Direct Answer: What Are Icu, Ics, Icw, and Icm?
Icu, Ics, Icw, and Icm are short-circuit performance ratings used for low-voltage circuit breakers, especially under IEC 60947-2 industrial circuit breaker practice. They describe different fault duties:
| Rating | Full name | What it answers | Typical unit |
|---|---|---|---|
| Icu | Rated ultimate short-circuit breaking capacity | What is the maximum short-circuit current the breaker can interrupt under ultimate test conditions? | kA RMS |
| Ics | Rated service short-circuit breaking capacity | At what short-circuit current is service performance verified after interruption? | kA RMS or % of Icu |
| Icw | Rated short-time withstand current | How much short-circuit current can a selectivity-category B breaker withstand for a stated delay time? | kA RMS / time |
| Icm | Rated short-circuit making capacity | What peak short-circuit current can the breaker close onto under specified conditions? | kA peak |
In practical selection, Icu must meet or exceed the prospective short-circuit current at the installation point. Ics matters when post-fault service continuity matters. Icw matters for selective coordination with intentional short-time delay. Icm matters when the breaker may close onto an existing fault.
If you are comparing miniature circuit breaker markings such as 6000, 10000, 6kA, or 10kA, read this page together with MCB Breaking Capacity: 6kA vs 10kA.
Key Takeaways
- Icu is the safety floor. It proves the breaker can interrupt a defined severe short-circuit event under ultimate breaking-capacity conditions, but it should not be treated as a service-continuity guarantee.
- Ics is the reliability rating. It is normally declared as a percentage of Icu, such as 25%, 50%, 75%, or 100%, and is critical for industrial panels where the breaker may need to remain usable after a fault.
- Icw is only relevant where short-time delay selectivity is part of the design. Category B breakers declare Icw; many Category A MCBs and standard MCCBs do not.
- Icm is a peak making rating. It is not the same type of value as Icu or Ics, which are RMS breaking values.
- The correct rating priority depends on the breaker role. A final branch breaker, motor feeder, main incomer, bus coupler, and transfer breaker do not stress the same rating in the same way.
Why IEC 60947-2 Uses Four Short-Circuit Ratings
A circuit breaker does more than one job during a fault. It may need to:
- interrupt an existing short-circuit current
- remain serviceable after clearing a fault
- carry fault current briefly while a downstream device trips first
- close onto a circuit where a fault is already present
These duties stress different parts of the breaker: contacts, arc runners, arc chutes, trip mechanism, housing insulation, terminals, and operating mechanism. One headline short-circuit number cannot describe all of that behavior. That is why IEC 60947-2 practice separates the discussion into Icu, Ics, Icw, and Icm.
This distinction is especially important for molded case circuit breakers, air circuit breakers, OEM panels, and industrial distribution boards where short-circuit performance and service continuity both matter.
What Is Icu in a Circuit Breaker?
Icu is the rated ultimate short-circuit breaking capacity. It is the maximum prospective short-circuit current that a circuit breaker can interrupt at a stated rated operational voltage under the relevant IEC 60947-2 test conditions.
In plain language, Icu answers:
Can this breaker safely interrupt the worst-case short-circuit current available at this point in the installation?
For example, if the prospective short-circuit current at a switchboard is 38 kA, a breaker with 25 kA Icu is not suitable at that point. A breaker with 50 kA Icu may be suitable from an ultimate interruption standpoint, provided the 50 kA value applies at the actual voltage, frequency, pole configuration, and installation conditions.
Icu Is Not a Reusability Promise
Icu is often misunderstood as “the breaker can clear this fault and go straight back into normal service.” That is not the right way to read it.
Icu proves ultimate interruption capability. After a severe fault near Icu level, the breaker may require inspection, testing, servicing, or replacement according to the manufacturer’s instructions and the site’s maintenance procedure. The breaker has performed its protective function, but serviceability is not the main message of Icu.
That is why industrial specifications rarely stop at Icu alone. They also review Ics.
What Is Ics and Why Is It Different from Icu?
Ics is the rated service short-circuit breaking capacity. It indicates the short-circuit current level at which the breaker is tested for service performance after interruption.
In many IEC 60947-2 datasheets, Ics is expressed as a percentage of Icu:
| Datasheet marking | How to read it |
|---|---|
| Icu = 50 kA, Ics = 25 kA | Ics is 50% of Icu |
| Icu = 50 kA, Ics = 37.5 kA | Ics is 75% of Icu |
| Icu = 50 kA, Ics = 50 kA | Ics is 100% of Icu |
The core difference is simple:
Icu asks whether the breaker can interrupt the maximum fault. Ics asks whether the breaker’s service performance is verified at a defined fault level.
The Ics Blind Spot
The most common selection error is choosing a breaker because its Icu looks large enough while ignoring Ics.
Example: a breaker marked Icu = 100 kA and Ics = 25% Icu has a service short-circuit breaking capacity of 25 kA. If it is installed on a bus where the prospective short-circuit current is much higher than 25 kA, the breaker may still clear a severe fault within its Icu rating, but the designer should not assume it remains suitable for continued service afterward.
In many industrial projects, the additional cost of a higher Ics rating is usually small compared with downtime, replacement labor, post-fault inspection, or a delayed restart. That does not mean every circuit needs Ics = 100% of Icu. It does mean Ics should be specified intentionally.
When High Ics Matters Most
High Ics, often 75% or 100% of Icu depending on project requirements and breaker family, deserves priority in:
- main distribution switchboards
- process plants and production lines
- data centers and critical infrastructure
- hospitals and emergency power systems
- OEM equipment where replacement access is difficult
- export panels where the end-user specification requires strong service continuity
For less critical final circuits, a lower Ics ratio may be acceptable if the protection strategy treats the breaker as a replaceable protective component after a major fault.
For standard selection context, see IEC 60898-1 vs IEC 60947-2 for MCBs.
What Is Icw in Circuit Breaker Selectivity?
Icw is the rated short-time withstand current. It is the RMS short-circuit current a breaker can withstand for a specified short duration without damage under stated conditions.
Icw exists because of selective coordination. In a selective system, the upstream breaker may intentionally delay tripping so the downstream breaker closest to the fault trips first. During that delay, the upstream breaker remains closed and carries fault current.
If the upstream breaker cannot withstand that current for the selected delay time, the selectivity scheme is not valid.
Category A vs Category B Breakers
IEC 60947-2 practice distinguishes breakers by selectivity category:
| Selectivity category | Short-time delay behavior | Icw relevance | Typical examples |
|---|---|---|---|
| Category A | No intentional short-time delay for short-circuit selectivity | Icw is normally not declared as a selection rating | Many MCBs and standard MCCBs |
| Category B | Designed for selectivity by providing short-time withstand current and associated delay | Icw is a key rating | ACBs and selected heavy-duty MCCBs |
This point is important for SEO and for engineering accuracy: not every IEC 60947-2 breaker will show Icw. If a breaker has no Icw value, it may simply be a Category A breaker rather than a product with “missing data.”
How to Read Icw Correctly
Icw must always be read with its time duration:
| Example marking | Meaning |
|---|---|
| Icw = 20 kA / 0.5 s | The breaker can withstand 20 kA for 0.5 seconds under specified conditions |
| Icw = 50 kA / 1 s | The breaker can withstand 50 kA for 1 second under specified conditions |
| Icw = 65 kA / 3 s | The breaker can withstand 65 kA for 3 seconds under specified conditions |
Do not compare Icw values without comparing the time. 50 kA for 0.5 s is not the same duty as 50 kA for 1 s.
For protection coordination, Icw should be reviewed together with trip-unit settings such as long-time, short-time, and instantaneous thresholds. The related guide on MCCB trip unit settings explains those settings in more detail.
What Is Icm Short-Circuit Making Capacity?
Icm is the rated short-circuit making capacity. It is the highest instantaneous peak current that a breaker can establish, or close onto, at rated voltage under specified short-circuit conditions.
Closing onto a fault can be more mechanically violent than many users expect. During an asymmetrical short circuit, the first current peak may be much higher than the RMS prospective short-circuit current. That peak creates strong electrodynamic forces that can stress contacts, latches, springs, conductors, and the breaker housing.
Icm is especially important for:
- automatic transfer schemes
- bus-tie and bus-coupler breakers
- remote reclosing
- generator-to-utility restoration
- incomers that may be closed after a fault has not yet been cleared downstream
Icm k-Factor Table
Under IEC 60947-2 practice, AC Icm is related to Icu by a standardized factor k, which depends on the short-circuit test power factor. Public engineering references commonly summarize the relationship this way:
Icm = k × Icu| Icu range | Test power factor cosφ | Icm relationship |
|---|---|---|
| 6 kA < Icu ≤ 10 kA | 0.5 | Icm = 1.7 × Icu |
| 10 kA < Icu ≤ 20 kA | 0.3 | Icm = 2.0 × Icu |
| 20 kA < Icu ≤ 50 kA | 0.25 | Icm = 2.1 × Icu |
| 50 kA ≤ Icu | 0.2 | Icm = 2.2 × Icu |
Example: if an AC circuit breaker has Icu = 100 kA, the corresponding making capacity relationship is commonly shown as Icm = 220 kA peak.
Icm = 2.2 × 100 kA = 220 kA peak
The critical point: Icm is a peak value. Icu and Ics are RMS breaking values. Do not compare them as if they were the same type of current.
All Four Ratings Side by Side
| Rating | Short name | Current type | Primary duty | Commonly declared for |
|---|---|---|---|---|
| Icu | Ultimate breaking capacity | RMS | Maximum short-circuit interruption | IEC 60947-2 circuit breakers |
| Ics | Service breaking capacity | RMS or % of Icu | Service performance after interruption | IEC 60947-2 circuit breakers |
| Icw | Short-time withstand current | RMS / time | Withstand during intentional delay | Category B breakers |
| Icm | Making capacity | Peak | Closing onto short circuit | Circuit breakers where making capacity is specified |
This table also explains why a datasheet may not show every value equally. Icu and Ics are central to industrial breaker selection. Icw becomes central when the breaker is used for short-time-delay selectivity. Icm becomes central when closing onto a fault is a realistic operating condition.
Icn vs Icu vs Ics: Why MCB Markings Look Different
Many users arrive at this topic after reading MCB markings. The confusion is understandable because different standards use different terms.
| Standard context | Common short-circuit term | Typical product context |
|---|---|---|
| IEC 60898-1 | Icn, rated short-circuit capacity | Household and similar MCBs |
| IEC 60947-2 | Icu and Ics | Industrial MCBs, MCCBs, ACBs, switchgear applications |
| UL / North American practice | Interrupting rating, AIR, SCCR context | UL-listed breakers, panelboards, industrial control panels |
An MCB marked C20 6000 may be using IEC 60898-1 style short-circuit marking. An industrial datasheet may list Icu and Ics under IEC 60947-2. These are related concepts, but not interchangeable labels.
If your project involves MCB selection, use the MCB selection guide for curve, rated current, pole count, and breaking-capacity decisions.
IEC 60947-2 vs UL 489: Do Not Convert Ratings One-to-One
For cross-region projects, it is tempting to treat IEC Icu as identical to UL interrupting rating or AIR. That is risky.
The concepts overlap because both describe short-circuit interruption capability. But IEC and UL standards use different test frameworks, markings, recovery-voltage assumptions, and certification contexts. A breaker with a certain UL interrupting rating is not automatically equivalent to a breaker with the same numerical IEC Icu value.
For export panels and multinational projects:
- specify the standard required by the project
- request certificates for the exact model and rating
- verify voltage-specific short-circuit ratings
- do not substitute IEC and UL ratings on a one-to-one numerical basis without engineering review
For US-specific terminology, see the VIOX guide on US low-voltage circuit breaker standards.
Which Rating Matters Most by Application?
| Application | First rating to verify | Second rating to verify | Third rating to verify | Notes |
|---|---|---|---|---|
| Final distribution / branch circuit | Icu or Icn | Ics, if IEC 60947-2 applies | Rated current and trip curve | Icw is usually not part of ordinary final-circuit selection |
| Motor feeder | Icu | Ics | Icm, if closing onto fault is credible | Also check motor starting current and coordination with contactor/overload relay |
| Industrial MCCB feeder | Icu | Ics | Trip-unit settings | High Ics improves post-fault service confidence |
| Main incomer | Icu | Ics | Icw, if time-delay selectivity is required | Often needs coordination study |
| ACB incomer or bus-tie | Icw | Icu/Ics | Icm | Selectivity and making duty become central |
| Transfer / automatic source switching | Icm | Icu | Ics | Closing onto an existing downstream fault is a realistic scenario |
The order changes because the breaker’s job changes. A branch breaker mainly needs correct interruption and trip behavior. A main incomer may need to delay. A transfer breaker may need to close onto an unknown downstream condition.
Complete Adequacy Check for Icu, Ics, Icw, and Icm
Use this sequence before approving a breaker for an industrial panel:
- Calculate prospective short-circuit current at the installation point. Include transformer impedance, cable impedance, upstream source contribution, and future system changes where relevant.
- Verify Icu or Icn against the calculated fault level. The rating must apply at the actual voltage and standard context.
- Add engineering margin where appropriate. A 10-20% margin is a common design practice in some projects, but it is an engineering decision, not a universal IEC rule.
- Verify Ics for service continuity. If uptime matters, do not select only by Icu.
- Verify Icw if intentional short-time delay is used. The Icw current and time must match the selectivity study.
- Verify Icm if closing onto a fault is credible. This is important for incomers, bus ties, transfer systems, and remote closing.
- Check manufacturer coordination data. Backup protection, cascading, and selectivity should be based on tested or published coordination tables, not assumptions.
For nameplate-level reading, the MCCB nameplate guide is the logical next step.
Common Mistakes When Reading Icu, Ics, Icw, and Icm
Mistake 1: Treating Icu as the Only Breaking Capacity That Matters
Icu is essential, but industrial reliability often depends on Ics. A breaker selected only by Icu may clear a fault but still require inspection or replacement before safe return to service.
Mistake 2: Assuming Ics Always Equals Icu
Ics can equal Icu, but it may also be 25%, 50%, or 75% of Icu depending on the breaker. Always read the datasheet line item.
Mistake 3: Expecting Every Breaker to Have Icw
Many Category A breakers do not declare Icw because they are not designed for intentional short-time-delay selectivity. If your protection scheme requires time delay, choose a Category B breaker with a suitable Icw value.
Mistake 4: Comparing Icm with Icu as If Both Were RMS Values
Icm is peak current. Icu is RMS breaking current. The larger number on Icm does not mean it is a larger version of Icu.
Mistake 5: Ignoring the Voltage Attached to the Rating
Short-circuit ratings are voltage-specific. A breaker may have different Icu/Ics values at different rated operational voltages. Always read the rating, voltage, pole configuration, and standard together.
Mistake 6: Mixing IEC and UL Terms Without Review
IEC Icu/Ics/Icw/Icm terminology and UL interrupting ratings are not one-to-one equivalents. Export projects need the correct certificate and datasheet for the target market.
FAQ
What is the difference between Icu and Ics?
Icu is the rated ultimate short-circuit breaking capacity. It shows the maximum short-circuit current the breaker can interrupt under ultimate test conditions. Ics is the rated service short-circuit breaking capacity. It shows the current level where service performance is verified after interruption. Icu is about ultimate interruption; Ics is about post-fault service confidence.
Is Ics always lower than Icu?
No. Ics may be lower than Icu, but it can also be equal to Icu. It is commonly expressed as a percentage of Icu, such as 25%, 50%, 75%, or 100%.
What does Icw mean on an MCCB?
Icw means rated short-time withstand current. It tells you how much short-circuit current a breaker can withstand for a specified time during a selective delay. It is mainly relevant for Category B breakers and should always be read with the stated time duration.
Why do some breakers not show Icw?
Many Category A breakers are not designed for intentional short-time-delay selectivity, so the manufacturer may not provide an Icw value. That does not automatically mean the datasheet is incomplete; it may mean the breaker is not intended for that application.
What is Icm in circuit breakers?
Icm is the rated short-circuit making capacity. It is the peak short-circuit current the breaker can close onto under specified conditions. It is important for transfer systems, bus ties, incomers, and remote closing operations.
Is Icm always 2.2 times Icu?
No. The relationship depends on the Icu range and the standardized test power factor. Public IEC 60947-2 summaries commonly show factors such as 1.7, 2.0, 2.1, and 2.2 depending on Icu range.
Do MCBs use Icu, Ics, Icw, and Icm?
Some industrial or dual-standard MCBs may list Icu and Ics under IEC 60947-2. Household and similar MCBs under IEC 60898-1 more commonly use Icn. Ordinary final-circuit MCBs usually do not use Icw as a practical selection rating.
Which rating is most important for MCCB selection?
Icu is the first safety check because it must cover the prospective short-circuit current. Ics is the key reliability check for post-fault service. Icw matters if the MCCB is used for short-time-delay selectivity. Icm matters if the breaker may close onto a fault.
Can a breaker be reused after interrupting at Icu?
Do not assume automatic reuse after an Icu-level fault. The breaker may require inspection, testing, servicing, or replacement according to the manufacturer’s instructions and site procedures. Ics gives a better indication of service-performance expectations after short-circuit interruption.
What happens if fault current exceeds Icu?
If the prospective short-circuit current exceeds the breaker’s rated ultimate breaking capacity, the breaker may fail to interrupt safely. Possible results include contact welding, case rupture, sustained arcing, or upstream device operation. The proper response is a higher-rated breaker, a tested backup protection scheme, or a redesigned protection architecture.
Related VIOX Guides
- Full Guide to Molded Case Circuit Breakers MCCBs
- How to Read MCCB Nameplates for Electrical Safety
- MCB Breaking Capacity: 6kA vs 10kA Selection Guide
- IEC 60898-1 vs IEC 60947-2 for MCBs
- MCCB Trip Unit Settings: Ir, Isd, Ii Explained
- VIOX MCCB Product Page
