In short. A short-circuit study calculates fault currents at each bus for the fault types and system states that matter. Its results set breaker and busbar ratings (maximum case), confirm that protection picks up (minimum case) and feed coordination and arc-flash studies.
Objectives
- Check equipment ratings: breaking capacity, making capacity, busbar and cable withstand.
- Provide fault data for protection settings.
- Provide data for arc-flash and earthing studies.
- Test the effect of adding sources or changing configuration.
Fault types
| Fault | Use |
|---|---|
| Three-phase | Usually the highest value; equipment ratings |
| Phase-to-phase | Minimum fault checks, generator protection |
| Phase-to-earth | Earth-fault settings, earthing design |
| Double phase-to-earth | Specific checks and relay testing |
Cases
- Maximum: all sources in, motors running, minimum impedances. See maximum and minimum studies.
- Minimum: weakest source, maximum impedance, motors off.
- Generator only: utility out, which gives the lowest fault current in many sites.
- Parallel: utility and generators together.
- Special states: maintenance configurations, ties closed.
Model components
- Utility: fault level or equivalent impedance. See utility fault level.
- Transformers: impedance and X/R. See transformer impedance.
- Generators: subtransient and transient reactances. See generator contribution.
- Motors: contribution during the initial cycles. See motor contribution.
- Inverter sources: limited current. See inverter behaviour.
- Cables and busducts: resistance and reactance.
Calculation standard
Studies commonly follow IEC 60909 or IEEE/ANSI methods. State the method used. See IEC calculation engineering.
Report content
- Fault current at each bus for each fault type and case.
- Comparison against equipment ratings with pass or fail and margin.
- Peak and breaking values as needed.
- Assumptions and data sources.
- Recommendations for any overstressed equipment.
Illustrative example
A site with a utility supply and one generator finds, in the maximum case, a fault level at the main board that is within the breaker rating. In the generator-only case, the fault current at the far feeders is lower than the instantaneous pickup of two breakers, so the study recommends setting changes. Both findings come from the same model.
Why a short-circuit study is structured
A single number for “fault level” is not enough. The study must define scenarios, the faults to apply, the contributions to include and the use of each result.
| Result | Used for |
|---|---|
| Maximum symmetrical and peak current | Breaking capacity, making capacity, bus bracing |
| Thermal let-through over clearing time | Cable and equipment withstand |
| Minimum fault current | Confirming protection sensitivity |
| Earth-fault current | Earth protection, touch voltage |
Fault types to calculate
- Three-phase for maximum breaking duty.
- Line-to-line, line-to-earth and double-line-to-earth for protection settings.
Scenarios on a generator site
- Utility only, maximum source.
- Utility only, minimum source.
- Generator only.
- Utility and generator in parallel, which gives the highest fault level.
Contributions to include
Utility, generators, running motors, and inverter sources with their limited current. Omitting motors understates the first-cycle duty; including motors that are not running overstates it. State the assumption.
Report format
Provide a table of fault level by bus and scenario, the equipment rating margin and the flagged items where rating is inadequate. See IEC calculation and fault level engineering.
Common mistakes
- Maximum case only.
- Omitting motor contribution.
- No check of cable withstand.
What to send us
Send the single-line diagram and equipment data. Send your application details. See also fault-level engineering.
Questions engineers ask
Why do I need a minimum case?
To confirm that protection will operate for the smallest credible fault.
Should I include motors?
Yes, for the maximum case, at least where motor load is significant.
What if equipment rating is exceeded?
Options include current limiting, higher-rated equipment, splitting the bus or changing the configuration.
How often should it be repeated?
After changes to sources, transformers or major loads.
This guide gives general principles. Studies must use verified data.