In short. The maximum case finds the highest fault current so equipment is not overstressed. The minimum case finds the lowest credible fault current so protection still operates and trips fast enough. Both are needed, and on generator-supplied systems the minimum case is often the critical one.
Assumptions that differ
| Item | Maximum case | Minimum case |
|---|---|---|
| Sources in service | All possible, including parallel generators and utility | Weakest credible combination (for example, a single generator) |
| Utility fault level | Maximum | Minimum |
| Voltage factor | Maximum | Minimum |
| Motors | Included | Excluded |
| Conductor resistance | At low temperature | At operating temperature |
| Transformer impedance | Lower tolerance | Upper tolerance |
| Fault type | Usually three-phase | Usually phase-to-phase or phase-to-earth |
| Fault resistance | Zero | May include arc or contact resistance |
How the results are used
- Maximum: breaker making and breaking capacity, busbar and cable withstand, arc-flash worst case, protective device upper limits.
- Minimum: relay pickup and sensitivity, clearing time at the lowest fault current, cable protection by the adiabatic check, earth-loop disconnection times. See cable protection.
Generator-only conditions
When the utility is lost, a single generator may supply the whole site. Fault current is then only a few times the generator rating and decays. Breakers and relays sized for the utility’s thousands of amps may take seconds, or fail, to clear a fault. This is the most common gap found in studies of standby systems. See generator contribution.
Workflow
- Define the operating configurations that could occur, including maintenance states.
- Identify the weakest and strongest configurations.
- Calculate each fault type at each bus in each case.
- Compare the maximum results with equipment ratings.
- Compare the minimum results with protection pickup and time requirements.
- Resolve conflicts: for example, a setting that suits both cases, or a second setting group.
Illustrative example
A distribution board shows a maximum fault current of 25 kA with the utility and a generator in parallel and about 3.5 kA with the generator alone. A feeder breaker with an instantaneous setting of 4 kA would operate in the first case but not the second, where the cable could overheat before the long-time element trips. The study recommends a lower setting under generator supply and a check of cable withstand.
Two studies, two purposes
| Study | Assumptions | Answers |
|---|---|---|
| Maximum | Highest utility level, all generators and motors on, minimum impedance tolerance, voltage factor 1.05 to 1.10 | Is the equipment strong enough? |
| Minimum | Lowest utility level, only essential sources, longest cable lengths, hot conductor resistance, voltage factor 0.95 to 1.0 | Will protection detect and clear the smallest fault? |
Worked comparison (illustrative)
A 415 V board shows 28 kA maximum with utility and generator in parallel, and 3.2 kA minimum with the generator alone. A breaker with an instantaneous setting of 6 kA clears the maximum fault but never trips instantly on the minimum. The long-time or short-time function must then clear the fault, which may take seconds. The study exposes this before it happens.
Details for the minimum case
- Use conductor resistance at fault temperature, which is higher than at 20 degrees C.
- Include arc resistance for arcing earth faults.
- Consider the far end of long feeders.
- Check earth-fault loop impedance for the disconnection time required.
Using the results
- Maximum values select breaker ratings and bus bracing.
- Minimum values verify pickup margins, normally at least 1.5 times pickup.
See coordination workflow and overcurrent protection.
Common mistakes
- A maximum-only study.
- Skipping the generator-only case.
- Mixing assumptions between cases.
What to send us
Send the single-line diagram and source data. Send your application details. See also short-circuit study architecture.
Questions engineers ask
Which case is more important?
Both. One protects equipment; the other makes sure protection works.
Why exclude motors in the minimum case?
They may not be running, and their contribution is brief, so excluding them gives a conservative lower value.
What if one setting cannot cover both?
Use setting groups, additional elements or different devices.
Does the minimum case include arc resistance?
It can, in some methods, since arcs reduce current.
This guide gives general principles. Cases should follow the chosen standard.