Running an arc flash study

An arc flash study calculates incident energy and the arc flash boundary at every bus, using IEEE 1584-2018. It runs the short-circuit calculation itself, so one run holds the fault currents, the protection evidence and the arc flash results together.

Arc flash studies are available on the Professional plan.

Before you run

Each bus needs enough information for the model to cover it. Set these on the bus:

Equipment class and enclosure rating, on the Equipment tab.
Electrode configuration, electrode gap, working distance and enclosure height, width and depth,

on the Study tab. Leave a field blank to use the equipment class value; type a value to use what you measured.

Main device isolated, on the Study tab. The class default follows ETAP: switchgear, cable

junction boxes and open air are isolated, so the run credits the bus's own main with clearing a bus fault. MCCs, switchboards, panelboards, other equipment and buses with no class are not, so the run credits the next device upstream. Choose Isolated or Not isolated when you have checked the enclosure; a cable on the drawing is not that evidence.

Equipment-class defaults appear in grey inside empty inputs and are used automatically. Set an equipment class to supply these defaults, or enter the geometry yourself. The results record which values came from the class.

The protective devices upstream of each bus need curves that can produce a clearing time. A device that only publishes an operating time, or whose curve provenance is unverified, gives the bus no clearing time and no result.

Pre-fault voltage

The arc flash study solves its own fault currents, so it has its own pre-fault voltage setting in Diagram settings, Arc Flash tab. It works the same way as the short-circuit setting and does not follow it. New projects start at a fixed 100% of nominal.

A fixed percent also sets the arc voltage. The IEEE 1584 equations use that percent of the bus's nominal voltage, as ETAP does, so a 208 V bus at 105% is calculated at 218.4 V. The IEC 60909 voltage factor leaves the arc voltage at nominal. Results, reports and labels still show the nominal voltage. IEEE 1584 covers 208 V to 15 kV at the calculated voltage, so a 208 V bus below 100% is reported as outside the model.

Choosing which buses are faulted

Every bus is faulted for short circuit and arc flash unless you turn it off. Leave out a bus that exists only to connect the drawing, such as a splice or tap point, so it gets no result, label or report row. There are three places to change it:

Diagram settings, the Faulted buses list on the Short Circuit tab and on the Arc Flash tab: one

row per bus with a box for that study. Search narrows the list, and the Excluded filter shows only buses left out. Ticking the column heading applies it to every bus showing.

Right-click a bus, or a selection of buses: Fault for Short Circuit and Fault for Arc Flash.
The bus's Study tab: the Fault for group.

Arc flash needs the fault current at the bus, so the two settings move together. Ticking arc flash also ticks short circuit, and unticking short circuit also unticks arc flash. The setting applies to every scenario. Changing it marks earlier results out of date; run the study again to update them.

If no bus is faulted for a study, it does not run. A notice says so, and Choose buses opens that study's tab in Diagram settings.

Running it

Select the scenario in the project header, then click Arc Flash beside Short Circuit and Load Flow. There is nothing to choose first: every other input is a project or bus setting. The study runs and reports how many buses came back with a result.

Reading the results

Open Results from the Arc Flash button's menu. The table lists every bus the run covered, with the bolted current, both arcing currents, the clearing device, the duration, incident energy and the boundary. Click any heading to sort; click a row to open that bus.

The heading counts three things: how many buses were calculated, how many are unavailable, and how many rest on a duration that did not come from the clearing device. A bus without a result stays in the table with its reason, so you can see which equipment the study does not speak for.

A bus that the scenario's open devices, open wires or out-of-service equipment cut off from every source is listed under No supply. It has no fault, so it has no incident energy in that scenario and nothing to fix. Run the scenario where it is fed to get its result.

When a utility has more than one fault case included in arc flash, the table gains a Utility case column naming the case each bus's values came from, and a bus without a result names the case that left it without one and that case's bolted current. An excluded-motor warning names the case that raised it. The report's results table has the same column, and its method lists the cases calculated. The fault labels on a TCC show the arcing currents of the case that governs, and none for a bus without a result. A project whose utilities each have one case shows no case anywhere.

On the diagram, each bus shows incident energy, the boundary and the clearing time under its short-circuit values. A bus without a result shows why instead. Use the eye control in the header to show or hide them, and Diagram settings to choose which values appear.

Clearing time

Some buses have no clearing time of their own. A bus fed directly by the utility, with no modelled device in front of it, is the common case.

The study uses the upstream protection to resolve clearing time automatically. If it cannot resolve a time, the bus shows the reason instead of an energy value. To give that bus a result, fix the protection or accept a duration for it. After each run, Messages counts the buses without a result, and a bus without one shows No arc flash result at the top of its panel.

When nothing on the drawing can clear a bus, the reason names the connection to put a fuse or breaker on. For a bus fed by a transformer that is the transformer primary, for example "between U-001 and TX-001": a bus with no equipment class counts its own main as inside its enclosure, so that main cannot clear an arc on its supply side. If the main has its own compartment, set Main device isolated to Isolated instead.

A fuse or breaker clears an arc only on its manufacturer's curve, so a device with no model chosen gives no clearing time; the study does not assume one from its rating. In the results window, select the link beside the bus, such as Choose F-001's model, to open that device's model list, then run the study again.

A fuse clears on its published total-clear curve, never its minimum-melt curve. When the arcing current is higher than the last point of that curve, the study uses the time at that last point, the fastest the manufacturer publishes, because a fuse clears no slower at a higher current. The curve is not extended, and the clearing time carries a flag reading "Held at the fuse curve's fastest published time" in the results window, on the diagram, in the report and on the label.

A breaker whose published curve ends below the arcing current is held the same way, with the flag "Held at the breaker curve's fastest published time". An arcing current up to 0.5% above a step in a breaker's curve, such as the top of its magnetic pickup band, uses the slower time at the top of the step, as ETAP does.

A documented device or utility clearing time replaces the device's own time. An exposure assumption is the longest a worker stays at the equipment, so it limits each case rather than replacing it: when the device clears an arcing current sooner, that case uses the device's time. For example, a breaker that trips instantaneously at the full arcing current but falls into its time delay at the variation-corrected current gives the full case its instantaneous time and the variation-corrected case the accepted exposure time. Only the case the assumption sets is flagged.

The two-second cap is off by default. Turning it on in the project's arc flash settings caps only buses that have an accepted exposure assumption, and every capped result is flagged. It never caps a device time, documented time or utility time on its own.

Accepting a duration

On the bus's Study tab, under Accepted duration, select Add accepted duration and enter:

Duration (s): the time in seconds.
Basis: Documented device time or Utility documented time replaces the device's time

for both cases. Exposure assumption is the longest a worker stays at the equipment, and limits each case as described above.

Source: where the time comes from, such as a relay settings letter.
Why it applies here: why the time holds at this bus.
Worker movement, for an exposure assumption only: whether the worker can move away and what

is in the way.

Select Accept to save it. EKX records you as the accepting engineer, with today's date, and records the acceptance again whenever you edit it. The next arc flash run uses the duration, and every result it sets is flagged "Accepted duration" in the results window, on the diagram, in the report and on the label. Select Remove to delete it; the bus then shows its reason again on the next run. A duration supplied through the API or an import is accepted the same way, by the user who sends it.

Utility fault cases

A utility letter often gives more than one fault level: a strong case for equipment duty and a weak case, which can give a higher incident energy because the protection clears more slowly at a lower current. Enter each case on the utility's Study tab and choose the studies it is included in (see Utility feeds).

The arc flash study calculates every case included in arc flash. With several utilities, it calculates every combination of their included cases, up to 16. At each bus it reports the case with the highest incident energy, and every value in that row, from the arcing currents and the clearing device to the duration and its flags, comes from that case. The arc flash boundary is the largest across all the cases, and a warning any case raises, such as excluded motor current above 10% of the bolted current, is shown with the case that raised it.

When any included case has no result at a bus, the bus has no result: the highest energy is unknown. The bus names that case and its bolted fault current, for example a weak case whose arcing current is below the 0.5 kA that IEEE 1584 covers at 208 to 600 V, or below the first point of the device's curve. Decide whether that case belongs in arc flash; to report the other cases, untick Arc flash for it on the utility's Study tab.

A project whose utilities each have one case is calculated exactly as before.

Worst case across scenarios

When a project has operating scenarios, such as a tie open or closed or a main out of service, the labelled energy should be the worst of them. Open Manage Scenarios... from the scenario selector and click Short Circuit and Arc Flash under Run all scenarios, or choose Worst case at the top of the results window and click Run all scenarios. Both run short circuit and arc flash in every scenario, Base included.

At each bus the scenario with the highest incident energy governs, the same rule as for utility fault cases, and every value in its row comes from that scenario. A bus with no supply in a scenario is skipped for that scenario; a bus without a result in any other scenario has no worst case and names that scenario. The Worst case view adds a Scenario column, and a Duty column with the highest short-circuit current at the bus in any scenario, which can come from a different scenario than the energy. See the worst case across scenarios for when it goes out of date.

What the study does not cover

Faults at load terminals, single-phase and direct-current arcs, and voltages above 15 kV.
Systems with more than one independent source, including sources that converge on one device.
Buses a running motor of 50 hp or more can feed. A motor in another part of the system, one that

is out of service, or one you have recorded as stopped does not make a bus unavailable.

Buses fed from the output of a variable-frequency drive.
Arc flash at overloads mounted on motor control devices.

Motors under 50 hp are left out of the arc flash fault current, so they never make a bus unavailable. The short-circuit results on the drawing still include them, which is why a bus's arc flash bolted current can be lower than its short-circuit current.

A bus in one of these systems is reported as unavailable with the reason, rather than given an approximate number.

Reports and labels

The report's arc flash section covers the inputs, the method, the assumptions, the per-bus results and the buses without one. Export it from Export > Study report as Arc Flash Analysis, or as part of a combined report. It uses the latest arc flash run for the scenario on screen and marks that run stale if the project changed after it. Labels print one 4 by 6 inch landscape page per bus, with a red DANGER header at or above your project's threshold and a safety orange WARNING header below it. A bus without a result, including one with no supply in the run's scenario, gets no label and is named on the cover page instead.

To print labels, click Print labels in the arc flash results window. A new tab shows the cover page and the labels of the latest run; click Print. In a project with scenarios besides Base, labels print from the Worst case view, and each footer names the scenario the numbers came from. When the run is out of date, or a scenario has no result, the sheet prints no labels and says what to run again.

Choose the label standard, the danger threshold, who completed the study, the label date and the length unit in the project's arc flash settings.