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Knowledge Base
Technical reference for electrical engineering standards, definitions, and best practices
What is a Short Circuit?
A short circuit is an abnormal low-impedance connection between two nodes of an electric circuit that are meant to be at different voltages. This forces excessive current to flow through the path of least resistance, bypassing the intended load.
In low-voltage installations supplied by transformers, the short-circuit current has two components:
- AC component (symmetrical) — the steady-state sinusoidal current, expressed as RMS value. This is the value used in most standard calculations.
- DC component (asymmetrical) — a transient decaying component superimposed on the AC waveform, caused by the inductive nature of the circuit. It decays exponentially and is worst at the moment of the fault.
The prospective short-circuit current is the current that would flow if the fault were created by an ideal connection of negligible impedance, without any change to the supply.
Short-Circuit Current Ratings
IEC 60947-2 defines four key short-circuit performance ratings for low-voltage circuit breakers. Each answers a different question about fault handling:
Icu
Rated Ultimate Short-Circuit Breaking Capacity
What it answers: What is the maximum short-circuit current the breaker can interrupt under ultimate test conditions?
The highest prospective fault current a circuit breaker can clear at rated operational voltage. After clearing a fault at Icu, the breaker does not have to remain serviceable — it may need inspection or replacement. Verified by a simple dielectric test after the O-t-CO sequence.
Think of it as the absolute safety limit — the breaker won't explode or cause a fire, but it might be damaged.
The highest prospective fault current a circuit breaker can clear at rated operational voltage. After clearing a fault at Icu, the breaker does not have to remain serviceable — it may need inspection or replacement. Verified by a simple dielectric test after the O-t-CO sequence.
Think of it as the absolute safety limit — the breaker won't explode or cause a fire, but it might be damaged.
Ics
Rated Service Short-Circuit Breaking Capacity
What it answers: At what short-circuit current is service performance verified after interruption?
The fault current level at which the breaker is tested and confirmed to remain fully operational after clearing the fault. Expressed as a percentage of Icu (25%, 50%, 75%, or 100%). Verified by O-t-CO-t-CO sequence, followed by dielectric withstand and temperature rise tests.
In plants where downtime must be minimized, select breakers based on Ics — the breaker will be ready for immediate re-closure.
The fault current level at which the breaker is tested and confirmed to remain fully operational after clearing the fault. Expressed as a percentage of Icu (25%, 50%, 75%, or 100%). Verified by O-t-CO-t-CO sequence, followed by dielectric withstand and temperature rise tests.
In plants where downtime must be minimized, select breakers based on Ics — the breaker will be ready for immediate re-closure.
Icw
Rated Short-Time Withstand Current
What it answers: How much short-circuit current can the breaker withstand for a stated delay time without damage?
The RMS current a breaker can carry for a specified short duration (0.05s, 0.1s, 0.25s, 0.5s, or 1s) without sustaining damage. This is critical for Selectivity Category B breakers that use intentional time-delay for coordination with downstream devices.
Only relevant for ACBs and heavy-duty MCCBs with short-time delay. Most MCBs and standard MCCBs (Category A) do not declare Icw.
The RMS current a breaker can carry for a specified short duration (0.05s, 0.1s, 0.25s, 0.5s, or 1s) without sustaining damage. This is critical for Selectivity Category B breakers that use intentional time-delay for coordination with downstream devices.
Only relevant for ACBs and heavy-duty MCCBs with short-time delay. Most MCBs and standard MCCBs (Category A) do not declare Icw.
Icm
Rated Short-Circuit Making Capacity
What it answers: What peak short-circuit current can the breaker close onto (make) under specified conditions?
The highest instantaneous peak current the breaker can establish when closing onto an existing fault. This is always greater than Icu and is expressed in kA peak (not RMS). Icm = n × Icu, where n depends on the test power factor.
Critical for transfer systems, bus ties, incomers, and remote closing operations where the breaker might close onto a downstream fault.
The highest instantaneous peak current the breaker can establish when closing onto an existing fault. This is always greater than Icu and is expressed in kA peak (not RMS). Icm = n × Icu, where n depends on the test power factor.
Critical for transfer systems, bus ties, incomers, and remote closing operations where the breaker might close onto a downstream fault.
Ics vs Icu — The Key Distinction
Icu — Ultimate Breaking
- Maximum fault current the breaker can interrupt
- Breaker may be damaged after clearing
- Tested with O-t-CO sequence
- Only dielectric test after (safety check)
- About safety — won't cause fire/explosion
Ics — Service Breaking
- Fault current level for verified serviceability
- Breaker remains operational after clearing
- Tested with O-t-CO-t-CO sequence
- Full dielectric + temperature rise tests after
- About reliability — ready for re-closure
Test Sequences
The difference is in the rigor of the test:
Icu: O — t — CO → Dielectric test only
Ics: O — t — CO — t — CO → Dielectric + temperature rise + overload verification
O = Opening (break) operation | CO = Close-Open (make then break) | t = time interval (minimum 3 minutes)
A breaker that has cleared a fault at Icu may have welded contacts, eroded contact material, and altered overload trip characteristics. Always inspect before reuse. A breaker that cleared at Ics is confirmed fit for service.
Standard Ratios: Ics as % of Icu
IEC 60947-2 Table 1 defines the minimum Ics percentages allowed for each utilization category:
| Ics (% of Icu) | Category A Instantaneous | Category B Time-Delayed |
|---|---|---|
| 25% | ✔ | — |
| 50% | ✔ | ✔ |
| 75% | ✔ | ✔ |
| 100% | ✔ | ✔ |
In European industrial practice, it is common to specify Ics = Icu (100%). Most major manufacturers (ABB, Schneider, Siemens) offer the majority of their range with Ics = Icu.
Utilization Categories
| Category | Behavior | Icw | Typical Devices |
|---|---|---|---|
| A | No intentional short-time delay for selectivity. Instantaneous tripping on fault. | Normally not declared | MCBs, standard MCCBs |
| B | Designed for selectivity with intentional short-time delay. Withstands fault current for a defined period. | Key rating (kA / time) | ACBs, heavy-duty MCCBs |
Icm — Short-Circuit Making Capacity
Icm is the peak current the breaker can safely close onto. It is always greater than Icu because of the DC offset in the first cycle of a fault. Under IEC 60947-2:
Icm = n × Icu
Where n depends on the test power factor (which relates to the X/R ratio of the fault circuit):
| Icu Range | Test cos φ | n Factor | Example |
|---|---|---|---|
| 6 kA < Icu ≤ 10 kA | 0.50 | 1.7 | 10 kA → 17 kA peak |
| 10 kA < Icu ≤ 20 kA | 0.30 | 2.0 | 20 kA → 40 kA peak |
| 20 kA < Icu ≤ 50 kA | 0.25 | 2.1 | 50 kA → 105 kA peak |
| Icu > 50 kA | 0.20 | 2.2 | 65 kA → 143 kA peak |
If the actual X/R ratio at the installation point is higher than the standard assumes, the real peak current (Ipk) can exceed Icm. The general formula from IEC 60909 is: n = sqrt(2) × (1.02 + 0.98 × e^(-3R/X)). Always verify Icm ≥ Ipk for your specific fault study.
When does Icm matter?
- Transfer switches / bus ties — breaker may close onto an existing downstream fault
- Remote closing — operator not present to assess fault conditions
- Incomers — closing onto busbar fault
- If your plant cos φ is lower than the standard value (near transformers/generators), you must verify Icm ≥ Ipk
- If your plant cos φ is higher than the standard value, Icm check is not critical
Prospective Short-Circuit Current Calculation
At the secondary terminals of a MV/LV distribution transformer, the three-phase symmetrical short-circuit current can be calculated using the transformer's rated parameters:
Isc = In × 100 / Usc
where In = transformer rated current, Usc = short-circuit impedance voltage (%)
Isc = (kVA × 1000) / (sqrt(3) × VLL × Zpu)
where Zpu = Usc% / 100, VLL = line-to-line secondary voltage
Example
For a 1000 kVA transformer, 400V secondary, Usc = 6%:
In = 1000 × 10³ / (sqrt(3) × 400) = 1443 A
Isc = 1443 × 100 / 6 = 24.05 kA
Isc = 1443 × 100 / 6 = 24.05 kA
Downstream Points
At any point downstream, the cable impedance adds to the transformer impedance, reducing the fault current:
Isc(distant) = Isc(transformer) × Ztransformer / (Ztransformer + Zcable)
The simplified formula above assumes the MV source impedance is negligible (infinite bus). For more accurate studies, include upstream source impedance using the IEC 60909 method.
Relevant IEC Standards
| Standard | Scope | Key Ratings |
|---|---|---|
| IEC 60947-2 | Industrial circuit-breakers (skilled/instructed persons, ≤1000V AC / ≤1500V DC) | Icu, Ics, Icw, Icm |
| IEC 60898-1 | Household MCBs (uninstructed persons, residential/commercial) | Icn, Ics |
| IEC 61439-1 | Low-voltage switchgear and controlgear assemblies (panels, switchboards) | SCP rating, Icw for assemblies |
| IEC 60909 | Short-circuit current calculation method (all voltage levels) | Ik", Ik, Ipk, m/n factors |
| IEC 60947-3 | Switch-disconnecters | Icw (1s preferred) |
Practical Selection Guide
When selecting a circuit breaker, verify these ratings in order of importance:
| Application | 1st to Verify | 2nd to Verify | 3rd to Verify | Notes |
|---|---|---|---|---|
| Final distribution / branch circuit | Icu or Icn | Ics | Rated current + trip curve | Icw usually not relevant |
| Motor feeder | Icu | Ics | Icm (if closing onto fault) | Check motor starting current |
| Industrial MCCB feeder | Icu | Ics | Trip-unit settings | High Ics improves post-fault confidence |
| Main incomer | Icu | Ics | Icw (if time-delay selectivity) | Often needs coordination study |
| ACB incomer / bus-tie | Icw | Icu / Ics | Icm | Selectivity + making duty central |
| Transfer / auto source switching | Icm | Icu | Ics | Closing onto existing downstream fault |
Always specify Ics = Icu — what is the point of a circuit breaker that can't be readily reused after tripping? Most quality manufacturers offer 100% ratio as standard.
Ratings vary with voltage. A breaker rated Icu = 100 kA at 415V may drop to 85 kA at 525V. Always check the rating at your specific operational voltage. Icw is also time-dependent — a longer withstand time means a lower current rating.
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