Running short-circuit analysis

This guide shows you how to run short-circuit analysis on your single-line diagram and interpret the results.

Before you start

Ensure your diagram has:

At least one power source (utility feed or generator)
Complete network with all components connected
Utility feed short-circuit MVA and X/R ratio specified
Generator subtransient reactance (X''d) if applicable
Transformer impedance percentages
All component voltage ratings

Missing source impedance data causes analysis to fail.

Running the analysis

Open your project in the SLD editor.
Verify all sources have impedance data.
Click Short Circuit in the header.
EKX runs the validated three-phase maximum-current short-circuit calculation on the scenario selected in the header.
Results appear on the single-line diagram.

Every bus is faulted unless you leave it out. To skip a bus, untick it in the Faulted buses list on the Short Circuit tab of Diagram settings, or right-click it and clear Fault for Short Circuit. A bus left out shows no short-circuit values and gets no arc flash result. See choosing which buses are faulted.

Rerunning after changes

While short circuit results are shown on the canvas, they rerun by themselves after any change that affects them. Updating short circuit... shows in the header while the new results are calculated, and the earlier fault currents stay on screen until they are replaced. Results that were already stale when you open the project rerun the same way. Arc flash results do not rerun by themselves; run arc flash again after changes.

Results stay gray, with Results stale in the header, only when they cannot refresh by themselves: the automatic rerun failed, or you can view the project but not edit it. Hover over Results stale to see why the rerun failed. The next change that affects the results tries again, or click Short Circuit.

Selecting fault type

Three-phase fault (recommended for equipment rating):

Highest fault current
Use for breaker interrupting capacity
Most conservative

The current production workflow supports three-phase maximum-current analysis only. Phase-to-phase, single-phase, minimum-current, and all-fault-type workflows require separate validation before they are enabled.

Viewing results

Each faulted bus shows:

Initial symmetrical fault current (kA)
Peak fault current (kA)
Fault MVA

Choose which of these appear under Diagram Settings > Display Labels. A value compared to an equipment rating turns amber near the rating and red over it.

The diagram shows the scenario selected in the header. To find each bus's highest fault current in any scenario, run short circuit and arc flash in every scenario from Manage Scenarios... and read the Duty column of the arc flash results window's Worst case view, which names the scenario each value came from. It lists the buses studied for arc flash; the API returns every faulted bus. See the worst case across scenarios. Equipment ratings on the diagram and in the report are still checked against the selected scenario's results.

Interpreting fault currents

High fault currents (>20 kA at 480V):

Near utility connection or large generators
Require high interrupting capacity breakers (65kA, 100kA AIC)
May need current-limiting fuses
Higher arc flash hazard

Medium fault currents (10-20 kA):

Middle of distribution system
Standard industrial breakers (25kA, 42kA AIC)
Typical for most equipment

Low fault currents (<10 kA):

End of long feeders
Far from sources
Lower interrupting capacity acceptable
Verify protection sensitivity

Compare fault currents to breaker interrupting capacity ratings. Breaker AIC must exceed fault current at installation point.

Equipment verification

Circuit breakers:

Interrupting capacity ≥ fault current
Example: 20 kA fault requires ≥22 kA AIC breaker
Use next standard size up for safety margin

Buses and switchgear:

Short-circuit withstand rating
Based on peak current and duration
Verify bracing adequate

Cables:

Short-circuit withstand: I²t rating
Duration typically 0.1-1.0 seconds
Larger cables have higher withstand

Coordination verification

The Protection view can plot time-current curves and screen nominal curve pairs for crossings or tight spacing. A full short-circuit coordination workflow would also:

Plot device time-current curves.
Mark fault currents on the plot.
Verify upstream device is slower than downstream at fault current.
Maintain separation for selectivity.

After a short-circuit study, open the Protection tab, select a protection device, then open Latest study to review its calculated operating times at reachable fault locations. The result identifies the fault location, function, current through the device, and whether the value is minimum-melt time, operating time, or a verified total-clearing time. If only a magnetic pickup is known, EKX reports that pickup was reached without inventing an operating time. If the installed characteristic or relevant short-circuit inputs changed after the run, EKX keeps the historical result visible, labels it stale, and requires a rerun before use.

EKX attributes current from the cable terminal or transformer winding at the protection device's location. Where sources converge, a downstream device is evaluated once using its combined physical current; devices on the individual source legs use their respective branch currents.

> Important: Results use the nominal symmetrical current attributed to the device and the effective device rating for the study scenario. They support rating-selected point curves, fixed ETU groups, thermal-magnetic and fuse functions, plus recognized relay inverse-time and instantaneous functions. If EKX cannot uniquely prove the current through a device, such as on an ambiguous meshed or same-bus path, Latest study shows the current as unavailable instead of substituting the fault-bus current. Only documented, unmodified manufacturer or reviewed digitized fuse total-clear curves are marked usable as clearing time. This remains a screening aid, not a completed protection-coordination or arc-flash study.

Arc flash considerations

Fault currents determine arc flash incident energy:

Higher fault current:

Higher incident energy
Greater arc flash hazard
More extensive PPE required
Larger arc flash boundary

Lower fault current:

Lower incident energy
Reduced hazard
Less PPE needed

The arc flash study uses these fault currents as its input. It performs its own short-circuit calculation so that one run holds the fault currents, the protection evidence and the arc flash results together. See Running an arc flash study.

Troubleshooting failed analysis

"No sources defined":

Add utility feed or generator
Verify source has impedance data

"Missing transformer impedance":

Set transformer Z% in properties
Use typical value if nameplate unavailable

"Nothing in the drawing joins BUS-X to a utility, generator or slack bus":

Connect the named bus to the system, or delete it
A bus cut off only by an open device, an open wire or an out-of-service component is not an error. It shows No supply with no fault current, and the rest of the system solves.

"Calculation did not converge":

Verify all impedances are positive, non-zero
Check for missing component data
Review any issues shown before the calculation runs

Understanding short-circuit - Short-circuit fundamentals
Fault types - Different fault scenarios
Protection devices - Breaker sizing based on fault currents