Arc Flash

Your questions answered with Zia Salami: Arc flash mitigation design

During this arc flash mitigation webcast, a variety of questions were raised; read answers to those items not covered during the live event

By Zia Salami, Ph.D. September 29, 2021
Courtesy: CDM Smith

De-energizing electrical equipment results in the safest conditions for electrical workers. However, in most industry applications, de-energizing the electrical system may not be practical and in some cases, may result in an even greater safety hazard.

Reducing arc flash to electrical personnel is an evolutionary process. No individual solution will eliminate all of the hazards of working on energized equipment. Electrical engineers must understand the codes, standards and design requirements when engineering a facility’s electrical power system distributions for arc flash mitigation. The mitigation techniques involve taking steps to minimize the level of hazard associated with an arc flash event. Many factors go into this process and engineers must thoughtfully consider the design and overall mitigation strategy.

During the Sept. 16, 2021, webcast on “Arc flash mitigation design,’ several items were left unanswered. Read some of the responses here.

Responses are from Zia Salami, Ph.D., Management Specialist – SME: Electrical Power System, CDM Smith, Charlotte, North Carolina.

Question: In general, owners, facility managers, engineers and contractors can all contribute to providing a safer place for personnel to work. The process starts with adequate and reliable engineering design, followed by proper installation of equipment, training and maintenance procedures. Is the C-plot area function also available for SKM?

Zia Salami: I am not sure, but I don’t believe so. As I mentioned in our presentation, this concept is by ETAP.

Question: Should old arc flash labels be removed and replaced with new labels reflecting the current system arc flash calculation results? If yes, how often?

Zia Salami: Yes, every five years or if there is a change in the system. NFPA 70E: Standard for Electrical Safety in the Workplace states “The incident energy analysis shall be updated when changes occur in the electrical distribution system that could affect the results of the analysis. The incident energy analysis shall also be reviewed for accuracy at intervals not to exceed five years.”

Question: Does the engineering software (i.e., ETAP, SKM, etc.) have the ability to include arc flash mitigation equipment?

Zia Salami: No, not using mitigation equipment, however, you can utilize the software to perform study with different mitigation methods as we did in our presentation.

Courtesy: CDM Smith

Courtesy: CDM Smith

Question: Is it OK to have overlapping curves below 0.1 second (for selective coordination)?

Zia Salami: In general, upstream and downstream protective devices overlapping at low voltage is common in industry, especially for molded case circuit breaker type protective devices. Therefore, there is a chance that the upstream protective device trips first and this should be evaluated between minimum and maximum available faults. A MCCB typically does not have an instantaneous time delay to make the adjustment and resolve overlapping issues. I typically recommend to select a different type of protective device with more adjustable settings such as power circuit breaker if there is a history of tripping within these locations.

Question: What is the cost addition for ZSI feature?

Zia Salami: Not sure. That depends on complexity of the system, number of protection zone, distance between zone, communication method and more.

Question: If there is arc-resistant switchgear, is an arc flash study still required?

Zia Salami: Yes. An arc-resistant switchgear reduces the amount of energy to which the person is exposed to. However, it does not reduce the electrical energy stored in the circuit. The actual energy and ratings need to be obtained from the manufacturer. Arc flash results can be used to determine if the kA cycle rating of the switchgear is appropriate for the energy, which can be generated by the electrical system.

Question: Can you analyze optical sensor in ETAP?

Zia Salami: Yes, if you know the total fault clearing time at the location that you plan to evaluate. You can assign fixed FCT in ETAP and perform the study.

Question: In ETAP, do we need to set FCT to include breaker opening time plus arc flash relay time?

Zia Salami: No need, time delay is been assigned for each type protective device (e.g., circuit breaker, relay, etc.) and will be added to FCT.

Question: Is the C-plot area function also available for SKM?

Zia Salami: I am not sure, but I don’t believe so. As mentioned in our presentation, this concept is by ETAP.

Question: For changes in arc flash studies, define what a large motor load change is. Is, for example 20 to 40 horsepower is that a trigger to revise the study?

Zia Salami: NFPA 70E does not define what is meant by a change to the electrical distribution system. There are no hard and fast rules. Best to just rerun the analysis with the changed motor to be on the safe side. In your case, the fault current contribution from small motor may change from approximately 100A to 200A and may have real small impact to your system. The requirement to review the arc flash risk assessment is not a requirement to have an entire study conducted. The review may just be verification or short evaluation that led to the original assessment result.

Question: What if the table method is used for arc flash? Is an incident energy required to be on label?

Zia Salami: The arc flash personal protective equipment category method (i.e., table method) can only be used for specific equipment listed in the table with defined maximum available short circuit current, fault clearing time and minimum working distance, otherwise, the incident energy calculation method must be used. The table is not providing incident energy; therefore, it is not required to be on the label.

Question: Is it required to do arc flash analysis for 1,200 A, 120/208 V, 3-phase, 4 W service?

Zia Salami: IEEE 1584-2018 states “sustainable arcs are possible but less likely in three-phase systems operating at 240 V nominal or less with an available short-circuit current less than 2,000 A.” Therefore, it is up to you, I would recommend to have arc flash labels for these locations.

Question: Which mitigation method discussed is more cost-effective for existing power systems?

Zia Salami: In my opinion, more cost-effective approach is changing the existing settings when possible or replace with more adjustable protective device.

Question: If nothing has changed in the building during those five years, what part of the study do they need to re-run? Do they have to go out and field verify everything again? If that was part of the original study? 

Zia Salami: NFPA 70E states: “The incident energy analysis shall be updated when changes occur in the electrical distribution system that could affect the results of the analysis. The incident energy analysis shall also be reviewed for accuracy at intervals not to exceed five years.”

If nothing has been changed in the system (e.g., CB, conductor, transformer, ect) in the last five years, equipment have been maintained properly, and no change in utility short circuit contribution, the review could be in form of verification of the original assessment including new label with the new date.

Question: The presentation focused on energy at the motor control center bus. Do you ever focus on the line side of the “main” motor control center breaker?

Zia Salami: Analyzing the bus (only) was just for presentation due to limited time that we had during the webinar. However, the line side should also be analyzed for two reasons:

  • The main source protective device may or may not adequately isolated from the bus and may fail to operate and de-energizing the arc fault before it escalates into a line-side arc fault.
  • If it is determined that the main protective device is properly isolated (from the bus), however, you need to have a label for energized work on this equipment.

Question: Is the zone selective interlock between two breakers implemented by electronic program logics in those relays? And what exactly is the difference between ZSI versus Instantaneous trip setting and Instantaneous override? Lastly, can you describe those as well as the other mitigation method differential relaying?

Zia Salami: In general, zone selective interlock protection would require communication between each trip unit (typically low-voltage circuit breaker with solid-state trip device) via control cable that the first upstream device can interrupt with any delays introduced for selective coordination purposes and prevent it from tripping (instantaneous) when there is a fault at downstream feeders.

However, in this scheme, the instantaneous settings will be enabled when there is fault at the bus location (i.e., faster fault clearing time, FCT). The ZSI function can be available within solid state trip unit and either is fixed or with adjustable settings.

Differential relaying is also one of the effective methods for mitigation strategy. It can reduce the FCT within the zone of protection (e.g., bus, transformer, cable, etc.) quickly, typically around 10 to 30 msec and consequently lower the incident energy. Note that the total fault clearing time for a differential relay will be the sum of the relay operating time plus the breaker time. Also, it is important to pay close attention to CT polarity and identify which energizing CBs are interlocked with the relay.

Question: Does the current NFPA 70E: Standard for Electrical Safety in the Workplace still require that an arc flash risk assessment and determination of personal protective equipment by either incident energy estimation/calculation or use of tables be performed, but does not specify that a particular estimation/calculation method be used?

Zia Salami: That is correct. Either, but not both, methods are permitted to be sued. Of course, in my opinion, incident energy calculation will provide more accurate results since it’s reflecting your actual system, assuming we are using proper design input parameters.

Question: How do you mitigate the arc-flash hazard when the available short circuit current is just below the circuit breakers instantaneous region?

Zia Salami: I assume you mean that the FCT is long (since no instantaneous trip occurs), so incident energy is high. As we pointed out in the presentation, one approach is to adjust the instantaneous settings (i.e., lower value that shift time current curve to the left) if possible or adding another protective device such as fuse in series with the existing breaker. Of course, other mitigation method such as utilizing maintenance switch can be implemented if available. In general, every system is different, and we need to evaluate it differently.

Question: I am getting higher incident energy (using SKM) on the secondary of a 480 V to 480/277 V transformer fed from a 480 V power panel. What can I do to mitigate the rating down to a more manageable level?

Zia Salami: We have the same issue in most facilities since the protective device on the primary is designed to protect transformer and also not tripping on magnetizing inrush current (i.e., protective device time current curve to the right of magnetizing inrush). Therefore, in most cases, this causes longer FCT on the secondary side and consequently higher incident energy. If you can set your breaker settings as close as possible to achieve these two conditions that may help you. Also, there is a new technology, vacuum fault interrupter transformer that is built with microprocessor-based relay system that can reduce incident energy on the secondary side, of course this might be costly, but interesting technology.

Question: Can you explain the rational in the enclosure size you used in the example ETAP setup?

Zia Salami: The one I used in the presentation for C-Area plot was the enclosure size based on IEEE 1584-2018 typical values for motor control center and also enclosure dimensions upper and lower limits (in %) were based on ETAP default values. You always can adjust all design input parameter tolerances that I discussed (e.g., short-circuit current, voltage range, electrode configuration, gap variation, working distance, including enclosure dimension tolerances) based on your application.

Question: How accurate can your incident energy levels be downstream for all the variables (cables, fuses, breakers, transformers) if you do have control over in design if you are given an unverified model with unverified upstream variables (cables, fuses, breakers, transformers) that are outside of your control?

Zia Salami: In general, arc flash hazard assessment (incident energy calculation) cannot be reliable to use for personnel safety if you have unverified design input and unknown source references for major parameters such as utility short circuit current, transformer size and impedance, cable size and length especially for a low-voltage system, large motors (i.e., above 50 hp), on-site back up sources, and etc. Each of these major parameters should be based on either facility walkdown (data collection) or justified based on typical standards values, proper assumption or engineering judgment.

Question: Follow up on current limiting fuses: Can’t you use the manual calculations provided in NFPA 70E for estimating arc flash values at different times if you have a time current curve profile for a specific device?

Zia Salami: The current limiting fuse equation method is very limited, and it can only be used for systems with particular configurations. For example, the system should be radial, no multiple sources are allowed, voltage must be less than 600 volts or less, the fuse should be Class L and RK1, working distance 18 inch or higher, the bolted fault current fault location downstream from the fuse must be within the range of the equations provided. If all conditions are met, yes, you can use the manual calculation assuming you are not utilizing any software. For example, ETAP provides three method of handling current limiting fuses for arc flash calculations.

Zia Salami, Ph.D.
Author Bio: Zia Salami, Ph.D., Management Specialist – SME: Electrical Power System, CDM Smith, Charlotte, North Carolina.