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Knowledge Base
Technical reference for electrical engineering standards, definitions, and best practices
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.
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:
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 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 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 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:
O = Opening (break) operation | CO = Close-Open (make then break) | t = time interval (minimum 3 minutes)
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% | ✔ | ✔ |
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:
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 |
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:
where In = transformer rated current, Usc = short-circuit impedance voltage (%)
where Zpu = Usc% / 100, VLL = line-to-line secondary voltage
Example
For a 1000 kVA transformer, 400V secondary, Usc = 6%:
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:
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 |
🏗️ Design
Circuit breaker reference and panel layout design tools.