A circuit breaker is a switch that opens by itself when the current becomes dangerous for the circuit it protects, and that can be reset instead of replaced like a fuse. In industrial low-voltage installations you will meet three broad families: miniature circuit breakers (MCB), molded-case circuit breakers (MCCB) and, for the largest currents, insulated-case and air circuit breakers (ICCB / ACB). This practical guide explains how they differ, what the trip unit does, which ratings matter and how to read a breaker label before you order a replacement.
How a breaker protects a circuit
Every breaker has to do three things: carry the normal load current without overheating, detect an abnormal current, and open its contacts fast enough to interrupt that current safely. Two kinds of fault are covered:
- Overload: a current somewhat above the rating that lasts too long, for example a jammed conveyor or too many loads on one circuit. The breaker trips after a delay that gets shorter as the overload gets larger.
- Short circuit: a very high current caused by a fault between conductors or to ground. The breaker must trip almost instantly and extinguish the arc between its contacts.
The part of the breaker that decides when to trip is the trip unit (or release). The part that actually interrupts the current is the contact system with its arc chutes. A breaker's ability to interrupt a fault without being destroyed is its breaking capacity (IEC) or interrupting rating (UL/CSA).
The three families at a glance
| Family | Typical current range | Mounting | Trip unit | Typical use |
|---|---|---|---|---|
| MCB (miniature) | Fractions of an amp up to about 100-125 A | DIN rail or plug-in panelboard | Fixed thermal-magnetic, curve B, C, D... | Control circuits, lighting, receptacles, small motors, panel loads |
| MCCB (molded case) | From about 15 A up to a few thousand amps, by frame | Panel, bolted or plug-in, sometimes withdrawable | Thermal-magnetic (fixed or adjustable) or electronic | Feeders, machine mains, motor circuits, distribution panels |
| ICCB / ACB (insulated case / air) | Large frames, typically several hundred to several thousand amps | Fixed or drawout in switchgear and switchboards | Electronic, fully adjustable, often with metering and communication | Service entrance, main switchboards, large feeders, generator breakers |
Ranges are typical and vary by manufacturer and series; check the manufacturer's data.
Miniature circuit breakers (MCB)
MCBs are small, modular breakers, usually 18 mm wide per pole on a DIN rail. Their trip characteristics are fixed: the rated current and the curve letter (B, C, D, and on some products K or Z) are set at the factory. The curve defines the magnetic trip threshold as a multiple of the rated current, which is why a C-curve breaker is used where moderate inrush is expected and a D-curve breaker for transformers or motors with high inrush. See Miniature Circuit Breaker Trip Curves B, C and D Explained for details.
In North America, check whether a DIN-rail device is a branch-circuit breaker or a supplementary protector: see Branch Circuit Breakers (UL 489) vs Supplementary Protectors (UL 1077).
Molded-case circuit breakers (MCCB)
An MCCB has its mechanism and contacts enclosed in a molded insulating housing. It is built on a frame (the housing and contact system) that comes in several sizes, and each frame accepts a range of current ratings. Many MCCBs offer adjustable settings and accessories such as shunt trips, undervoltage releases, auxiliary and alarm contacts, motor operators and rotary handles for door mounting.
Insulated-case and air circuit breakers (ICCB / ACB)
At the top of the range, breakers are designed to be maintained and are often drawout units racked into switchgear. In IEC practice these are air circuit breakers covered by IEC 60947-2. In North America, insulated-case breakers are evaluated to UL 489 like MCCBs, while low-voltage power circuit breakers follow UL 1066 and the ANSI/IEEE C37 series. These large breakers usually have a stored-energy spring mechanism, an electronic trip unit and a short-time withstand rating that lets a downstream breaker clear a fault first.
The three breaker families at a glance (typical ranges).
Trip units: thermal-magnetic vs electronic
Thermal-magnetic
A bimetal strip heats up with the load current and bends until it releases the latch: this is the overload (thermal) element, with an inverse-time delay. A coil or magnetic yoke pulls the latch instantly when the current exceeds a high threshold: this is the short-circuit (magnetic) element. On MCBs both are fixed; on MCCBs the thermal and sometimes the magnetic setting can be adjusted on the front.
Electronic
Current transformers measure the current and a small processor compares it to programmable settings. The usual protection functions are named by letter:
- L (long time): overload protection, set as Ir with a time band.
- S (short time): delayed short-circuit protection, set as Isd with a delay, used for selectivity.
- I (instantaneous): undelayed short-circuit protection, set as Ii.
- G (ground fault): protection against ground-fault currents, set as Ig.
A trip unit described as "LSI" or "LSIG" has those functions. Electronic units give more precise settings, less sensitivity to ambient temperature and, on many models, current metering and trip history.
Thermal-magnetic versus electronic (LSI) trip characteristics, schematic.
Ratings you need to know
| Symbol | Meaning | Where you see it |
|---|---|---|
| In | Rated current: the current the breaker carries continuously under the reference conditions | All breakers |
| Ir | Long-time (overload) setting on an adjustable breaker, often a fraction of In | MCCB, ACB |
| Icn | Rated short-circuit capacity of a household-type MCB under IEC 60898-1 | MCB |
| Icu | Rated ultimate short-circuit breaking capacity under IEC 60947-2: the largest fault the breaker can interrupt, after which it may need replacing | MCB, MCCB, ACB |
| Ics | Rated service short-circuit breaking capacity: the fault the breaker can interrupt and still remain in service, stated as a percentage of Icu | MCB, MCCB, ACB |
| Icw | Rated short-time withstand current: the fault current the breaker can carry for a stated time without tripping, used for selectivity | ACB and some large MCCBs |
| Ue, Ui, Uimp | Rated operational voltage, insulation voltage and impulse withstand voltage | IEC-rated breakers |
| Interrupting rating (kA at V) | UL/CSA short-circuit interrupting rating, given at each marked voltage | North American breakers |
Two points are often missed. First, breaking capacity depends on voltage: the same breaker usually has a lower kA rating at 480 V than at 240 V. Second, the interrupting rating must be at least equal to the prospective short-circuit current at the point where the breaker is installed. If you are not sure what that current is, ask the person responsible for the installation's short-circuit study.
Frame size vs rating
The frame size (or frame rating) is the largest current rating the frame can carry. A 250 A frame, for example, may be offered with trip ratings from a fraction of that value up to 250 A. In UL catalogs the frame is often a letter or a family code inside the catalog number; in IEC catalogs it may appear as a rated uninterrupted current (Iu) or a frame designation. When you replace a breaker, the frame matters for the physical fit (mounting, terminals, handle, accessories) and the rating matters for the protection. Matching only one of the two is not enough.
How to read a breaker label
- Manufacturer and full catalog number. This is the key to everything else. Copy it completely, including suffixes: they often encode the trip unit, the terminals, the voltage or the approvals.
- Number of poles (1, 2, 3 or 4) and whether the neutral is protected.
- Rated current In, or the trip unit rating and its adjustment range.
- Curve letter on MCBs (B, C, D...).
- Voltage ratings: AC and/or DC, maximum voltage, frequency.
- Short-circuit ratings: Icn or Icu/Ics for IEC, kA at each voltage for UL/CSA.
- Standards and marks: IEC 60898-1, IEC 60947-2, UL 489, UL 1077, CSA C22.2 No. 5 and so on.
- Terminals: wire range, copper or aluminum, tightening torque, line and load marking.
- Reference temperature for the thermal calibration, which affects the actual trip point in a hot cabinet.
Work on electrical equipment must be done by qualified personnel following the applicable electrical code and the manufacturer's instructions.
Key takeaways
- MCBs are small, fixed-setting breakers for control and distribution circuits; MCCBs cover feeders and machine mains with adjustable options; ICCB/ACB units protect the largest circuits.
- Thermal-magnetic trip units are simple and fixed or semi-adjustable; electronic trip units offer L, S, I and G settings.
- Check In, the curve or settings, the voltage and the short-circuit rating at the installation voltage.
- Frame size and current rating are two different things.
- Always order by the full catalog number shown on the label.
Related parts at 3JIndustry
Find used MCBs, MCCBs and accessories in our Breakers collection, from brands such as Schneider / Telemecanique. All parts are used and covered by a 30-day warranty, with returns accepted within 30 days of delivery.
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