Short answer: Low-voltage switchgear vocabulary is small and badly taught. About twenty abbreviations cover the whole family tree — MCB, RCBO, RCCB, MCCB, ACB and the rest — and another twenty cover the numbers printed on the device: In, Icn, Icu, Ics, Ue, IΔn. Once you know which symbol answers which question, a datasheet stops being a wall and becomes a checklist. The glossary below is organised by the question each term answers, not alphabetically, because that is how you actually read a specification.
We are an independent trading company — not a manufacturer, and not an appointed distributor of any brand — and the definitions here are the ones we use when a customer sends a photo of a device instead of a code.
The families: which device does what
Every device in a low-voltage distribution board answers one of three questions: does it protect against overload and short circuit, does it protect people against earth leakage, or does it only switch. The family names encode that.
| Abbreviation | Full name | Standard | Protects against |
|---|---|---|---|
| MCB | Miniature circuit breaker | IEC 60898-1, with an IEC 60947-2 variant | Overload and short circuit, in final circuits, typically up to 63 A |
| MCCB | Moulded-case circuit breaker | IEC 60947-2 | Overload and short circuit at distribution level, with adjustable or fixed trip units |
| ACB | Air circuit breaker | IEC 60947-2 | The same, at the incoming end of a large board, with draw-out construction and a controller |
| RCCB | Residual current circuit breaker | IEC 61008-1 | Earth leakage only — no overload protection |
| RCBO | Residual current breaker with overcurrent protection | IEC 61009-1 | Earth leakage and overcurrent in one device |
| RCD | Residual current device | Generic term | The category that RCCBs and RCBOs belong to; not a product in itself |
| SPD | Surge protective device | IEC 61643-11 | Transient overvoltage, classified Type 1, 2 or 3 by the test waveform it survives |
| ATS / ATSE | Automatic transfer switch equipment | IEC 60947-6-1 | Nothing — it transfers load between two supplies |
| MPCB | Motor protection circuit breaker | IEC 60947-2 with IEC 60947-4-1 | Motor overload, short circuit and phase loss, with an adjustable thermal setting |
| Contactor | — | IEC 60947-4-1 | Nothing — it switches load, and its rating depends on the utilisation category |
Two confusions are worth killing early. An RCCB is not a “leakage breaker with overcurrent” — it has no overcurrent element at all, which is why a board built from RCCBs still needs MCBs beside them. And an ATS or a contactor is not a protective device: neither will clear a fault, and specifying one as if it would is a common error in small generator installations. The family comparison is covered at length in MCB vs RCCB vs RCBO, and the transfer-switch side in PC class vs CB class ATS.
The numbers: current, voltage and what they are not
This is where most mis-specification happens, because three currents and three voltages look interchangeable and are not.
| Symbol | Name | The question it answers |
|---|---|---|
| In | Rated current | The current the device is calibrated to carry continuously, and the number the thermal element is set to |
| Ie | Rated operational current | What a switching device can carry in a stated utilisation category — not the same as a breaker’s In |
| Ir, Isd, Ii | Long-time, short-time and instantaneous settings | The adjustable trip thresholds on an MCCB or ACB trip unit |
| Ue | Rated operational voltage | The system voltage the device may be used at; a 230/400 V device does not become a 127 V device by relabelling |
| Ui | Rated insulation voltage | The dielectric design limit — 500 V on a typical miniature breaker |
| Uimp | Rated impulse withstand voltage | The transient it survives without flashover, typically 4 kV on the same device |
| IΔn | Rated residual operating current | The earth-leakage threshold — 10, 30, 100, 300 or 500 mA |
| IΔm | Residual making and breaking capacity | The leakage current the device can make and break without damage |
| Icn | Rated short-circuit capacity (IEC 60898-1) | What a household MCB interrupts and remains serviceable afterwards — 6 kA and 10 kA are the common levels |
| Icu | Rated ultimate breaking capacity (IEC 60947-2) | The maximum fault an industrial breaker clears, after which it may not be reusable |
| Ics | Rated service breaking capacity | The fault it clears and remains in service afterwards, expressed as a percentage of Icu |
| Icw | Rated short-time withstand current | How much fault current it carries for a stated time without tripping — the number that makes selectivity possible |
The distinction that matters commercially is Icn against Icu. They are not two names for the same test: IEC 60898-1 and IEC 60947-2 define different regimes, and a device declared under one is not automatically declared under the other. On the industrial side, Icu and Ics come as a pair precisely because a breaker is allowed to be sacrificial at its ultimate rating but must remain serviceable at its service rating. That pair is why two datasheets quoting “50 kA” can describe very different devices. The two standards are compared in IEC 60898 vs 60947, and the practical consequence in 6 kA vs 10 kA.
Protection and coordination terms
Trip curve B, C, D. The magnetic band of a miniature breaker, expressed as a multiple of In. B trips magnetically at three to five times In, C at five to ten, D at ten to twenty. The letter is a statement about inrush tolerance, which is why lighting and socket circuits and motor circuits do not share one.
Type AC, A, B and F. What a residual current device can detect. AC responds to alternating leakage only; A adds pulsating direct current; B adds smooth direct current and is the type electric vehicle charging and some inverter loads require; F sits between A and B for mixed-frequency leakage. This is the specification that fails most often, because the housing looks the same across all four.
Selectivity. Coordination such that the device nearest the fault trips and the upstream device does not. It is achieved by current discrimination, by time discrimination, or by energy discrimination, and it is verified from the manufacturer’s tables rather than assumed from the rating ratio — a 100 A upstream breaker does not automatically discriminate with a 63 A downstream one. See selectivity between MCB and MCCB.
Cascading, or back-up protection. The deliberate use of a downstream device with a breaking capacity lower than the prospective fault current at its terminals, relying on the upstream device to share the interruption. It is permitted only where a tested combination is published, and “both are 6 kA” is not a tested combination.
Utilisation category. AC-1 for resistive load, AC-3 for squirrel-cage motors switched during running, AC-4 for plugging and inching duty. A contactor’s current rating is meaningless without the category, and the same frame carries very different currents under AC-1 and AC-4. See AC-1 vs AC-3 vs AC-4.
Installation and enclosure terms
Module width. The 9 mm unit that modular devices are built on, so a single-pole breaker occupies one module and a four-pole device four. Board capacity is quoted in modules and in ways, and those are different numbers.
DIN rail. The 35 mm symmetric rail that modular devices clip to. Mechanical compatibility is trivial; busbar and shroud compatibility is not.
IP rating. Two digits: solids first, water second. IP20 means finger-safe with no water protection, which is the normal rating for a device inside an enclosure, and it says nothing about the enclosure around it. See IP rating for enclosures.
Pollution degree. The environmental classification that sets creepage and clearance distances. Pollution degree 3, typical of industrial atmospheres, is why an industrial breaker cannot simply be substituted by a household device of the same current.
Ways versus modules. A “way” is a circuit position in a board; a module is a 9 mm slice of rail. A twelve-way board does not hold twelve single-pole devices if any of them are multi-pole, a residual current device, or an SPD.
Documents and marks
Type test report. The laboratory evidence behind a claimed rating, issued against a named edition of a standard and a named code. It is the document behind every other document.
Certificate of conformity. The market-specific instrument — a PVoC certificate, a SONCAP certificate, a SABER certificate, an EAC certificate — issued under a national programme. It answers a regulatory question, not a quality one, and it is quoted per exact ordering code.
Declaration of conformity. The manufacturer’s own statement against a directive or regulation, which is what the CE marking rests on for the products where a notified-body certificate is not mandatory.
CB report. A report issued under the IECEE mutual recognition scheme, useful as the technical basis for converting into national certificates in many markets — but not itself a market approval anywhere.
If a term in a datasheet is still unclear after this page, the fastest route is to send the code rather than the description: an ordering code resolves to a standard, a rating set and a certificate, while “40 amp breaker” resolves to nothing in particular. How to read an MCCB datasheet applies this vocabulary line by line, and the cross-reference maps a competitor’s code onto an equivalent. Product families are listed under MCB, MCCB and RCCB and RCBO; send your grid and destination through the enquiry form and we quote the code that works.
FAQ
Q: What is the difference between an MCB and an MCCB?
A: Both protect against overload and short circuit, but they are declared under different standards and built for different fault levels. An MCB is a modular final-circuit device, usually to 63 A, declared under IEC 60898-1 with an Icn rating. An MCCB is an industrial device declared under IEC 60947-2 with Icu and Ics ratings, adjustable trip settings and a much higher current range.
Q: Is an RCBO the same as an RCCB plus an MCB?
A: Functionally yes, physically not quite. An RCBO combines residual current and overcurrent protection in one device to IEC 61009-1, which saves a module and removes one set of terminals; an RCCB plus a separate MCB gives you two devices with independent ratings and separate replacement. The protection behaviour is comparable only if the RCBO’s overcurrent element is specified to match.
Q: What does Icu mean, and why is Ics always quoted with it?
A: Icu is the ultimate breaking capacity — the largest fault the device can interrupt, after which it may need replacing. Ics is the service breaking capacity — the fault it clears and remains usable after. Quoting Icu alone describes a device’s limit, not its behaviour.
Q: What is the difference between Type AC and Type A residual current devices?
A: Type AC detects alternating residual current only. Type A also detects pulsating direct current, which is why it is required where electronics, inverters or rectified loads are present. The physical device looks the same, so the type has to be read off the marking or the certificate.
Q: What does 6 kA on a miniature breaker actually mean?
A: It is the short-circuit current the device can interrupt and remain serviceable, declared under IEC 60898-1. Whether 6 kA is enough depends on the prospective fault current at the installation point, which comes from the supply authority or the transformer, not from the board’s total load.
Q: Why is a contactor rated in AC-3 rather than in amps?
A: Because the current a contactor can switch depends on what it is switching. AC-3 covers motor duty and is the rating used for motor starting; AC-1 covers resistive load and permits a much higher current on the same frame. A current figure without a utilisation category is not a specification.
Related: MCB vs RCCB vs RCBO · IEC 60898 vs 60947 · How to read an MCCB datasheet · AC-1 vs AC-3 vs AC-4 · Selectivity between MCB and MCCB · IP rating for enclosures · Cross-reference · MCCB range