Engineering Executive Summary: Selecting a Molded Case Circuit Breaker (MCCB) requires precise matching between the breaker’s short-circuit interrupting capacity (Icu / Ics in kA) and the facility’s prospective maximum fault current (Isc) under IEC 60947-2.
(Looking for basic definitions like “what does kA stand for” or kA-to-Amps conversion? Refer to our beginner’s guide: What Is kA in MCCB? Breaking Capacity & kA to Amps Explained).
1. Icu vs. Ics vs. Icm vs. Icw: Demystifying IEC 60947-2 Ratings
On any industrial Molded Case Circuit Breaker (MCCB) nameplate, you will encounter multiple short-circuit parameters defined under international standard IEC 60947-2. Mistaking these parameters can lead to costly over-specification or hazardous under-specification.
| Symbol | Full Standard Name | Standard Test Sequence | Operational Consequence | Selection Guidance |
|---|---|---|---|---|
| Icu | Rated Ultimate Short-Circuit Breaking Capacity | O - t - CO(Open – wait – Close/Open) | The breaker safely clears the fault, but internal contact erosion occurs. The device must be inspected and often discarded. | Must be ≥ calculated prospective fault current (Isc). |
| Ics | Rated Service Short-Circuit Breaking Capacity | O - t - CO - t - CO | The breaker safely clears the fault and remains fully functional for continuous operation without service interruption. | Expressed as % of Icu (e.g., 50%, 75%, 100%). Specify Ics = 100% Icu for data centers and critical industrial plants. |
| Icm | Rated Short-Circuit Making Capacity | Instantaneous Peak Closing | Verifies the breaker can close into an active short circuit without contacts instantly welding or exploding. | Typically 1.7 to 2.2 × Icu (governed by circuit power factor). |
| Icw | Rated Short-Time Withstand Current | Sustained for 0.5s, 1s, or 3s | The breaker withstands thermal and electrodynamic stress without tripping to let downstream breakers clear first. | Essential for Category B selective coordination switchboards. |
2. How to Calculate the Required kA Rating of an MCCB
To ensure system safety, the selected breaker’s breaking capacity (Icu) must exceed the maximum prospective symmetrical short-circuit current (Isc) calculated at its installation terminals.
The Infinite Bus Short-Circuit Calculation Formula:
- Stransformer: Transformer apparent power in kVA.
- VLL: Nominal secondary phase-to-phase voltage (e.g., 400V or 415V).
- %Z: Transformer percentage impedance (typically 4.0% to 6.0% for distribution transformers).
Step-by-Step Engineering Calculation Example:
Consider an industrial switchboard fed by a 1,000 kVA distribution transformer (415V secondary, 5% impedance):
- Calculate Transformer Rated Full-Load Current (In):
In = (1,000 × 1,000) / (1.732 × 415) ≈ 1,391 Amperes - Calculate Prospective Fault Current (Isc):
Isc = 1,391 / 0.05 = 27,820 Amperes = 27.82 kA - Apply Engineering Margin (+20% safety & motor contribution factor):
Target Breaking Capacity = 27.82 kA × 1.2 = 33.38 kA
Selection Result: The main incomer must use a circuit breaker rated for at least 36kA or 50kA at 415V. Installing a standard 25kA MCCB here would violate electrical safety regulations.
3. Standard MCCB kA Rating & Sizing Selection Table
Use this engineering selection matrix to determine the required breaking capacity based on transformer capacity (400V/415V, 50/60Hz systems):
| Transformer (kVA) | Impedance (%Z) | Busbar Fault (Isc) | Main Incomer MCCB (Icu) | Sub-Feeder MCCB (Icu) |
|---|---|---|---|---|
| 315 kVA | 4.0% | ≈ 11.4 kA | 18 kA – 25 kA | 16 kA – 18 kA |
| 500 kVA | 4.5% | ≈ 16.0 kA | 25 kA – 36 kA | 18 kA – 25 kA |
| 800 kVA | 5.0% | ≈ 23.1 kA | 36 kA – 50 kA | 25 kA – 36 kA |
| 1,000 kVA | 5.0% | ≈ 28.9 kA | 50 kA | 36 kA |
| 1,600 kVA | 6.0% | ≈ 38.5 kA | 50 kA – 65 kA | 50 kA |
| 2,000 kVA | 6.0% | ≈ 48.1 kA | 65 kA – 85 kA | 50 kA – 65 kA |
4. Selecting MCCBs for 65kA Available Fault Currents
When an engineering single-line diagram (SLD) specifies 65kA available fault current, pay special attention to breaker mechanical sizing:
- Frame Size Threshold: Small 125A frame breakers lack the arc chute volume to clear 65kA. Panel builders typically select a 250A frame or 400A frame MCCB equipped with an adjustable trip unit dialed down to the desired operational current. Explore our AC Adjustable Current MCCB series for flexible onsite trip adjustments.
- Demanding Ics = 100% Icu: In heavy industrial environments subjected to 65kA surges, always demand
Ics = 100% Icuto prevent having to replace the entire breaker assembly after an emergency trip.
5. Voltage Dependency: Why kA Changes from 400V to 690V
Breaking capacity is not an absolute number—it decreases significantly as operating voltage increases. Higher voltages generate more powerful recovery arcs across opened contacts.
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Operating Voltage (Ue) Breaking Capacity (Icu) Ics Ratio
220V/240V AC 85 kA 100%
380V/415V AC 50 kA 100%
440V/460V AC 42 kA 100%
690V AC 20 kA 100%
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Procurement Warning: Never request a “50kA breaker” without specifying operational voltage. A breaker delivering 50kA at 400V AC may only be certified for 20kA at 690V AC (common in solar PV and heavy mining applications).
6. 5 Critical Engineering Procurement Pitfalls
- Sourcing by Amps (In) Alone: Inquiring for “a 400A MCCB” allows distributors to supply entry-level, low-kA residential-tier breakers incapable of surviving industrial busbar faults.
- Accepting Low Ics Ratios: Buying breakers where Ics = 50% Icu to save cost forces your facility into costly downtime after a fault while waiting for replacement units.
- Ignoring Motor Feedback Contribution: Medium- and low-voltage induction motors act as generators during a short circuit, contributing an additional 4× to 6× full load current into the fault.
- Overlooking Busbar Bracing: Installing an 85kA MCCB inside a distribution board whose copper busbars are only braced for 30kA electrodynamic stress results in physical panel destruction.
- Mismatching AC and DC Ratings: Never install an AC-rated breaker on battery storage or solar strings. For energy storage projects, always specify certified DC Moulded Case Circuit Breakers.
7. What Technical Data to Provide for a Fast Factory Quotation
To help the NUOMAK engineering team match the exact breaker model and provide a direct factory quotation within 24 hours, please submit the following specifications:
- System Voltage & Frequency: (e.g., 400V AC, 690V AC, or DC voltage)
- Continuous Current Rating (In): (e.g., 100A, 250A, 400A, 800A, up to 1600A)
- Required Breaking Capacity (Icu / Ics): (e.g., 36kA, 50kA, 65kA, or 85kA at nominal voltage)
- Pole Configuration: 3-Pole or 4-Pole execution
- Trip Unit Architecture: Thermal-Magnetic Fixed, Thermal-Magnetic Adjustable (TMD), or Microprocessor Electronic Trip Unit (ETU / LSIG)
Frequently Asked Questions (FAQ)
What is the primary difference between Icu and Ics in an MCCB?
Icu (Ultimate Breaking Capacity) is the maximum short circuit the breaker can clear once before needing internal maintenance or replacement. Ics (Service Breaking Capacity) is the fault level the breaker can interrupt repeatedly and remain completely operational for continuous duty.
How do I calculate the short-circuit current for an MCCB?
Use the standard formula: Isc = kVA / [ √3 × V × (%Z / 100) ]. For instance, a 1,000 kVA, 415V transformer with 5% impedance produces approximately 27.82 kA of symmetrical prospective short-circuit current.
Can I install an MCCB with a higher kA rating than required?
Yes. Installing a breaker with higher interrupting capacity (e.g., specifying a 50kA MCCB where available fault current is only 28kA) is technically safe and adds safety headroom, though it comes at a higher unit cost.
What does “Ics = 100% Icu” mean on a specification sheet?
It means the circuit breaker is certified to clear 100% of its ultimate short-circuit rating and remain fully operational without contact replacement or performance degradation, critical for continuous uptime in hospitals and data centers.
Request Certified High-Breaking MCCBs from NUOMAK
NUOMAK manufactures IEC 60947-2 compliant Molded Case Circuit Breakers with breaking capacities up to 85kA at 415V AC. We support panel builders and electrical wholesalers worldwide with direct factory pricing and technical compliance support.
