest currents at the top of the system, with the short-time withstand needed for selectivity. Choosing well means reading the right ratings at the right voltage: Icu and Ics against the fault level, Icw and utilization category where selectivity depends on delay, and trip-unit settings against the cable. Coordination is confirmed from manufacturers’ tested tables. Finally, keep device ratings and assembly ratings separate. A breaker is only as good as the switchboard it is installed in, and the reverse is equally true.

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Relevant standards

IEC standards are the primary reference. Confirm the applicable edition for each project, because national wiring rules may modify or replace them.

Standard	What it covers	Why it matters
IEC 60947-2:2024: Low-voltage switchgear and controlgear, Part 2: Circuit-breakers	Circuit-breakers for use by instructed or skilled persons, including MCCBs and ACBs; Icu, Ics, Icw, Icm; utilization categories A/B	The source of the ratings compared in this article. The 2024 edition does not classify breakers by design, so specify by ratings rather than by the MCCB or ACB label
IEC 60898-1: Circuit-breakers for overcurrent protection for household and similar installations, Part 1: AC operation	MCBs for use by uninstructed persons; B/C/D characteristics; Icn and Ics	Applies to MCBs on final circuits; the B, C or D curve must suit the load and Icn must suit the fault level at the board
IEC 60947-1: General rules	Common definitions and general requirements for LV switchgear and controlgear	Provides the shared terms behind the ratings of breakers, switches and contactors
IEC 61439-1 / IEC 61439-2 / IEC 61439-3: LV switchgear and controlgear assemblies	Assembly ratings InA, Inc, RDF, Icw, Ipk, Icc; design verification	Breaker ratings alone do not rate the board: the assembly ratings and verification evidence must be specified as well
IEC 61009-1: RCBOs	MCBs with residual current protection	Relevant where a final circuit needs overcurrent and residual current protection in one device
IEC 60947-4-1: Contactors and motor-starters	Contactors and motor-starters	Relevant to coordination between the breaker and the starter on motor feeders
IEC 60364-4-43: Protection against overcurrent	Installation rules for overcurrent protection: Ib ≤ In ≤ Iz and the 1.45 × Iz condition	Links the breaker rating to the cable: the breaker must protect the cable as well as the load
IEC 60909-0: Short-circuit currents in three-phase AC systems	Calculation method for prospective short-circuit currents	The calculated fault level at each point sets the minimum breaking capacity (Icu, Ics) to specify