Icu vs Ics vs Icw

Icu is the maximum prospective fault current a circuit breaker is rated to interrupt under its ultimate breaking-capacity test sequence. Ics is the fault current it is rated to interrupt under the service breaking-capacity sequence, where continued service capability is verified. Icw is a different quantity altogether: the RMS fault current that equipment can carry for a stated short duration — typically 1 second — without unacceptable damage. Icm covers closing onto an existing fault.

The consequence is that "50 kA panel" is not yet a specification. Until the document states whether 50 kA is the prospective fault current at the installation point, the breaker Icu, the breaker Ics, an Icw with its duration, or the short-circuit withstand of the complete assembly, four different panels can all be quoted against the same line — at four different prices.

The Question Behind “415 V, 2500 A, 50 kA”

A customer sends an RFQ stating:

“415 V PCC Panel, 2500 A, 50 kA.”

At first glance, this appears clear.

But from an engineering point of view, an important question remains:

What exactly does 50 kA mean?

Does it mean:

• The incoming ACB must have 50 kA Icu?
• The ACB must have 50 kA Ics?
• The breaker must withstand 50 kA for 1 second?
• The main busbar assembly must withstand 50 kA for 1 second?
• The entire panel must have an applicable short-circuit withstand rating?
• The prospective fault current at the panel location is 50 kA?
• Or has someone simply copied “50 kA” from an older project specification?

These are not the same requirement.

Understanding the difference between Icu, Ics, Icw, Icm and the short-circuit withstand rating of the complete assembly is essential when specifying PCC, MCC, PMCC and other LT panels.

This guide explains these terms practically for consultants, EPC engineers, plant engineers, procurement teams, panel builders and electrical designers.

Quick Answer: Icu vs Ics vs Icw

The simplest way to remember the three terms is:

Rating	What It Basically Tells You
Icu	Maximum short-circuit current the circuit breaker is rated to interrupt under its ultimate breaking-capacity test
Ics	Short-circuit current the breaker is rated to interrupt under the service breaking-capacity test sequence, with continued service capability verified according to the product standard
Icw	RMS short-circuit current the equipment can carry for a specified short duration without unacceptable damage
Icm	Peak short-circuit current the device can make/close onto under specified conditions
Assembly short-circuit withstand	Capability of the complete switchboard/busbar assembly under the applicable IEC 61439 verification conditions

IEC 60947-2:2024 is the current IEC product standard covering applicable low-voltage circuit breakers.

The most important point is:

Breaker breaking capacity and complete panel short-circuit withstand are related, but they are not the same rating.

First Understand What a Short Circuit Does to an LT Panel

Under normal operation, an LT panel may carry:

800 A

1600 A

2500 A

or

4000 A

continuously.

During a short circuit, current may suddenly increase to tens of thousands of amperes.

For example:

50 kA = 50,000 amperes RMS

That fault current creates two major stresses.

1. Thermal Stress

High current produces intense heating.

The heating effect increases approximately with current squared and fault duration.

Therefore, the difference between:

50 kA for 0.1 second

and

50 kA for 1 second

is extremely important.

2. Electrodynamic / Mechanical Stress

Short-circuit current also produces large electromagnetic forces between conductors.

The peak current during the initial part of the fault can be significantly higher than the RMS value.

These forces can stress:

• Busbars
• Supports
• Joints
• Breaker connections
• Insulators
• Structural parts

That is why short-circuit design is not simply about making a thicker copper busbar.

The complete current path and mechanical support system matter.

What Is Prospective Short-Circuit Current?

Before selecting breakers or switchboard ratings, determine the prospective short-circuit current at the location where the panel will be installed.

This is the fault current that the electrical system could supply under the specified fault condition before the protective device interrupts it.

It depends on factors such as:

• Utility/network strength
• Transformer rating
• Transformer impedance
• Number of transformers
• Parallel sources
• Generator contribution
• Cable impedance
• Busduct impedance
• Motor contribution
• System voltage
• Operating configuration

Therefore:

Fault level belongs to the electrical system.

The breaker and panel ratings are then selected to safely handle that fault condition according to their applicable requirements.

Example: Why Transformer Size Matters

Consider a transformer feeding an LT PCC panel.

If transformer impedance is relatively low, available fault current at its LV terminals can be high.

If transformer impedance is higher, available fault current generally reduces.

But transformer-only calculation may not be enough where the system contains:

• Parallel transformers
• Utility contribution
• DG sets
• Large motors
• Bus couplers
• Multiple operating configurations

A worked figure helps here. On a 1000 kVA, 415 V transformer at approximately 5% impedance, full-load current is around 1390 A and the symmetrical fault current at the LV terminals works out to roughly 28 kA from the transformer alone — before any utility, generator or motor contribution is added. That is why a 50 kA specification on a single 1000 kVA incomer usually tells us the number was inherited from an older document rather than calculated. The actual figure must still come from the project short-circuit study.

For an important industrial installation, use the project's short-circuit study rather than relying on a generic transformer approximation.

What Is Icu?

Icu = Rated Ultimate Short-Circuit Breaking Capacity

Icu is a circuit-breaker characteristic defined under IEC 60947-2.

It indicates the maximum prospective short-circuit current that the breaker is rated to interrupt under the specified ultimate breaking-capacity test sequence.

For example:

A breaker may be marked:

Icu = 65 kA at 415 V

That means its ultimate short-circuit breaking capacity at that stated voltage is 65 kA under the applicable standard conditions.

Important: Icu Depends on Voltage

A breaker does not necessarily have one universal Icu value at every voltage.

The same MCCB may have different short-circuit breaking capacities at:

• 380 V
• 415 V
• 440 V
• 500 V
• 690 V

Therefore, never specify only:

“Breaker Icu = 65 kA.”

Also verify:

At what operating voltage?

A 65 kA rating at 415 V does not automatically mean the breaker has the same rating at 690 V.

Does an Icu Test Mean the Breaker Must Remain Fully Serviceable Afterwards?

This is where the distinction between Icu and Ics becomes important.

Icu represents the breaker's ultimate breaking-capacity performance under the relevant IEC 60947-2 test sequence.

Ics represents its service short-circuit breaking capacity under a different, more demanding sequence.

Therefore, for a critical installation:

Do not look only at Icu.

Look at Ics too.

What Is Ics?

Ics = Rated Service Short-Circuit Breaking Capacity

Ics indicates the level of fault-current interruption at which the breaker completes the service breaking-capacity test sequence specified by IEC 60947-2.

Ics is commonly expressed as:

• 25% of Icu
• 50% of Icu
• 75% of Icu
• 100% of Icu

depending on the breaker design and product range.

For example:

Breaker A

Icu = 50 kA

Ics = 25 kA

Breaker B

Icu = 50 kA

Ics = 50 kA

Both breakers have the same headline:

50 kA Icu

But their service short-circuit performance is different.

That difference may be important in applications where continuity and post-fault serviceability matter.

Why Ics Matters in Industrial Plants

Imagine an MCCB experiences a major fault.

The breaker successfully interrupts it.

That is obviously the first requirement.

But the plant engineer's next question is:

What condition is the breaker in after clearing that fault?

In critical applications such as:

• Process plants
• Refineries
• Utilities
• Data centers
• Continuous manufacturing
• Infrastructure facilities

the service breaking-capacity characteristic can influence breaker selection.

Therefore:

Two breakers with the same Icu should not automatically be treated as technically identical.

Compare Ics too.

Example: Icu vs Ics

Suppose prospective fault current at a feeder location is:

32 kA

Option 1:

Icu = 50 kA

Ics = 25 kA

Option 2:

Icu = 50 kA

Ics = 50 kA

Both have an Icu higher than the prospective 32 kA fault current.

However, their service breaking-capacity ratings differ.

Whether the project requires Ics to equal or exceed the prospective fault level depends on the specified design philosophy, applicable rules and required continuity.

The important point is that procurement should not compare both breakers only using:

“50 kA breaker.”

What Is Icw?

Icw = Rated Short-Time Withstand Current

Icw answers a different question.

Instead of:

“How much fault current can the breaker interrupt?”

Icw asks:

“How much RMS fault current can the equipment carry for a specified short time without unacceptable damage under the applicable conditions?”

For example:

Icw = 50 kA / 1 s

means something very different from:

Icu = 50 kA

Why Is Icw Important?

Icw becomes particularly important when selectivity is required.

Consider:

Main ACB → Busbar → Downstream MCCB

A short circuit occurs on a downstream feeder.

Ideally:

The downstream MCCB should trip first.

The upstream ACB may intentionally have a short-time delay to allow the downstream breaker to clear the fault.

During this delay, the upstream ACB and associated current path must withstand the fault current.

That is where short-time withstand capability becomes important.

Example: Selective Protection

Suppose:

• Main ACB = 3200 A
• Downstream ACB/MCCB = 800 A
• Available fault current = 45 kA
• Upstream short-time delay = 0.3 seconds

During the downstream fault, the main ACB may remain closed intentionally for that short period.

Therefore, asking only:

“Main ACB Icu?”

is incomplete.

The protection engineer must also consider:

Can the upstream device safely withstand the fault during the intentional delay?

That is a key reason Icw matters in ACB applications.

Icu and Icw Are NOT Interchangeable

Suppose a breaker has:

Icu = 65 kA

but:

Icw = 50 kA for 1 second

This does not represent a contradiction.

It means:

• The breaker has a defined ultimate interruption capability.
• Its short-time carrying capability has a separate rating.

Different functions.

Different test requirements.

Different engineering meaning.

Why MCCB and ACB Specifications Often Look Different

Air circuit breakers used as main incomers are frequently required to provide:

• Short-time delay
• Selective coordination
• High short-time withstand

Therefore, Icw can be a major specification parameter.

MCCBs are often designed to clear high fault current rapidly and may use strong current-limiting behaviour rather than intentionally carrying very high fault current for long delays.

Icw is particularly important for ACB applications, while the design approach for many MCCBs emphasizes rapid opening and current limitation.

That is why you should not select an MCCB and ACB using exactly the same mental checklist.

What Is Icm?

Icm = Rated Short-Circuit Making Capacity

Consider a breaker that is currently open.

Unfortunately, a short circuit already exists downstream.

Someone—or an automatic control system—commands the breaker to CLOSE.

The breaker now closes directly onto the fault.

At the instant of making, the current can reach a large asymmetrical peak.

Icm addresses the breaker's ability to make onto that short circuit under specified conditions.

RMS kA and Peak kA Are Not the Same

This is another common source of confusion.

When someone says:

“Fault level = 50 kA”

they are usually referring to an RMS symmetrical short-circuit current.

But the instantaneous peak current during the initial fault can be significantly higher.

That peak matters because electromagnetic force increases strongly with current.

This is why:

• Busbar supports
• Bracing
• Breaker connections
• Enclosure structure

must be able to deal with the mechanical forces associated with short-circuit conditions.

Do not compare a peak-current value directly with an RMS value as if they are the same quantity.

Now the Most Important Question: What Does “50 kA Panel” Mean?

This phrase is widely used.

But technically it is incomplete unless the context is clear.

When a consultant writes:

“PCC shall be 50 kA rated”

the panel manufacturer should understand or clarify:

Is it:

50 kA prospective fault current at the panel location?

Or:

50 kA breaker Icu?

Or:

50 kA breaker Ics?

Or:

50 kA Icw for 1 second?

Or:

50 kA short-circuit withstand of the complete assembly?

These requirements need to be coordinated, not assumed.

Breaker Rating Does Not Automatically Prove the Complete Panel Rating

This is one of the most important points in this article.

Suppose the incoming ACB has:

Icu = 100 kA

Does that automatically make the switchboard:

100 kA rated?

No.

The complete assembly contains much more than the ACB:

• Main busbars
• Vertical busbars
• Connections
• Supports
• Joints
• Functional units
• Protective conductor
• Enclosure structures

IEC 61439-1:2020 provides the general construction, technical-characteristic and verification framework for LV assemblies, while IEC 61439-2:2020 establishes the specific requirements for power switchgear and controlgear assemblies.

Therefore:

A high-breaking-capacity breaker installed inside a weaker assembly does not magically increase the short-circuit capability of the complete switchboard.

We have opened older water-treatment and process-plant boards where the incomer was a high-frame ACB, but the vertical dropper busbars ran on insulator supports spaced far wider than the assembly design intended, with tap-off joints left on a single bolt. Nothing in that arrangement was visible from the breaker nameplate. The mechanical forces during a fault act on the supports and joints, so the weakest part of the current path sets the real capability of the board — not the strongest component in it.

Circuit-Breaker Standard vs Panel Assembly Standard

This distinction should always be clear.

Circuit Breaker

Relevant international product standard:

IEC 60947-2:2024 — Low-voltage switchgear and controlgear — Circuit-breakers.

This is where circuit-breaker characteristics such as Icu, Ics, Icw and related performance are addressed.

Complete LT Panel / PSC Assembly

Relevant assembly standards include:

IEC 61439-1:2020 — General Rules

and

IEC 61439-2:2020 — Power Switchgear and Controlgear Assemblies.

This distinction prevents a common procurement mistake:

Component compliance ≠ automatic assembly compliance.

Indian Standards: What Should Indian Projects Reference?

For India, BIS currently lists:

IS/IEC 61439 (Part 2):2020

for power switchgear and controlgear assemblies.

For low-voltage circuit breakers, BIS currently lists:

IS/IEC 60947 (Part 2):2016

which was reviewed in 2022.

This creates an important practical point for Indian specifications:

The current international IEC edition of IEC 60947-2 is 2024, while the BIS catalogue currently lists the Indian adoption as IS/IEC 60947 Part 2:2016.

Therefore, specifications should clearly state which standard/version is contractually applicable instead of simply writing:

“As per latest IEC/IS.”

What Is Short-Circuit Withstand of the Complete Assembly?

For an LT switchboard, the current path includes much more than the breaker.

Consider:

ACB terminal → connection busbar → main busbar → vertical busbar → outgoing device

Every part of this path experiences fault stress.

The assembly design must therefore account for:

• Thermal stress
• Electrodynamic stress
• Busbar support strength
• Joints
• Conductor spacing
• Protective circuits
• Installed components
• Assembly construction

IEC 61439 requires applicable design verification of the assembly rather than simply assuming performance from individual component labels.

50 kA for 1 Second vs 50 kA for 3 Seconds

These are very different requirements.

A busbar system carrying:

50 kA for 1 second

experiences substantial thermal energy.

Requiring the same system to carry:

50 kA for 3 seconds

creates a significantly greater thermal duty.

Therefore never write only:

“Busbar withstand = 50 kA.”

Specify the duration if a short-time withstand rating is required.

Examples:

50 kA RMS for 1 second

or

65 kA RMS for 1 second

according to the actual project requirement and verified assembly system.

Do not automatically specify 3 seconds because it sounds safer.

A higher-duration requirement can materially influence:

• Busbar size
• Support design
• Panel system
• Cost

and may provide no useful benefit if the protection system clears faults much faster.

In our experience, a 3-second requirement on an LT assembly is usually copied across from an HT switchgear specification, where longer durations are conventional. When we query it, the protection philosophy on the same project often shows an upstream short-time delay of 0.2 to 0.4 seconds — so the busbar and support system is being sized for a duty roughly an order of magnitude beyond what the relay settings will ever ask of it. Our recommendation: define the short-time withstand duration from the actual selectivity study, and specify 1 second unless the study genuinely demands more.

Why Protection Settings Affect Short-Circuit Design

Short-circuit rating and protection coordination should not be designed independently.

Consider an ACB with an intentional short-time delay.

Increasing that delay may improve downstream selectivity.

But the fault now exists for longer before the upstream breaker operates.

Therefore, protection engineers balance:

Selectivity

against

Equipment withstand

and

Fault-clearing time

The protection study should check the withstand capability of:

• Busbars
• Cables
• Breakers
• Transformers
• Other equipment

against the proposed protective-device settings.

What Is Current Limitation?

Some circuit breakers can limit the peak and energy that pass downstream during a short circuit by opening extremely rapidly.

This is known as current-limiting behaviour.

It can reduce stress on downstream equipment.

However, any use of:

• Cascading
• Back-up protection
• Current limitation

must be based on properly documented manufacturer-tested or manufacturer-published combinations and applicable standards.

Do not assume:

“Upstream breaker is 100 kA, so every downstream 25 kA breaker is automatically safe.”

The combination must be validated.

What Is Cascading or Back-Up Protection?

In some coordinated systems, an upstream protective device can increase the effective short-circuit protection capability of a downstream device through a tested combination.

This can sometimes allow a downstream breaker with a lower standalone breaking capacity to be used where the prospective fault current is higher than that device's individual rating.

But this is not a calculation to guess.

Use the switchgear manufacturer's published:

• Back-up tables
• Cascading tables
• Tested combinations
• Voltage-specific data

and follow the project specification.

For critical industrial PCC/MCC systems, this should be deliberately engineered and documented.

Icu vs Ics vs Icw — Practical Example

Consider a main LV board with:

System voltage: 415 V

Transformer: 2500 kVA

Prospective fault current at main board: 48 kA

Required assembly withstand: 50 kA for 1 second

A proposed ACB has:

Icu = 65 kA at 415 V

Ics = 65 kA at 415 V

Icw = 50 kA for 1 second

At first review:

• Icu exceeds the prospective fault current.
• Ics exceeds the prospective fault current.
• Icw matches the specified short-time duty.

But the engineering review is still not finished.

You must also verify:

Does the complete switchboard assembly have applicable short-circuit verification for the required duty?

Because breaker performance alone does not verify:

• Main busbar
• Connections
• Supports
• Vertical distribution
• Assembly construction

That final step is frequently missed.

Another Example: A High-Icu Breaker Does Not Fix a Low-Rated Panel

Suppose:

Breaker

Icu = 100 kA

Assembly system

Applicable short-circuit withstand = 50 kA / 1 s

Project prospective fault current

65 kA

Can we call the panel:

100 kA rated because the breaker is 100 kA?

No.

The complete assembly must be suitable for the actual fault requirement under the relevant assembly design and verification framework.

This is why procurement should never accept a panel's short-circuit claim only by reading the breaker label.

What About Incoming and Outgoing Breakers?

Not every breaker in a panel necessarily sees the same prospective fault current.

Fault current reduces as system impedance increases.

A main incomer near a transformer may experience a high fault level.

A downstream feeder at the end of a long cable may experience considerably less.

Therefore, technically optimized systems may use different breaking capacities in different locations.

However, this should come from the short-circuit study.

Do not reduce breaking capacity simply to save cost without calculation.

Bus Couplers Can Change the Fault Level

Suppose the plant has:

Transformer A → Bus A

and

Transformer B → Bus B

with a bus coupler.

When the bus coupler is open, the fault contribution may come mainly from one source.

If the system permits both transformers to operate in parallel, the fault level can increase.

Therefore, the short-circuit study should evaluate all permitted operating conditions.

A switchboard that is adequate with the bus coupler normally open may not necessarily be adequate if both sources are paralleled.

This is especially relevant for:

• Large industrial plants
• Data centers
• Refineries
• Utilities
• Multi-transformer substations

We have reviewed pharma and chemical plant SLDs where two 1000 kVA transformers each contributed approximately 28 kA, and the panel specification asked for 32 kA. That figure is adequate while the bus coupler stays open, but not for the brief paralleled condition the same SLD permitted during changeover. Our recommendation: state the bus-coupler operating philosophy — open transition, closed transition, or sustained parallel — on the same sheet as the kA figure. This is the item we push back on at clarification stage, because the answer can change the busbar system, not just the breaker selection.

DG Contribution Must Also Be Considered

Generator-based systems can also contribute fault current.

The magnitude and decay characteristic differ from a utility transformer source.

Where DGs can operate:

• In parallel with other generators
• In parallel with utility
• Through a common emergency bus

the short-circuit study should evaluate those configurations.

Do not calculate only the transformer fault level and ignore permitted generator operating conditions.

Motor Contribution to Fault Current

Large running induction motors can temporarily contribute current into a nearby short circuit.

In plants with many large motors, this contribution can affect the initial fault level.

A proper short-circuit study should therefore model major motor contributions where relevant.

Again, this reinforces an important principle:

The panel manufacturer should be given the calculated fault level, not asked to guess it from transformer kVA alone.

What Should a Consultant Put in an LT Panel Specification?

Instead of writing:

“Panel shall be 50 kA.”

a better specification may identify:

• Prospective short-circuit current at the panel location
• Required assembly short-time withstand rating
• Required duration
• Breaker Icu requirement
• Breaker Ics requirement where specified
• Breaker Icw requirement where applicable
• Required peak/making capability
• Applicable IEC/IS standards
• Required design-verification evidence
• Protection/selectivity requirements

For example:

“The main LV switchboard shall be suitable for the calculated prospective short-circuit current at the installation point. The assembly shall have applicable short-circuit withstand verification for not less than 50 kA RMS for 1 second, with associated peak withstand capability in accordance with the applicable IEC 61439 requirements. Installed circuit breakers shall have breaking and short-time withstand characteristics suitable for the calculated fault level and approved protection/selectivity study.”

This is much more technically meaningful than:

“All breakers shall be 50 kA.”

What Should the Buyer Ask the Panel Manufacturer?

During technical evaluation, ask:

1. What is the panel's short-circuit withstand rating?

Not just the breaker rating.

2. For what duration?

For example:

1 second?

3. What design-verification evidence supports that rating?

Ask what assembly system/configuration the evidence covers.

4. Does the proposed panel remain within that verified design?

This is important if the manufacturer has changed:

• Busbar dimensions
• Busbar spacing
• Support arrangement
• Enclosure
• Connections
• Device installation

5. What are the incomer breaker's Icu and Ics at the actual operating voltage?

Not at another voltage.

6. What is the breaker Icw?

Particularly for selective main incomers.

7. Are downstream breaker ratings suitable for their calculated fault levels?

8. Is cascading/back-up being used?

If yes, ask for the manufacturer's applicable combination data.

How to Read a Breaker Nameplate

A modern industrial ACB or MCCB may show several ratings.

For example:

Ue — Rated operational voltage

Ui — Rated insulation voltage

Uimp — Rated impulse withstand voltage

In / Ie — Current-related ratings depending on device marking

Icu — Ultimate short-circuit breaking capacity

Ics — Service short-circuit breaking capacity

Icw — Short-time withstand current

Icm — Short-circuit making capacity

ACB and MCCB nameplates carry Ics, Icw, Icu and Icm as separate markings precisely because they represent different characteristics — if one number covered all four duties, only one number would be printed.

Do not combine them into one generic:

“kA rating.”

Does Bigger kA Always Mean Better?

Not necessarily.

Suppose your calculated fault level is:

28 kA

and the required project margin is satisfied by an appropriately verified:

36 kA / 50 kA system

depending on the specified architecture.

Selecting:

100 kA

everywhere may increase:

• Cost
• Breaker frame selection
• Panel-system constraints

without materially improving system performance.

On the other hand, underspecifying short-circuit capability creates a serious safety and reliability risk.

The objective is:

Correctly rated—not maximally rated and not minimally guessed.

What Safety Margin Should Be Used?

There is no universal rule such as:

“Calculated fault current × 1.25 = required breaker kA.”

The required rating should be determined according to:

• Applicable standards
• Project design criteria
• Utility requirements
• Future system configuration
• Calculation uncertainty
• Equipment ratings
• Client specifications

If future parallel transformers or sources are planned, those operating conditions should be considered where applicable.

Do not hide future-system uncertainty behind a random percentage.

Model the expected system.

Common Short-Circuit Rating Mistakes

Mistake 1: Calling Breaker Icu the Panel Rating

A 65 kA ACB does not automatically create a 65 kA assembly.

Mistake 2: Ignoring Ics

Two breakers with the same Icu may have different service breaking capacities.

Mistake 3: Ignoring Icw

This can create problems in selective systems using intentional short-time delays.

Mistake 4: Writing “50 kA” Without Duration

For short-time withstand, duration matters.

Mistake 5: Checking the Breaker at the Wrong Voltage

Breaking capacities can change with operating voltage.

Mistake 6: Calculating Fault Level From Transformer kVA Only

Parallel sources, cables, DGs and motors can change the result.

Mistake 7: Ignoring Bus-Coupler Operating Conditions

Parallel sources may significantly increase the fault level.

Mistake 8: Assuming Bigger kA Is Always Better

Over-specification can add cost without engineering benefit.

Mistake 9: Using Cascading Without Manufacturer Evidence

Back-up protection should use validated device combinations.

Mistake 10: Not Connecting Protection Coordination With Withstand Rating

A longer short-time delay means equipment has to withstand fault current longer.

Practical Short-Circuit Selection Checklist

Before approving an LT panel, confirm:

System Study

✓ System voltage known ✓ Transformer rating known ✓ Transformer impedance known ✓ Utility contribution considered ✓ DG contribution considered ✓ Motor contribution considered where relevant ✓ Parallel operating conditions assessed ✓ Fault current calculated at panel location

Circuit Breaker

✓ Icu checked at actual voltage ✓ Ics checked ✓ Icw checked where applicable ✓ Icm/making capability considered ✓ Trip unit selected ✓ Protection settings coordinated

Assembly

✓ Main busbar short-circuit rating verified ✓ Short-time duration defined ✓ Peak withstand considered ✓ Connections included in verified design ✓ Busbar supports suitable ✓ Protective circuit included ✓ Applicable IEC 61439 evidence reviewed

Coordination

✓ Downstream breakers checked ✓ Selectivity assessed ✓ Cascading/back-up documented where used ✓ Cable withstand checked ✓ Transformer/equipment withstand considered

Only after these points are coordinated should the panel be called technically suitable for the required fault level.

Design Verification vs Routine FAT

Short-circuit withstand of an IEC 61439 assembly is fundamentally a design-verification characteristic.

It is not practical or appropriate to subject every manufactured customer panel to a destructive high-current short-circuit test during routine FAT.

IEC 61439-1 establishes the assembly verification framework, while IEC 61439-2 applies those requirements specifically to power switchgear and controlgear assemblies.

During project FAT, the useful approach is to verify:

• Offered assembly configuration
• Applicable design-verification evidence
• Busbar construction
• Connections
• Supports
• Installed breaker ratings
• Protection settings
• Workmanship
• Routine verification records

The objective is to ensure that the manufactured panel remains consistent with the applicable verified design.

What About CPRI / ERDA Test Evidence?

For Indian projects, consultants and end users may require independent laboratory evidence for selected assembly tests.

Where such evidence is specified, the buyer should review more than the laboratory name.

Check:

• Standard
• Test performed
• Rated current
• Fault current
• Duration
• Peak current
• Busbar configuration
• Panel construction
• Device arrangement
• Scope of the report
• Applicability to the offered assembly

Avoid the vague question:

“Is this CPRI approved?”

Ask instead:

“What short-circuit verification evidence applies to the actual panel system and rating being offered?”

That is a much stronger technical question.

How Wisdom Techno Solutions Approaches Short-Circuit Rated Panels

At Wisdom Techno Solutions, the required short-circuit rating should begin with the project's electrical-system data and not with a standard breaker BOM.

Wisdom Techno Solutions is a licensed partner for three design-verified switchgear platforms: Rittal Ri4Power (Rittal Solutions Partner since 2021 — the Ri4Power system carries Intertek ASTA design verification to IEC 61439-1 and -2 Edition 3, with copper and aluminium busbar systems up to 6300 A), C&S CX (Licensed Partner since 2023, up to Form 4B Type II) and Siemens SIEPAN Elite/8PU (IEC 61439 Solutions Partner since 2024). We build to each system's verified configuration and routine-verify every assembly in-house at our Vadodara, Gujarat facility, for industrial, EPC and infrastructure projects across India. That places the declared Icw and Ipk withstand values inside a verified design envelope.

For PCC, MCC, PMCC and other LT switchboards, relevant inputs include:

• System voltage
• Transformer rating
• Transformer impedance
• Calculated fault level
• Number of sources
• Bus-coupler philosophy
• Generator operation
• Required rated current
• Required short-time withstand
• Protection/selectivity philosophy
• Preferred switchgear make
• Applicable IEC/IS requirements

The breaker, busbar and overall assembly should then be coordinated around that requirement.

Depending on the project, we review:

• ACB/MCCB Icu
• Ics
• Icw
• Fault-level requirement
• Busbar configuration
• Short-circuit withstand
• Breaker selectivity
• Protection settings/interfaces
• Applicable assembly verification evidence
• Customer FAT requirements

The objective is not simply to write:

“65 kA”

on the datasheet.

The objective is to understand what must withstand 65 kA, what must interrupt it, for how long, and under which operating condition.

FAT Checks Related to Short-Circuit Design

A routine FAT does not reproduce the short-circuit design test.

However, important items can still be checked.

Verify:

• Correct ACB/MCCB make and model
• Correct breaking capacity
• Correct trip unit
• Correct breaker frame
• Correct busbar configuration
• Busbar supports
• Joint workmanship
• Phase spacing
• Protective earth connections
• Approved drawings
• Applicable verification documents
• Protection settings
• Interlocks
• Bus-coupler philosophy

In our FAT experience, the most common short-circuit-related finding is not a wrong breaker — it is a trip unit whose short-time delay has been left at a factory default of around 0.4 seconds on a board specified for 1 second withstand, with a downstream feeder that would have cleared in well under 0.1 seconds. The hardware is correct and the setting quietly extends the fault duty. We therefore treat the recorded short-time settings as a FAT deliverable and check them against the approved selectivity study, not only against the relay's own range.

We perform applicable in-house FAT using calibrated testing equipment, and we support customer/consultant witnessing or TPI when required by the project.

For short-circuit performance, FAT should therefore confirm that the manufactured panel matches the approved and applicable verified design rather than attempting to recreate destructive short-circuit testing on the finished project panel.

A Better Way to Compare Two Panel Quotations

Suppose two manufacturers quote:

Manufacturer A

2500 A PCC — 50 kA

Manufacturer B

2500 A PCC — 50 kA

Are they technically equal?

Not enough information.

Ask both for:

Parameter	Manufacturer A	Manufacturer B
Assembly withstand	?	?
Duration	?	?
Incomer Icu	?	?
Incomer Ics	?	?
Incomer Icw	?	?
Busbar material/configuration	?	?
Applicable design verification	?	?
Breaker selectivity	?	?
Cascading used?	?	?

Only then does:

₹ Price A vs ₹ Price B

become a meaningful commercial comparison.

Without technical normalization, the cheaper quotation may simply contain a different short-circuit design.

Conclusion

The term:

“50 kA panel”

sounds simple.

But good short-circuit engineering requires much more clarity.

First calculate the prospective fault current.

Then understand what each rating means.

Icu tells you about the circuit breaker's ultimate breaking capability.

Ics tells you about its service breaking capability.

Icw tells you how much short-circuit current equipment can withstand for a defined short duration.

Icm deals with making onto a short circuit.

And the complete assembly short-circuit withstand tells you whether the switchboard system—not merely the breaker—has been designed and verified for the required fault duty.

Do not select a panel from one kA number.

Check:

• Fault level
• Voltage
• Breaker Icu
• Breaker Ics
• Breaker Icw
• Duration
• Peak current
• Busbar withstand
• Assembly verification
• Protection selectivity
• Source operating configurations

The correct question is not:

“Is your breaker 65 kA?”

It is:

“Is the complete electrical system—from the breaker through the busbar assembly—correctly rated and coordinated for the prospective fault current at this installation?”

That is the question that leads to a safer and better-engineered LT panel.

Planning a PCC, MCC, PMCC or high-fault-level LT panel?

Share your SLD, transformer data, calculated short-circuit level, protection philosophy and technical specification with Wisdom Techno Solutions for project-specific switchboard engineering and technical evaluation.

Related Guides

• Breaker selectivity, discrimination and cascading
• Busbar sizing for LT panels
• LT panel technical specification and RFQ guide
• IEC 61439 design vs routine verification
• Product page: IEC 61439 type-tested panels

Frequently Asked Questions

What is Icu in a circuit breaker?

Icu is the rated ultimate short-circuit breaking capacity of the breaker under the applicable IEC 60947-2 test conditions. It represents the maximum prospective short-circuit current for which the breaker has the specified ultimate interruption rating.

What is Ics?

Ics is the rated service short-circuit breaking capacity. The applicable test sequence verifies a more demanding service-duty interruption performance than the Icu test sequence.

Is Ics always equal to Icu?

No. Ics is declared by the manufacturer as a proportion of Icu — commonly 25%, 50%, 75% or 100% — depending on the breaker design and range. Two devices both marked 50 kA Icu can therefore have service breaking capacities of 25 kA and 50 kA respectively, which is not visible from the headline figure. Where post-fault continuity matters, compare the declared Ics values at the actual operating voltage.

What is Icw?

Icw is the rated short-time withstand current: the RMS current the device can carry for a specified short duration under the applicable standard test conditions without unacceptable damage. It answers a carrying question rather than an interrupting question, and it is meaningless without its duration — Icw is always quoted as a current and a time, such as 50 kA for 1 second. Icw matters most for main incomer ACBs that hold in deliberately during an intentional short-time delay so that a downstream breaker can clear the fault selectively.

What does 50 kA for 1 second mean?

It means the applicable equipment or assembly has a specified short-time withstand duty of 50 kA RMS for one second under its defined conditions. This is not the same as a 50 kA breaker Icu.

Is Icu the same as panel short-circuit rating?

No. Icu is a circuit-breaker characteristic. The complete switchboard assembly has separate applicable requirements and verification under the IEC 61439 framework.

If an ACB has 100 kA Icu, is the panel automatically 100 kA rated?

No. Icu is a property of that single device, whereas the fault current also passes through the breaker connections, main busbars, vertical droppers, joints, supports, functional units and the protective conductor. Every part of that current path must be suitable and verified for the required fault duty under the IEC 61439 framework. A high-Icu breaker fitted into a weaker assembly does not raise the rating of the switchboard — the weakest part of the current path sets the real capability.

Which is more important: Icu or Ics?

Both describe different aspects of breaker short-circuit performance. Icu relates to ultimate breaking capacity; Ics relates to service breaking capacity. The project specification and required continuity philosophy should determine the required values.

Why is Icw important for ACBs?

Main ACBs may intentionally delay tripping to allow downstream breakers to clear faults selectively. During that delay, the ACB must withstand the short-circuit current, which makes Icw particularly important.

What is Icm?

Icm is the rated short-circuit making capacity — the peak current the breaker is rated to close onto when a short circuit already exists downstream. Because it is a peak value including the asymmetrical component, it is numerically higher than the RMS breaking capacity of the same device. Icm becomes relevant wherever a breaker can be closed by an operator or by an automatic scheme onto a circuit whose condition is unknown, such as an auto-changeover or a DG closing sequence.

Does breaker kA rating change with voltage?

Yes, it commonly does. The same MCCB frame may carry different declared breaking capacities at 380 V, 415 V, 440 V, 500 V and 690 V, and the figure generally reduces as the operational voltage rises. A device described in a quotation as "65 kA" may therefore be considerably lower at 690 V. Always read the manufacturer's declared value at the actual system voltage, and state that voltage alongside the kA figure in the specification.

Is 50 kA always enough for an LT panel?

No. 50 kA means nothing until it is compared with the prospective short-circuit current calculated at that panel's installation point. A main board fed by parallel transformers can require more than 50 kA, while a downstream board at the end of a long cable run may need considerably less. The required rating must come from the project short-circuit study and the applicable design criteria, not from an earlier project's specification.

How is LT panel fault level calculated?

The short-circuit study starts from the source voltage and adds the impedances of the utility network, the transformers, and the cables or busducts between the source and the panel. Contributions from parallel transformers, generators and large running motors are included wherever the operating philosophy permits those configurations. As a rough indication, a 1000 kVA, 415 V transformer at approximately 5% impedance gives around 28 kA at its LV terminals — but a generic figure like this is not a substitute for the study, which must also cover every permitted switching arrangement.

Can two transformers in parallel increase fault level?

Yes, materially. Two sources feeding the same bus contribute in parallel, so the bus fault current approaches the sum of their individual contributions — two 1000 kVA transformers at approximately 5% impedance, each contributing roughly 28 kA, can take the bus well past a 32 kA board rating. Every permitted parallel configuration must therefore be included in the short-circuit study, including brief closed-transition overlaps during changeover. A board that is adequate with the bus coupler open is not automatically adequate with it closed.

Can a lower-kA downstream breaker be used with a higher-kA upstream breaker?

Potentially, where a valid manufacturer-tested cascading/back-up combination applies. It should not be assumed without documented coordination data.

Which IEC standard applies to industrial circuit breakers?

IEC 60947-2 is the IEC product standard for low-voltage circuit breakers, and the current international edition is IEC 60947-2:2024. It defines the test sequences that sit behind the Icu, Ics, Icw and Icm markings on an ACB or MCCB nameplate. It covers the device only — the assembly the device is installed in is governed by the separate IEC 61439 series.

Which IEC standard applies to an LT switchboard?

IEC 61439-1:2020 provides the general rules for low-voltage switchgear and controlgear assemblies, and IEC 61439-2:2020 applies those rules specifically to power switchgear and controlgear assemblies such as PCC, PMCC and MCC boards. Between them they set the construction requirements, the rated characteristics including short-circuit withstand, and the design-verification and routine-verification framework. This is why component compliance with a breaker standard does not by itself demonstrate compliance of the complete assembly.

What Indian standard applies to LV circuit breakers?

The BIS catalogue currently lists IS/IEC 60947 (Part 2):2016, reviewed in 2022, as the Indian adoption covering low-voltage circuit breakers. The current international edition of IEC 60947-2 is 2024, so the Indian and international editions are not aligned on the same revision. Indian specifications should therefore name the standard and edition that is contractually applicable instead of writing "as per latest IEC/IS", which leaves the requirement open to interpretation at evaluation stage.

What Indian standard applies to power switchgear assemblies?

BIS lists IS/IEC 61439 (Part 2):2020 for power switchgear and controlgear assemblies, which corresponds to IEC 61439-2:2020. Part 2 is always read together with the general rules, so the general-rules part applies alongside it. Where a project cites both IS and IEC, name the specific parts and editions so that the design-verification evidence being asked for is unambiguous.

What should I mention in an LT panel RFQ regarding short-circuit rating?

State the calculated prospective short-circuit current at the panel location, and separately the required assembly short-circuit withstand current with its duration — for example 50 kA RMS for 1 second. Then give the device requirements: Icu at the actual operating voltage, Ics where post-fault continuity matters, and Icw where the incomer will use an intentional short-time delay. Finally name the applicable IEC/IS parts and editions, the protection and selectivity philosophy, and the design-verification evidence you expect to review. A line reading only "all breakers shall be 50 kA" leaves every one of these points to be assumed by the panel builder.