In short. Every change that adds a source, changes impedance or paralleling, or raises equipment ratings can alter fault levels. A short-circuit study checks that switchgear, cables and protection can survive and clear the highest fault, and that protection still operates at the lowest. If any trigger below applies, a study is needed.
Why it matters
- Breakers and busbars have a rated breaking and withstand capacity. A fault above it can destroy equipment and harm people.
- Settings depend on fault level for selectivity.
- Arc-flash energy depends on fault current and clearing time.
- Low fault levels, such as on generator or inverter sources, may fail to trip protection.
Triggers: when a study is needed
| Trigger | Why |
|---|---|
| New or additional generator or transformer | Adds fault contribution |
| Paralleling sources or closing a tie | Fault levels add |
| Utility supply change | Source impedance changes |
| Adding large motors | Motors contribute to the fault |
| Adding BESS, UPS or solar inverters | Inverter contribution and protection behaviour |
| Replacing or adding switchgear | Ratings must be verified |
| Changing protection settings or devices | Selectivity and clearing times |
| Island or microgrid operation | Low fault levels |
| No up-to-date study exists | Unknown margin |
Data to collect
- Utility fault level or source impedance, with the minimum and maximum.
- Transformer ratings, impedance and vector group.
- Generator ratings and reactances.
- Motor sizes and types.
- Cable types, sizes and lengths.
- Switchgear ratings and protection settings.
- Single-line diagram showing all operating configurations.
What the study produces
- Maximum fault currents for equipment rating checks.
- Minimum fault currents for protection sensitivity.
- Results for each source and tie configuration.
- Recommendations: equipment upgrades, impedance limiting, setting changes.
See short-circuit study architecture and IEC short-circuit calculation.
Quick screening view (illustrative)
A 1,250 kVA, 5 per cent impedance transformer can supply about 35 kA at 415 V to a bolted fault, before adding motor contribution and the source. If the downstream board’s breakers are rated 25 kA, a study is clearly needed. This is a screening estimate, not a design value. The study replaces the guess with a calculated answer.
After the study
Keep the model, assumptions and results with the single-line diagram. Update when anything changes. Feed results into the protection coordination study. See protection study selection.
Do you need a short-circuit study?
| Trigger | Why it needs a study |
|---|---|
| Adding a generator in parallel with the utility | Fault level rises |
| Paralleling several generators | Combined contribution |
| Bus above about 30 to 50 kA, or unknown | Breaker ratings uncertain |
| New or replaced transformer | Impedance changes the level |
| Protection settings being set or changed | Needs minimum and maximum levels |
| Arc-flash labelling requirement | Requires fault current data |
| Old plant with unknown ratings | Safety |
Quick screen (illustrative)
For a 1,000 kVA, 5 percent transformer: rated current 1,391 A, fault current up to about 27.8 kA on an infinite source. If the board’s breakers are rated 25 kA, they are inadequate. The screen flags the need before any detailed work. Adding a 500 kVA generator with 15 percent reactance contributes about 4.6 kA, raising the total to about 32 kA.
What the full study provides
- Maximum levels for equipment ratings.
- Minimum levels for protection sensitivity.
- Peak and thermal values for bus bracing and cables.
Data to collect
Utility fault level, transformer data, generator reactances, motor contributions and cable lengths. See short-circuit study architecture and fault level engineering.
Common mistakes
- Checking only maximum fault level.
- Studying only normal configuration.
- Using an outdated utility fault level.
What to send us
Send the single-line diagram, equipment data and utility details. Send your application details.
Questions engineers ask
Do I need a short-circuit study for a single generator?
If it connects to existing switchgear or a utility supply, check ratings and protection. For small systems a simple verification may suffice.
Why does the minimum fault level matter?
Protection must detect the smallest credible fault, such as on generator-only supply.
How often should it be updated?
Whenever the system changes, and on a periodic review.
Who performs the study?
A power systems engineer using recognised software and standards such as IEC 60909.
Figures are illustrative. A real study requires verified data.