The two ETAP bus calculations show the same reported 30 kA short-circuit current and 24 kA arc current, but their incident-energy results diverge because the study selected different fault-clearing times. In the reported configuration, the upstream-device search stopped after three bus levels: one bus found the transformer fuse and used an 83-second clearing time, while the other could not find that fuse four levels away and used ETAP’s 0.1-second default. Check the protective-device path and clearing-time result before treating a large energy difference as a convergence problem.
Which reported values should you compare first?
Start with the output values for each faulted bus, not with a change to the connecting impedance. In this case, both buses showed 30 kA short-circuit current and 24 kA arc current, while the calculated incident energies were 11.99 cal/cm² and 8421 cal/cm². Equal current results do not prove that the arc-flash calculations used the same protective device or fault-clearing time.
For each bus, locate the fault-clearing time (FCT) and the protective device ETAP used to determine it. The reported configuration used the PRI XFMR fuse for one bus, with an FCT of 83 seconds. For the other bus, the fuse was four bus levels upstream, outside a configured search limit of three levels; ETAP assigned an FCT of 0.1 second. This difference explains why the energy results diverged even though the reported current values matched.
The 83-second value is a key diagnostic finding, not proof that the actual fuse will take that long to clear the modeled fault. Confirm the selected device and its operating time against the device data, time-current information, and study assumptions. The reported 0.1-second value is likewise a software fallback in this configuration, not confirmation that the real protection will clear in 0.1 second.
Does the signal pattern point to clearing time or current?
Arc-flash energy depends on the modeled arcing conditions and the duration of the arc before protection clears it. When the reported arc current matches but energy differs drastically, compare the selected protective device and FCT before investigating small impedance differences. The comparison below maps the useful readings to their likely diagnostic meaning.
| Signal or result | Source to inspect | Wrong-value symptom |
|---|---|---|
| Short-circuit current | Fault result at each bus | A mismatch directs attention to the network model, fault location, or study setup. |
| Arc current | Arc-flash result for each bus | A mismatch means the energy comparison is not isolating a clearing-time difference; compare the modeled arc conditions. |
| Protective-device selection and FCT | Device identified for the fault and its calculated clearing time | Different devices, a missing upstream device, or an unexpected FCT can produce a large energy difference. |
| Upstream-device search depth | Study configuration for the number of bus levels searched | A limit shorter than the device path can make the program stop before reaching the intended fuse or breaker. |
| Bus path and impedance | One-line topology, bus levels, and modeled connection | An artificial impedance or added bus can change the device-search path and expose a configuration boundary. |
Use the pattern as a decision aid rather than as a substitute for the detailed report. If short-circuit or arc current differs, investigate the electrical model and arc calculation inputs. If those currents match but FCT or device selection differs, trace the protection search first. If all three match but energy still differs, compare the remaining arc-flash calculation inputs and outputs for the two buses.
How can the device-search limit create this result?
ETAP searches upstream from a faulted bus to identify a protective device and determine its fault-clearing time. The configured search depth counts bus levels along that path. A device beyond the configured limit may not be selected, even when the one-line contains it and another nearby fault location can find it.
In the reported study, the search was configured for three bus levels above the fault. The PRI XFMR fuse was within that reach for one PDC, so ETAP used its reported 83-second FCT. The fuse was four levels away for the other PDC, so it fell outside the search and ETAP used the reported 0.1-second default. The different energy values followed from different protection timing inputs, not necessarily from different available fault current.
Small network edits can change this search result. Inserting an impedance element may introduce an additional bus level between a fault and its upstream device. The source reports that inserting a small impedance above the top bus made the fuse four levels away and caused ETAP to use the 0.1-second default there as well. That behavior can look like an impedance-sensitive numerical problem when the actual change is the protection-device search path.
Should you treat 0.001 ohms as a harmless bus connection?
The modeled arrangement used 0.001 ohms for R, X, and Y between buses because ETAP would not directly connect the bus bars. That value was intended to represent a very small impedance, and both buses returned the same reported short-circuit and arc currents. However, a small impedance does not make two bus objects identical in the software model. Bus boundaries can matter to the upstream-device search even when the current results appear unchanged.
Do not use a smaller impedance as the first fix for a large energy discrepancy. The reported case was initially attributed to convergence, and another suggested explanation was zero impedance. The later diagnosis identified a device-search configuration error instead. Changing the impedance may alter the one-line path or create another bus level without correcting the actual protective-device selection.
Model the connection in a way that represents the real electrical arrangement and is supported by the software, then inspect both current results and protection paths. If a bus-to-bus connection must be approximated with an impedance element, record the modeled R, X, and Y values and examine whether that element changes the number of buses between each fault location and its protective device. Treat any resulting change in FCT as a modeling or configuration change that must be verified.
What readings separate a topology issue from a protection issue?
Follow the signal path from network to result: the one-line defines the buses and impedance; the fault calculation determines available current; the arc-flash calculation produces arc current; the protection search identifies a device and FCT; and the incident-energy result reflects the modeled arc duration and conditions. Compare those stages in order, so a later output does not distract from an earlier difference.
- Read the one-line path. Trace from each faulted bus upstream to the intended fuse or breaker. Count the bus levels and note any impedance elements or inserted buses.
- Compare short-circuit results. Confirm that the reported current at the compared buses matches. If it does not, resolve the fault-model or topology difference before drawing conclusions from energy.
- Compare arc-current results. Record each arc current. If these differ, check the arc-flash inputs and calculation conditions rather than attributing the whole energy difference to clearing time.
- Read the selected device and FCT for each fault. Confirm that ETAP found the intended protective device. Compare its reported operating time with the study’s device data and fault conditions.
- Check the configured search depth. Compare the setting with the number of bus levels to the intended upstream device. If the device lies beyond the limit, increase the search depth only as needed to include the actual path, then recalculate.
- Re-run and inspect the detailed results. Confirm that both bus calculations now use the intended protective device and plausible FCTs. Recheck current and energy outputs, and investigate any remaining discrepancy using the inputs that still differ.
How should you correct and verify the reported configuration?
For the reported case, the resolving branch is the protection search, not an arbitrary impedance adjustment. Set the upstream-device search depth so the intended protective device lies within the search path for each faulted bus. Then verify that ETAP identifies the same intended fuse or breaker where the physical protection arrangement warrants it. Do not force matching device selection if the two buses are actually protected by different devices.
After changing the setting, recalculate each fault location and inspect the detailed results individually. Confirm:
- The intended upstream protective device appears for each bus.
- The FCT is taken from the correct device and matches its applicable operating data.
- The short-circuit and arc-current results remain consistent with the modeled network.
- The energy discrepancy has been addressed by a justified protection-time result rather than by relying on the software fallback.
- Any added impedance or bus remains an accurate representation of the installation, and the resulting bus-level count is understood.
Review the 83-second FCT especially carefully against the fuse’s applicable curve and modeled fault current. A very long time can dominate calculated incident energy, but changing the search depth only makes the software reach a device; it does not validate the device data or prove the resulting value is physically correct. Likewise, replacing an unlocated device with a 0.1-second default can produce a deceptively low energy result.
Why do the two ETAP energy results differ?
Why does ETAP show different arc-flash energy for buses with the same arc current?
Arc current alone does not determine incident energy; the selected protection and fault-clearing time matter. In the reported case, ETAP used an 83-second FCT for one bus and a 0.1-second default for the other because the upstream fuse was outside the configured three-level search for that bus.
Why does a small impedance change the ETAP arc-flash result?
A small impedance element can add a bus level or change the upstream path that ETAP searches. Compare the selected protective device and FCT before concluding that the impedance caused a convergence problem.
Why did ETAP use a 0.1-second clearing time?
In the reported configuration, ETAP used 0.1 second when the intended fuse was beyond the three-bus-level search limit. Confirm the selected device and search depth in the study results rather than treating 0.1 second as the actual protection time.
When should I stop changing the model and contact ETAP support?
Stop adjusting impedance when the device-selection path or reported FCT remains unexplained after checking the one-line, search depth, and detailed results. Escalate to ETAP technical support with the study configuration, one-line, fault location, selected device, and FCT outputs so they can review the model behavior.