Circuit Protection
MCB vs MCCB: what the difference actually means for your panel
The difference is not size, it is the standard each device is built and tested to — and that difference propagates into the trip characteristic, the short-circuit markings and everything you can do with the device in a panel.
Ask three people what separates a miniature circuit breaker from a moulded-case circuit breaker and you will get three answers about physical size. Size is a consequence, not the definition. The real distinction is that the two are built and tested to different standards, written for different kinds of user — and that difference propagates into every number on the datasheet.
The standard is the difference
IEC 60898-1 covers “circuit-breakers for overcurrent protection for household and similar installations”, and “and similar” carries weight — a great many light industrial and commercial final circuits are correctly protected by 60898 devices. The standard assumes the device will be operated by someone with no electrical training, will not be adjusted, and will not be serviced. Everything follows from that: the trip characteristic is sealed, and the marked performance is a single figure the installer compares against a calculated fault level.
IEC 60947-2 assumes the opposite. The device sits in an assembly designed by an engineer and set to co-ordinate with the devices around it, so the standard permits — and the products exploit — adjustable settings, a wider accessory range, and a more detailed statement of short-circuit performance.
Trip characteristics, and why B, C and D exist
Both families protect against two different events with two mechanisms in one housing. A sustained overload is detected by a thermal element, classically a bimetal strip, acting over seconds to minutes. A short circuit is detected by a magnetic element acting in milliseconds.
For a 60898 MCB the thermal end of the characteristic is defined by the standard, not by the manufacturer: the device must not trip within the conventional time at 1.13 times rated current, and must trip within it at 1.45 times. The conventional time is one hour up to and including 63 A, two hours above. That tight, specified band is what lets an installer treat two manufacturers’ 60898 devices as equivalents.
The magnetic end is where the letter comes from.
| Type | Instantaneous range | Typical application |
|---|---|---|
| B | 3 to 5 × In | Resistive and lighting circuits, long cable runs, low available fault current |
| C | 5 to 10 × In | General distribution, small inductive loads, lighting with driver inrush |
| D | 10 to 20 × In | Transformers, larger motors, anything with substantial magnetising or charging inrush |
The point of the letter is not sensitivity in the abstract; it is discriminating between a fault and a legitimate inrush. A transformer energised at the wrong point on the wave draws a large, brief current that is not a fault, and a Type B breaker will very likely see it as one. A Type D rides through it and still opens in milliseconds on a real fault, because real fault currents exceed even 20 × In in most installations. So the letter is an inrush question first — but you must also confirm the fault current available at that point is high enough to reach the chosen band, or the instantaneous element never operates and the thermal element clears the fault slowly instead.
IEC 60947-2 does not use the B/C/D letters. An MCCB’s characteristic is stated by the manufacturer, and on most frames both the thermal setting (Ir, the long-time pick-up) and the magnetic setting (Im, the instantaneous pick-up) are adjustable. Larger frames replace the thermomagnetic mechanism with an electronic trip unit offering long-time, short-time, instantaneous and often earth-fault elements, each with its own pick-up and delay. Those ranges are properties of the specific device: the only honest source is its datasheet and published time/current curves.
The short-circuit numbers are not the same numbers
This is the commonest source of confusion, because both families print a kA figure on the front.
A 60898 device is marked with Icn, its rated short-circuit capacity, conventionally shown in a rectangle on the case. One standard test sequence and one set of acceptance criteria mean Icn is directly comparable between manufacturers.
A 60947-2 device is marked with Icu, the rated ultimate breaking capacity, and Ics, the rated service breaking capacity. Icu is what the breaker can interrupt while remaining safe; it need not be fit for further service afterwards. Ics is what it can interrupt and still carry on working. IEC 60947-2 lists preferred values for Ics as 25 %, 50 %, 75 % or 100 % of Icu, and the ratio is a property of the individual device. A separate guide covers breaking capacity in detail; the point here is that comparing an MCB’s Icn with an MCCB’s Icu is a category error.
What else changes in practice
| Aspect | MCB (IEC 60898-1) | MCCB (IEC 60947-2) |
|---|---|---|
| Intended user | Ordinary persons; not maintained | Skilled or instructed persons |
| Protection setting | Fixed; selected by type letter and In | Adjustable thermal and magnetic; electronic trip units on larger frames |
| Short-circuit marking | Icn | Icu and Ics |
| Mounting | DIN rail, modular widths | DIN rail on small frames; plate or withdrawable on larger frames |
| Connection | Cable terminals sized for the rating | Cable lugs, busbar connection, terminal shrouds |
| Accessories | Auxiliary contacts, shunt trip, undervoltage release | The same, plus motor operators, door-interlocked handles, earth-fault modules |
| Typical position | Final circuits, sub-distribution | Incomers, sub-mains, motor feeders, transfer schemes |
Discrimination and cascading
Discrimination (selectivity) means that on a fault only the breaker immediately upstream of it opens, leaving the rest of the installation live. Achieving it needs the upstream characteristic to sit clear of the downstream one across the whole current range of interest, which usually means an upstream device with an adjustable short-time delay. Manufacturers publish discrimination tables for their own device pairs, and those tables are the authority — not a visual comparison of two curves.
Cascading (back-up protection) is the opposite move: installing a downstream device where the prospective fault current exceeds its own breaking capacity, because an upstream current-limiting device acts with it to clear the fault. It is valid only for combinations the manufacturer has type-tested and published. It is not a calculation, and it does not transfer between manufacturers or between ranges.
Choosing between them
- Design current and cable capacity. The breaker’s rated current sits between the two, with the cable protected across the whole disconnection time.
- Prospective short-circuit current at that point. This sets the minimum breaking capacity. If the figure exceeds what the modular ranges offer, the answer is an MCCB regardless of load current.
- Does the protection need setting or co-ordinating? If it must discriminate with something downstream, or be tuned on commissioning, it needs an adjustable characteristic.
- What does the inrush look like? This sets the MCB type letter, or the MCCB’s magnetic setting.
- What connects to it, at what ambient? Busbar connection, large cables, remote operation, door interlocking and an undervoltage release all push towards an MCCB frame; both families derate above their reference ambient.
How to check this on the product page
Every breaker on this site carries a specification table under the product image. The fields that answer the questions above are:
- Standard — tells you which family you are looking at, and therefore how to read the rest.
- Rated current (In) and number of poles.
- Tripping characteristic — the B/C/D letter, or the trip-unit type.
- Breaking capacity — read the label carefully: Icn, or Icu and Ics.
- Rated operational voltage (Ue), since breaking capacity is quoted against a voltage.
Each value is a link: clicking one filters the catalogue to every product sharing it. Where a figure is absent we do not hold a verified value for it — take it from the manufacturer’s datasheet and ask us if you would like it confirmed. Add your shortlist to a quote request; the sales desk confirms stock, lead time and price against live availability.
A closing note on responsibility
This guide explains what the ratings mean. It cannot select a device for your installation. The prospective fault current at each point, the disconnection times required by the installation standard in force, cable sizes and thermal withstand, the discrimination scheme, the earthing arrangement and the environment are all properties of a specific design. Final selection is the responsibility of the designing engineer, working from actual installation data and the manufacturer’s published data for the specific device. RMS can assist with selection, supply datasheets and check a proposed combination against the manufacturer’s published tables — but that assistance supports competent design, it does not replace it.