In short. Fault level is the current that flows in a short circuit. It sets the minimum breaking capacity of devices and busbar withstand (maximum case) and also checks that protection will pick up (minimum case). Generators give a lower, decaying fault current, so both cases matter.
What contributes to the fault current
- Utility: set by the supply fault level or short-circuit MVA.
- Transformers: limit current according to impedance.
- Generators: contribute by subtransient reactance, then transient, then synchronous values as the fault current decays.
- Motors: contribute briefly during a fault, important on motor-heavy boards.
- Cables: add impedance and reduce fault current with distance.
Simple estimates
For a transformer, the approximate fault current on the secondary is its full-load current divided by its per-unit impedance. For a generator, the initial symmetrical value is approximately its full-load current divided by the subtransient reactance in per unit.
If ≈ IFL ÷ Zpu
These are screening values. A full study includes source impedance, cables and motor contribution.
Maximum and minimum cases
| Case | Use |
|---|---|
| Maximum | Breaker breaking capacity, busbar and cable withstand, equipment ratings |
| Minimum | Check that protection picks up and trips in time, important on generator supply |
Generator behaviour
A generator’s short-circuit current falls over a short time as flux decays. An alternator without a way to sustain excitation can fall below the overcurrent pickup, and the fault is not cleared. Excitation systems that provide short-circuit support, such as a PMG, help. See alternator reactance and short circuit.
Using the result
- Compare maximum fault level with breaker ratings. See device selection.
- Check busbar short-time withstand.
- Use minimum fault level to confirm pickup and speed.
- Feed results to coordination and arc-flash studies.
Illustrative example
A 1,250 kVA transformer at 5 percent impedance gives a secondary fault current of roughly twenty times full load. The same board on a 1,000 kVA generator with 15 percent subtransient reactance might see only about seven times its full-load current, decaying toward about three times. A relay set for the utility case may not trip on the generator alone, so a separate setting is needed.
A worked fault-level calculation (illustrative)
Consider a 1,000 kVA, 11 kV/415 V transformer with 5 percent impedance, supplied from a strong utility so that source impedance is small.
Rated current = 1,000 ÷ (√3 × 0.415) ≈ 1,391 A
Fault current ≈ Rated current ÷ impedance = 1,391 ÷ 0.05 ≈ 27.8 kA
So switchgear on the secondary must be rated above roughly 28 kA, with margin. Adding a weaker source impedance lowers this slightly, but design practice uses the higher figure.
Generator fault current
A generator’s short-circuit current is driven by its reactances. Initially (subtransient) it may be several times rated current, then decays toward the lower steady-state value, which depends on excitation. For a set with 15 percent subtransient reactance, the initial fault is about 6.7 times rated current. After a short time it may fall to around three times rated with a field-forcing exciter, or lower without. Protection must work with these shapes. See excitation where relevant.
Why both maximum and minimum matter
| Case | Used for |
|---|---|
| Maximum fault level | Equipment withstand and breaking capacity |
| Minimum fault level | Confirming protection will trip on the smallest fault, for example on generator only |
Contribution from motors
Running motors feed current into a fault for a few cycles. In plants with large motor load, add this to the fault level for breaker making and breaking duty.
Report content
- Assumptions and source data.
- Fault levels at each bus, maximum and minimum.
- Equipment ratings compared with results.
- Recommendations.
Related study: utility fault level and source impedance.
Common mistakes
- Checking only the maximum case.
- Ignoring changes after adding a generator or transformer.
- Using the nameplate impedance without checking tolerance.
What to send us
Send the single-line diagram, transformer and generator data and cable lengths. Send your application details. See also fault current basics.
Questions engineers ask
Why does fault current matter?
It decides whether devices can safely interrupt a fault and whether protection will operate.
Is generator fault current always lower?
Usually, compared with a strong utility, and it decays with time.
Should motors be included?
On motor-heavy boards, yes, because they feed current into a fault briefly.
Do I need to redo the study after changes?
Yes, after adding sources, transformers or large motors.
Values here are illustrative screening figures. A full study is needed for design.