ABB Emax 2 Icw Rated Short-Time Withstand Current Values Guide

What is ABB Emax 2 Icw rated short-time withstand current? ABB Emax 2 Icw is the short-time withstand current rating defined under IEC 60947-2, specifying the maximum fault current — ranging up to 100 kA across the E1.2–E6.2 frame sizes — that the breaker sustains for 1 s or 3 s without damage, enabling upstream-downstream selectivity in low-voltage power systems. Undersizing Icw against prospective fault current causes thermal or electrodynamic failure during intentional time-delay coordination, voiding selectivity schemes and exposing busbars to uncontrolled fault energy. This guide covers the IEC 60947-2 definition distinguishing Icw from Icu and Ics, Icw values across the full Emax 2 range, the adiabatic heating argument behind the 1 s and 3 s time bases, real selectivity coordination examples, performance class comparisons, and required Icw calculation methodology.

Published: 2026-04-27 | Last updated: 2026-04-27 | By Stoklink Engineering Team

What Icw Actually Means in IEC 60947-2 — and Why It Differs from Icu and Ics

Engineers often conflate the ABB Emax 2 Icw with Icu (rated ultimate short-circuit breaking capacity) or Ics (rated service short-circuit breaking capacity). They are not the same thing, and treating them interchangeably will get you into trouble during a coordination study.

Here is the practical distinction. Icu says "I can clear 65 kA, once." Icw says "I can sit there carrying 50 kA for a full second while the downstream MCCB clears the fault, and I'll do it again tomorrow." For time-current selectivity in a main-tie-main configuration, Icw is what matters. Icu is irrelevant for selectivity if the breaker has already tripped on instantaneous.

In our experience reviewing coordination studies for industrial substations, the most common mistake is specifying a frame on Icu alone — typically because the SLD shows a 50 kA prospective fault and someone picks a 65 kA Icu device without checking the 1 s withstand. When the actual fault clearing time of the downstream device is 200–400 ms, that selection often fails to coordinate.

For complete technical specifications and Icw test verification methodology applicable to the ABB Emax 2 range, refer to IEC 60947-2 Low-voltage switchgear standard.

Icw Values Across the Emax 2 Range: E1.2 to E6.2

ABB publishes Icw values for each ABB Emax 2 frame at both 1 s and 3 s, and the values vary by performance class (B, N, S, H, L, V). The L and V classes use current-limiting technology and therefore have lower Icw — that is the trade-off with current limitation. The B, N, S, and H classes are true selective breakers with high Icw.

E1.2 frame (up to 1600 A)

The E1.2 is the smallest Emax 2 frame and covers 630–1600 A. Typical Icw values:

• E1.2B: Icw = 42 kA / 1 s, 36 kA / 3 s (Icu = 42 kA at 415 V)
• E1.2N: Icw = 50 kA / 1 s, 36 kA / 3 s (Icu = 66 kA at 415 V)

For projects where 42 kA is sufficient at the incomer, the ABB 1SDA070701R1 E1.2B 630 Ekip Dip LI 3p F F is a typical choice for a 630 A feeder. Stepping up the rating, the ABB 1SDA070781R1 E1.2B 1000 covers 1000 A applications with the same 42 kA Icw, and the ABB 1SDA070861R1 E1.2B 1600 sits at the top of the E1.2 frame.

E2.2 frame (up to 2500 A)

• E2.2B: Icw = 42 kA / 1 s, 42 kA / 3 s (note: B-class E2.2 is symmetric across 1 s and 3 s for ratings up to 2000 A)
• E2.2N: Icw = 66 kA / 1 s, 50 kA / 3 s
• E2.2S: Icw = 85 kA / 1 s, 65 kA / 3 s
• E2.2H: Icw = 100 kA / 1 s, 85 kA / 3 s
• E2.2L: Icw = 15 kA / 1 s (current-limiting class — much lower withstand by design)

The ABB 1SDA070981R1 E2.2B 1600 is a common choice when an E1.2 frame is undersized for thermal margin or when more terminal real-estate is needed in the cubicle. For 2000 A, the ABB 1SDA071021R1 E2.2B 2000 is the workhorse.

E4.2 and E6.2 frames

• E4.2S: Icw = 85 kA / 1 s, 65 kA / 3 s (rated up to 4000 A)
• E4.2H: Icw = 100 kA / 1 s, 85 kA / 3 s
• E6.2H: Icw = 100 kA / 1 s, 100 kA / 3 s (rated up to 6300 A)
• E6.2X: Icw = 120 kA / 1 s, 100 kA / 3 s (highest performance in the range)

Why the 1 s and 3 s Distinction Matters: The Adiabatic Heating Argument

The reason Icw is rated at specific times in the ABB Emax 2 datasheets — and not as a single number — is rooted in the physics of conductor heating during a fault. During a short-circuit event, current flows for so brief a period that essentially no heat is dissipated to the surroundings. The conductors and contacts behave adiabatically, meaning all the I²t energy goes into raising their temperature.

Take a concrete example. An E2.2H rated at 100 kA / 1 s carries an I²t of 10¹⁰ A²·s during the test. At 3 s, the same breaker is rated 85 kA, giving I²t = 2.17 × 10¹⁰ A²·s — over twice the energy. The 3 s test is therefore the more thermally severe condition, and the rated current must drop to keep the contact temperature within the post-test reusability limit specified by IEC 60947-2 Annex T.

What we typically see in the field: engineers assume that a 100 kA / 1 s rating means "the breaker handles 100 kA forever." It does not. If your fault clearing time at the upstream incomer is going to be 1.5 s due to a deliberate selectivity delay (a so-called "ZSI bypass" scenario), you must interpolate, and the 100 kA value is no longer valid.

Interpolating Icw between rated test points

For times between the standard test points, IEC 60947-2 permits adiabatic interpolation using the constant-I²t assumption:

I_permitted(t) = I_cw(1s) × √(1/t)

So an E2.2H at 100 kA / 1 s permits roughly 70.7 kA at 2 s and 57.7 kA at 3 s — but the manufacturer's tested 3 s value of 85 kA is what you cite, since the contact metallurgy actually performs better than the simple adiabatic estimate.

Applying Icw to Selectivity: A Real Coordination Example

Selectivity (also called discrimination) means that under a fault condition, only the breaker closest to the fault opens — the upstream device must hold. To achieve this with time-current coordination, the upstream ABB Emax 2 breaker's Icw must exceed the prospective fault current AND its short-time delay (S-function) setting must be longer than the downstream breaker's total clearing time.

Consider a 2500 kVA, 22/0.4 kV transformer feeding a main switchboard. Prospective short-circuit at the LV terminals: about 60 kA. Downstream feeders use Tmax XT4 MCCBs with total clearing time around 25 ms on instantaneous trip.

The incomer needs:

• Icu ≥ 60 kA at 415 V
• Icw ≥ 60 kA / 1 s (so it can hold the fault while the downstream MCCB clears, plus margin for inrush during selectivity studies)
• S-function delay set to ~100 ms — well above the MCCB's 25 ms but well below the breaker's 1 s withstand

An E2.2N at 66 kA / 1 s Icw fits cleanly. An E2.2B at only 42 kA / 1 s does not — that is a hard rejection. For data center main distribution boards where 100 kA fault levels are common, the E4.2H or E6.2H frames are typical, as discussed in ABB Emax 2 in Data Centers: MDB Design, Redundancy and Uptime Considerations.

The pitfall of S-function bypass at high faults

A subtlety engineers often overlook: many Ekip trip units include an instantaneous override at currents far above the S-function pickup, to protect the breaker itself. If your S setting is 8 × In with a 200 ms delay, but the trip unit auto-bypasses to instantaneous at 12 × In, then on a 50 kA fault on a 2000 A breaker (25 × In), the breaker trips instantly — selectivity is lost. Always check the ABB curve datasheet for the explicit override threshold, or refer to ABB Emax 2 Nuisance Tripping: Root Causes, Diagnostic Steps and Fixes for related diagnostic approaches.

Comparing Icw Across Performance Classes

The same physical ABB Emax 2 frame is offered in multiple performance classes — B, N, S, H, L, V — and the differences come down to contact materials, arc chute design, and trip unit configuration. Here is a practical comparison for the E2.2 frame:

Note that Ics = 100% of Icu across all Emax 2 classes — this is one of the strongest selling points of the Emax 2 versus competitor frames. After clearing a fault at full Icu, the breaker remains in service. Some engineers argue that this distinction is academic since you should always investigate after a major fault, but in my experience the practical reliability gain is real, particularly in continuous-process plants where unplanned downtime is measured in tens of thousands of dollars per hour.

For a deeper specifications comparison across the full range, see ABB Emax 2 Full Technical Specifications: Current Ratings, Breaking Capacity and Dimensions. For competitive benchmarking, ABB Emax 2 vs Schneider MasterPact MTZ is a useful read.

Calculating the Required Icw for Your Application

The required Icw for an ABB Emax 2 selection depends on three inputs: prospective fault current at the breaker location, the maximum delay required to achieve selectivity with downstream devices, and the safety margin policy of your organization (typically 10–20%).

For a complete sizing methodology including thermal current, breaking capacity, and Icw checks together, see How to Size ABB Emax 2: Step-by-Step Calculator for LV Distribution Panels.

Common Field Mistakes with Icw — and How to Avoid Them

In our experience reviewing customer specifications for ABB Emax 2 applications, three Icw-related errors come up repeatedly.

1. Confusing Icw with Icm (peak making capacity)

Icm is the peak asymmetric current the breaker can close onto a fault, per IEC 60947-2 §4.3.5.3. It is typically 2.1–2.2 × Icu RMS. Icw is RMS symmetric. Mixing these up by a factor of 2 in fault calculations is not unusual, and it always errs in the wrong direction.

2. Ignoring the Ue derating

Icw and Icu values quoted in ABB's main catalog are at 415 V (or 690 V, depending on the table). At 690 V, Icu drops — for the E2.2H, from 100 kA at 415 V to 85 kA at 690 V. Icw is generally less voltage-dependent than Icu but still varies. Always cross-check the Ue column.

3. Specifying the wrong frame for generator applications

On a generator with X"d of 12% and a 2 MVA rating at 400 V, the initial subtransient fault current is high but decays in roughly 100–200 ms. Specifying for the steady-state fault is too low — the breaker may trip on the subtransient peak. Specifying for the subtransient is over-conservative and expensive. The right answer lies between, and Icw at 1 s is usually the correct anchor point.

For 1250 A applications often seen in genset feeders, the ABB 1SDA070821R1 E1.2B 1250 is a common selection where 42 kA Icw is sufficient. For 800 A genset breakers, the ABB 1SDA070741R1 E1.2B 800 covers most cases. For LSI trip-unit configurations supporting selectivity (which is exactly where Icw matters most), the ABB 1SDA070782R1 E1.2B 1000 Ekip Dip LSI is preferred over LI variants.

Icw Testing: What ABB Actually Does in the Lab

The IEC 60947-2 test sequence for Icw is brutal, and the ABB Emax 2 must pass it to earn its rated value. The breaker is closed onto a calibrated short-circuit source. Current rises in a controlled asymmetric waveform — first half-cycle peak per IEC 60947-2 Table 11, declining to symmetric RMS — and is held for the rated time. After the test, the breaker must:

• Open on command and remain operable
• Show no welding of contacts that prevents subsequent operation
• Pass dielectric verification at 2 × Ue
• Carry rated current Iu without exceeding the temperature rise limits in IEC 60947-2 Table 7

What ABB does beyond the standard: each Emax 2 frame is type-tested at multiple voltage levels (415 V, 525 V, 690 V) and at both 1 s and 3 s durations. The published values represent the worst-case verified envelope, not nameplate optimism. In our experience auditing manufacturer test reports, ABB's Emax 2 numbers are conservative — meaning the breakers typically survive the test with margin, which is reassuring when your application sits at 90% of the rated value.

One detail worth knowing: the test is performed with the breaker mounted in its specified enclosure (or a representative enclosure), with the published terminal connections and torques. Field installations that deviate — undersized busbars, loose torque, inadequate ventilation — will not achieve nameplate Icw. We have seen one industrial site where a measured fault was below the breaker's Icw, yet the breaker failed because the busbar bracing was inadequate and the magnetic forces deformed the connection before the breaker could clear. Icw is a breaker rating, not a switchboard rating.

Mechanical vs thermal failure modes

During an Icw event, the breaker faces two simultaneous stresses. Mechanically, the peak asymmetric current (Ipk, roughly 2.1–2.2 × Icw RMS) creates electromagnetic forces between phases that can exceed several hundred kilonewtons in a 100 kA frame. Thermally, the I²t energy heats the contacts toward their melting point. The 1 s test is mechanically dominant; the 3 s test is thermally dominant. That is why some frames have asymmetric ratings between the two durations — the contact metallurgy and the bracing structure are optimized differently.

Icw and the IEEE / NEMA Perspective

For engineers working in North American markets or with mixed IEC/ANSI specifications, the ABB Emax 2 Icw concept maps roughly to the "short-time current rating" defined in IEEE C37.13 for low-voltage power circuit breakers (LVPCB). The numerical equivalence is close but not identical:

• IEC 60947-2 Icw at 1 s ≈ IEEE C37.13 short-time rating at 30 cycles (0.5 s), adjusted
• NEMA AB-1 covers MCCBs and uses a different framework — generally not applicable to ACBs
• UL 1066 governs LVPCBs in North America with a 30-cycle rating that is functionally similar to Icw / 0.5 s

The Emax 2 is type-tested to both IEC 60947-2 and UL 1066, so dual-listed breakers exist for global projects. For a North American 480 V switchboard requiring 65 kA short-time rating per UL 1066, the equivalent Emax 2 selection would be the E2.2H or E4.2H, depending on continuous current. The published 1 s Icw values transfer cleanly when converted to RMS symmetric basis.

Practical Selection Workflow Using Icw

Here is the workflow we use when sizing ABB Emax 2 incomers in coordination studies:

Step 1. Calculate the maximum prospective three-phase symmetric fault current at the breaker location, per IEC 60909. Include motor contribution if motor load exceeds ~25% of transformer rating.

Step 2. Determine the longest selectivity delay required at the breaker location. For a single-tier scheme with downstream MCCBs, this is typically 100–200 ms. For two-tier schemes (incomer → feeder ACB → MCCB), it can reach 500–800 ms.

Step 3. Apply the adiabatic relationship to calculate equivalent Icw at 1 s: I_required(1s) = I_fault × √(t_delay / 1).

Step 4. Add a 10–20% margin per your engineering policy.

Step 5. Select the frame and performance class whose 1 s Icw equals or exceeds this value, then verify the 3 s Icw is also adequate if the delay scheme could ever exceed 1 s.

Step 6. Verify Icu meets or exceeds the prospective fault. This is usually automatic since Icu ≥ Icw for all Emax 2 classes, but check anyway.

For a 4000 A incomer on a 100 kA bus with 500 ms selectivity delay: I_required(1s) = 100 × √0.5 = 70.7 kA, plus 15% margin = 81.3 kA. The E4.2S at 85 kA / 1 s is a fit; the E4.2H at 100 kA / 1 s gives more headroom. Most engineers, including us, would pick the H class for the headroom — the cost difference is small relative to the consequences of a misselection.

Related Reading

• What Is the ABB SACE Emax 2? Features, Models and Key Benefits
• ABB Emax 2 Full Technical Specifications: Current Ratings, Breaking Capacity and Dimensions
• How to Size ABB Emax 2: Step-by-Step Calculator for LV Distribution Panels
• ABB Emax 2 vs Schneider MasterPact MTZ: Technical Specs, Features and Price Compared

For browsing the full range, see the Air Circuit Breakers collection at Stoklink. For complementary protection devices in the same panel, the Miniature Circuit Breaker collection, Residual Current Device collection, and Relay collection are useful resources.

Conclusion

Icw is the parameter that decides whether your selectivity scheme works on paper but fails in reality. Specify it correctly and your switchboard rides through a downstream fault, isolating only the affected feeder. Specify it carelessly — by anchoring on Icu alone or by ignoring the 1 s vs 3 s distinction — and you can have a switchboard that nominally meets fault levels but trips out the entire incomer on every downstream fault. The Emax 2 range gives you a clear performance ladder from 42 kA / 1 s on the E1.2B up to 120 kA / 1 s on the E6.2X, with verified 3 s values for sustained backup duty.

Pick the frame from the continuous current. Pick the performance class from the Icw. Verify the trip unit's S-function delay envelope sits inside the Icw withstand. Then sleep well at night. For the complete selection methodology — including dimensions, accessories, and maintenance — see the ABB SACE Emax 2 Air Circuit Breaker: Selection, Application and Maintenance Guide, the pillar reference for engineering teams working with this range.