Ten Most Common Errors in Arc Flash Studies
Each year we see dozens of arc flash studies ranging from first attempts to submittals from seasoned veterans in the industry. With an eye towards improving the overall results in the field, we offer this list of “Ten Most Common Errors.” This is not a criticism of anyone or any group; it is more of an effort to bring some insight to areas that may not be that well understood, including changes in the standards, the phenomenon of arc flash, and the application of analytical tools toward that end.
#1 - The Lack of Understanding the “Arc Flash Paradox.”
The counter-intuitive nature of the arc flash hazard tends to impede the acceptance of arc flash as a hazard by some of us with a long history in the industry. Put simply, within the same system you can have buses that have high current and high arc incident energy AND other buses that have lower current but that calculate a higher arc incident energy. Incident energy has a significant relationship to the time required to clear the arc.
Comparing Bus-4 to Bus-2 in the example below, Bus-4 has a higher incident energy in spite of having available fault current at Bus-4 that is only 20% of the fault current available at Bus-2. The length of time to clear the arc in this case has a significant bearing on the calculation.
#2 - “Infinite source” is Not the Worst Case
Nor should it be used in place of actual utility data when calculating arc flash results. Often times, the local utility company cannot or does not respond to a request for short circuit data as required for an arc flash hazard assessment. From the early days of power system design, the engineer responsible for the electrical system was tasked with the requirement to have the system be able to safely protect itself against the worst-case short circuit current that it could possibly see from the utility source. The concern was due to the short circuit rating for each piece of equipment in the system. Each piece of equipment needed to be able to survive in the case of a short circuit fault. The results of an equipment duty analysis will determine if any system element has the potential for being over-stressed in the case of a worst-case fault at that location. In that case, using an infinite source would be valid to determine system integrity. In the case of arc flash analysis, however, the use of a higher short circuit current value than is really expected could potentially yield a lower result for arcing incident energy since the higher current value of the infinite source would cause the protective devices to trip quicker than the same circuit exposed to actual or estimated fault current at the utility connection. This can be shown in the framework of an EasyPower study by using the Scenario Manager to compare results.
#3 - Not all buses are created equal!
The long-awaited release of IEEE 1584-2018 promised improved alignment between model results and the test data upon which the model is built. Part of this improvement is the inclusion of a number of parameters that were not included in the original arc flash calculation method. Included in the updated standard is a table of typical values for each of these parameters along with equipment type and different voltage levels.
It is therefore a good practice to ensure that the data collection team can identify the type of switchgear or panelboard and at the same time recognize if there is a significant difference between the typical values listed in the standard and record the actual values for:
- Working distance
- Electrode gap
- Enclosure size
- Electrode configuration
In EasyPower, the default values for each of these parameters are included in the Arc Flash Hazard Default table in the Device Library. If the actual system diverges from the default values significantly, it is incumbent on the person creating the digital model of the system to update the recorded values during model creation.
Arc Flash Hazard Default Values
Typical Parameters in the Arc Flash Hazard Tab
#4 - Why Not Put 6 Inches Working Distance on the Label, Since That is What My Hand Will See?
It is expected that the results of the arc flash study will be reviewed by management with an eye towards verifying that all hot work required in the plant will be necessary and conducted with the utmost focus on employee safety. As such, all hot work should be conducted under the auspices of properly completed Hot Work Permit. Labels do not authorize hot work. The working distance on the label is generally based upon the distance from the bus to the head or thorax of a worker while the panel cover is being removed. It is really a convenient number to permit the qualified worker to understand the relative magnitude of the hazard when the cover is removed. Additional information on the label is discouraged to prevent any misunderstanding of this restriction.
#5 - Why Calculate Equipment Duty on Plant Equipment That Has Already Been in Operation For 25 Years?
If it has been working for that long without an issue, why bother? Right? SCCR (short circuit current rating) is the maximum short-circuit current an electrical component can safely withstand without causing shock or fire hazard. In general, the SCCR for electrical switchgear or panels is based on the lowest component’s SCCR in that panel. Protective devices included in the one-line diagram must be linked to the datasheet for that element located in the Device Library. As long as the one-line element is linked to the Device Library (meaning it is verified using the Calculate button on the Short Circuit tab of the Data dialog box), EasyPower calculates the equipment duty during a short circuit analysis.
Bus Bracing Missing in the Data Dialog Box for an MCC
The withstand rating of switchboards, panels, and MCC enclosures is usually listed on the nameplate of the installed element. In EasyPower, this rating should be entered in the dialog box for the element to be included in the equipment duty calculation. Consideration should be given to system changes over the life of the installation. Changes in load size and location, increases in fuse sizes, breaker trip settings, as well as unannounced changes in utility short circuit values could render the original system design guard bands inadequate. Even if the SCCR for some elements may not have been entered during the original arc flash study, it seems prudent to include the data during study updates.
#6 - We Do Not Need a Coordination Study on Protective Devices During This Update. We Did It Five Years Ago
You may be correct unless:
- Some blown fuses were swapped out with a different manufacturer’s product or values because they were available during urgent times.
- Relay or LVPCB trip settings were tweaked to eliminate nuisance tripping.
- Large system loads have been added, moved, or deleted.
At a minimum, you should check the Sequence of Events. PowerProtector™ is the name of the EasyPower module used for plotting TCC curves and verifying coordination. When system fault current is displayed during a single bus fault, EasyPower illuminates an icon labeled Sequence of Events. A simple click on that icon displays the chronological trip order of all device between the faulted single bus and the energy sources affected, providing a quick and efficient way to verify device coordination.
#7 - I Always Calculate Arc Incident Energy Including the Main Breaker Because I Don’t Think It Will Fail
The IEEE 1584 standards (both 2002 and 2018) recommend excluding the main breaker of a panel or switchgear when there is not an arc barrier between the incoming conductors to the main breaker and anyone working downstream on the main bus. The reason has less to do with the reliability of the breaker itself and is more based on the fact that once established, an arcing fault will produce a plasma ball, often several millimeters in diameter. If that plasma ball contacts any electrified bus work, the arc can propagate and potentially render the protective device useless. Verifying the existence of an arc barrier between the incoming conductors and the downstream bus is the only way to recommend including the main breaker.
Worst-Case Arc Flash Hazards Indicating the Option to Exclude Main Breaker and Use the Integrated Method to Calculate Arc Flash
#8 - Half-cycle (Momentary) Current is the Worst-case Fault Current, so I Use It to Calculate Arc Incident Energy
Recognizing that the available fault current may change over time, IEEE 1584-2018 recommends that a dynamic assessment of fault current be used to accommodate changes that may be the result of reducing motor contribution, load drops, or other changes in fault current. In EasyPower, the recommendation is to use the Integrated Method.
In the EasyPower Integrated Method, during:
- The first cycle of the fault, momentary current is used for energy integration.
- The next 8 cycles, interrupting (5-cycle) current is used to calculate energy.
- For any additional time to clear the protective device, 30-cycle current is used.
9 & 10 Deal with labels, which are the source of a large number of frequent errors. These are two of the most frequent:
#9 - I Want My Labels to Show the HRC #, in Spite of the NFPA 70E Changes
NFPA 70E revision history includes the following changes:
- The 2012 version had Hazard Risk Category (HRC) tables.
- The intent was to calculate incident energy and then use the table to determine the PPE needed.
- The 2015 version changed HRC to PPE category.
- The label may have calculated Incident Energy OR PPE category, but NOT BOTH.
- Allows for a “Site Specific” option for companies to define their own categories.
- The 2018 version repeats label restrictions.
- The industry is migrating to align training for labels and PPE rating to reflect the incident energy.
While the standard is voluntary, the industry is moving to align with the NFPA 70E recommendations to label PPE with incident energy. So, the reasons (if any) to stay with using the HRC # on labels should be reconsidered. We recommend:
- Set a timetable to achieve NFPA 70E alignment.
- Decide between Site Specific or Incident Energy to determine PPE.
- Update the safety manual and training to follow NFPA 70E,
#10 - I Want to Put Maintenance Mode incident Energy on the Label
For at least the last 10 years, hardware companies have made available a feature that enables the user to temporarily set the trip time of a relay or LVPCB unit to be much quicker and provide a safer, lower arc hazard on the protected bus. This feature can be enabled by a switch on the front panel or an electric control signal routed to the back of the panel and is called Maintenance Mode. This is a method of hazard reduction preferred over just adding more PPE for the worker at the higher energy level. It also falls into the category of hot work. Therefore, the information should not be on a label. Instead, the maintenance procedure, correct PPE, and instructions to use the Maintenance Mode feature should be approved and listed on the Hot Work Permit.
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