In short. The utility is the largest source in most short-circuit studies, and it is represented by its fault level (short-circuit MVA or kA) at the connection point and its X/R ratio. You need both a maximum and a minimum value, ideally from the utility in writing.
What to request
- Maximum and minimum three-phase short-circuit level at the point of connection, in MVA or kA.
- Single-phase to earth fault level or the earthing arrangement at the utility transformer.
- X/R ratio at the connection point.
- Voltage range and tolerance.
- Any future change expected, such as network reinforcement.
Converting fault level to impedance
If the utility gives short-circuit MVA at the connection voltage, the equivalent source impedance in ohms is calculated from the voltage and the MVA.
Zsource = U² ÷ Ssc
Where U is the line voltage and Ssc the short-circuit apparent power. The reactance is usually most of this value, with the resistance found from X/R. Refer the impedance to the voltage level you study using the transformer ratio.
Maximum versus minimum
| Value | Used for |
|---|---|
| Maximum fault level | Equipment ratings, arc-flash worst case |
| Minimum fault level | Protection sensitivity and trip time at the weakest condition |
The utility’s fault level can change with network configuration, generation dispatch and reinforcement, so ask for both values and a stated validity.
Effect of the transformer
For an LV board fed by a transformer, the fault level on the LV side is limited mainly by transformer impedance, and a stronger utility raises it only a little. The utility’s value matters more at higher voltage. See transformer impedance.
When utility data is not available
- Use a conservative assumption: infinite bus for the maximum case.
- For the minimum case, assume a weak source, and flag the assumption.
- Record the assumption in the study report and update when data arrives. See inputs and validation.
Illustrative example
A utility states 250 MVA maximum and 150 MVA minimum at 11 kV. The corresponding source impedances differ by about 40 percent. A 1,250 kVA, 6 percent transformer at the site sees a fault level on its LV side that is mostly set by the transformer, so the utility range changes the LV fault current only modestly, but changes the 11 kV breaker duty significantly.
Getting the utility source right
The utility is represented by its fault level at the point of supply, either in MVA or kA, plus an X/R ratio. The distribution company can state these values; request them in writing for both maximum and minimum conditions.
Source impedance Zs = Un² ÷ S”k, referred to the study voltage
Worked example (illustrative)
An 11 kV supply with 250 MVA fault level: Zs = 11² ÷ 250 = 0.484 ohm at 11 kV. Referred to 415 V on a 1,000 kVA, 5 percent transformer: transformer impedance is 0.05 × (0.415² ÷ 1.0) = 8.6 milliohm; source impedance at 415 V is 0.484 × (0.415 ÷ 11)² ≈ 0.69 milliohm. The source adds only about 8 percent to total impedance, so the transformer dominates the fault level.
Why minimum matters
| Case | Effect |
|---|---|
| Maximum utility level | Sets equipment rating |
| Minimum utility level | Sets protection sensitivity, especially earth-fault tripping |
What to request from the utility
- Maximum and minimum three-phase fault level at the connection point.
- X/R ratio.
- Zero-sequence data for earth-fault study.
- Planned changes that could alter the level.
Weak grids
Where the utility is weak, voltage dip from motor starting is larger and generator interaction is stronger. Treat the utility as a source with finite impedance, not an infinite bus.
See short-circuit architecture and transformer contribution.
Common mistakes
- Using only the maximum value.
- Forgetting that data changes with network reinforcement.
- Mixing MVA and kA without voltage reference.
What to send us
Send the utility’s letter or data sheet. Send your application details. See also short-circuit study architecture.
Questions engineers ask
What does short-circuit MVA mean?
The apparent power the source could deliver into a three-phase fault at the connection point.
Why is X/R important?
It affects the DC component and peak current, and the breaker’s duty.
Who provides the data?
The distribution or transmission utility, usually on request for an interconnection or design.
Can I assume an infinite bus?
For the maximum case it is conservative. For the minimum case it is not.
This guide gives general principles. Use data supplied by the utility.