Your questions answered: How to design electrical systems to reduce arc flash incident energy

Existing arc flash codes and standards like IEEE 1584 and 1584.1 present models that electrical engineers can use to estimate incident energy levels in power distribution equipment, but they provide limited direction regarding how equipment construction should be considered in the calculations. The NFPA 70E hierarchy of risk control describes that the most effective ways are designing/specifying to remove the hazard at its source, minimize the severity of hazard and blocking the hazard from affecting employees. 

Questions not answered during the Aug. 6, 2019, webcast “How to design electrical systems to reduce arc flash incident energy” are answered here.  

Presenters: 

• Erhan Cokal, P.E., Offer Marketing & Launch Leader, Schneider Electric 
• Dakota Blair, EcoStruxure Power Business Development Specialist, Schneider Electric 
• Rod West, Senior Staff Engineer, Schneider Electric 
• Timothy Faber, Engineering Fellow – Low Voltage Overcurrent Protection, Schneider Electric 

Question: How do you calculate arc flash over for power plant equipment at multivoltage levels during design stage? 

IEEE 1584-2018, IEEE C2 (the National Electrical Safety Code), NFPA 70E and other sources provide guidelines for arc-flash analysis for various types of systems. Even in the design stage when final parameters may change, calculations may be performed to help identify locations with potentially high incident energy levels so that mitigation strategies can be defined. As with other electrical calculations, the arc-flash study should be updated as the design evolves. 

Question: What should the arc flash warning labels look like and what is the minimum info to be given? 

See NFPA 70E-2018 130.5(H) and the NEC 110.16(B) for arc-flash label requirements.  

Question: What are some other design strategies being employed to reduce the arc flash incident energies at the main switchboard or main panel? 

ArcBlok focuses online side and passive AFIE reduction technology. It is available only Model 6 MCC now, but may be expanded for the other low-voltage equipment in the future. 

Question: What are the impacts of the IEEE 1584 revision? 

IEEE 1584-2018 provides a new calculation model that takes into account additional system variables and provides additional accuracy. Results may change relative to the previous edition of IEEE 1584, but exactly how depends on several variables — energy levels may increase, decrease or stay roughly the same. 

Question: Would transfer switches be grouped with the switchboard product classification? 

From a “compartmentalization” standpoint, possible. 

Question: Should ArcBlok also be put on top of the breaker where the busbars exist? 

ArcBlok is appropriate for all power connections for which the line side of the main device is the highest priority. While all forms of connections can be considered, we have elected to start with cables as they seemed the most straightforward and were appropriate for the MCC application. 

Question: How can a barrier be put in place to eliminate inadvertent contact with terminals that people would be servicing? 

The ArcBlok system incorporates a wireless thermal sensor on each phase, thus reducing or eliminating the need for workers to access the line side connections while they are energized. 

Question: So, how does NFPA 70E enforced upon the installing contractor if 70E recognized by Occupational Safety and Health Administration for the property owner and facilities department and not by the authority having jurisdiction? 

NFPA 70E requires the employer to implement and document an overall electrical safety program. The role of an AHJ has a broad definition that varies across jurisdictions and responsibilities. Where public safety is primary, the AHJ may be a federal, state, local or other regional department or individual such as a building official or electrical inspector. In many 70E circumstances the property owner or his or her designated agent (electrical contractor) assumes the role of the AHJ. 

Question: In the future, when infrared testing is done on these types of arc isolation breaker modules, can these breakers be viewed easily for heat signature? Is there any concern about reinstalling the shield, if removed for this test, by the field electricians? 

Thermal monitoring is a simple and effective means to determine if the electrical connections require attention. As an alternative to infrared thermography, Square D provides wireless thermal sensors on these connections that allow monitoring to be done outside the arc flash boundary of the equipment and without the need to remove covers or open doors. 

Question: Do you have any animation and/or video with arc-flash blast happening with the ArcBlok installed? 

Yes, please see the recorded presentation, it is not easy to recognize, but slight light is visible. Or visit https://www.schneider-electric.us/arcblok. 

Question: How is absence of voltage verification performed when the arc–resistant shell is installed? 

The ArcBlok system provides protection from both shock and arc flash hazards, which otherwise exist on the line side of the main device. With an ArcBlok system installed, once the main device is locked out and absence of voltage on the load side is verified (either via the permanently mounted absence of voltage tester, a portable test device or both), an electrically safe work condition exists inside the main section of the MCC. 

Question: What happens at the other end of the conduits that are being used as an exhaust vent? Is there a minimum conduit size and or length to ensure exhaust gasses on the opposite end are not harmful? 

There is not minimum conduit size or length requirement.  

Question: Is test setup a 3–phase bolted fault? 

The fault is 3–phase initiated by a 10-gauge wire per the IEEE C37.20.7 standard (IEEE Guide for testing switchgear rated up to 52 kilovolts for internal arcing faults). 

Question: Was there any internal damage in the third video to the equipment? 

There is light arcing damage to the power connectors and some soot on the phase barriers, similar to a circuit breaker.  

Question: Will there be an option for infrared windows in the ArcBlok? How can infrared testing be accomplished with this product? 

No, IR windows are incompatible with internal arcing. Thermal sensors installed in the ArcBlok provide with temperature reading for the line side power connectors. Load side could be IR scanned if desired. 

Question: Where does the “exhaust” go? Does this get piped outdoors with conventional electrical conduit? 

The exhaust is similar in nature to a typical circuit breaker. In current designs, it is brought to the exterior of the gear in an area not proximate to a user. It will normally contain little incident energy and can be exhausted into the vicinity of the electrical equipment without incident. No need for an exhaust plenum. 

Question: Please talk about how much larger the equipment will become with ArcBlok.  

No impact. The same footprint as standard Square D Model 6 MCC. 

Question: What is the purpose of the perforated/vented cover panel in front of the arc–resistant shell? 

Part of enclosure ventilation system for thermal performance. 

Question: Are you working on similar technology for medium– and high–voltage? 

The question is being considered in R&D. 

Question: Were the example explosions including the already-required NEC arc energy reduction turned on? 

No NEC energy reduction was used or needed. 

Question: Where is it vented for bottom feed equipment? 

Back plate and front covers.  

Question: ArcBlok is designed to eliminate a fault before the upstream overcurrent device can open. The videos show no observable evidence of the arc flash event. What notification of the event can be implemented as the user may not otherwise know it occurred as power was not lost? 

We are working on it. Techniques to measure overcurrent, leakage current or light are prime candidates. 

Question: How do we know that the absence of voltage tester is working correctly, i.e., with a handheld meter you test it first on a known good source first? 

An AVT is not a simple voltage indicator. It is a test device specifically designed and used to verify the absence of voltage of the conductors or circuit parts at the work location. The AVT should meet the requirements listed in Exception 1 to 120.5 in NFPA 70E-2018. Among those is the requirement that the AVT “… is verified as operating satisfactorily on any known voltage source before and after verifying the absence of voltage.” 

Question: Is this technology going to be made available in I-Line panelboards or switchboard construction as opposed to just Model 6 MCC? 

Several variants are being studied. The details of which will be announced. 

Question: Is there an Easergy, TH110 device for medium–voltage equipment? 

Yes. 

Question: What is the cost premium for a breaker with versus without the ArcBlok? 

ArcBlok is a part of Model 6 MCC. It will be priced based on your lineup configuration. 

Question: Have the details been given to major calculation software vendors already (i.e., SKM Version 9)? 

Calculation results would vary depending on specific configuration considered — e.g., line side of main, load side of main, ArcBlok cover on, ArcBlok cover removed, etc. All the major software packages are flexible enough to allow entry of user-defined clearing times that allow for calculation of incident energy levels in any configuration desired. 

Question: Would the ArcBlok MCC be practical for a 480-volt, 3-phase, 3-wire system under 600 amps? 

It is. Even though AFIE might be low, it will still reduce the risk of hazard to the personnel and the equipment. 

Question: Does the AVT change the category rating on the equipment? 

The AVT allows a worker to test for presence of voltage without opening doors or removing covers from equipment. As such, it protects by reducing exposure risk. If an arc flash were to occur in energized equipment, the severity of that event is not affected by presence or absence of an AVT. 

Question: Will NEC still require arc flash studies and labeling? 

Yes, the marking requirements in 110.16(B) of the NEC are still required. Note that as described in the exception to this section, equipment labeling performed in accordance with NFPA 70E 130.5(H) also satisfies the requirement. 

Question: When will ArcBlok be available for main breakers in switchboards? 

ArcBlok is being considered for other products within our portfolio. Right now, we are developmental phase. Its introduction to market will be announced in the future. 

Question: Does the ablative coating need to be refreshed after an arcing fault occurs? 

No. It is very similar to the materials used in circuit breakers and is capable of multiple short circuits. 

Question: Explain again how this device extinguishes the arc. 

By ensuring that the arc must be of a minimum length we assure that it also has a minimum viable energy. With the normal cyclic decrease in current and with the help of the ablatives and exhaust, we can remove more energy from the arc than it can generate effectively making it nonviable.  

Question: In regards to arc flash labeling does Schneider Electric suggest labeling the ArcBlok cover with the line side incident energy? 

The ArcBlok cover should be labeled with the available incident energy based on the clearing of the upstream overcurrent protective device in order to inform workers of the potential hazard when the cover is removed. 

Question: Is the arc vault only for MCCs? What about main service entrance switchgear or any switchgear? 

ArcBlok is being considered for other products within our portfolio. Right now, we are developmental phase. Its introduction to market will be announced in the future. 

Question: How is ArcBlok technology accounted for by traditional arc flash computer programs? 

Calculation results would vary depending on specific configuration considered — e.g., line side of main, load side of main, ArcBlok cover on, ArcBlok cover removed, etc. All the major software packages are flexible enough to allow entry of user-defined clearing times that allow for calculation of incident energy levels in any configuration desired. 

Question: How do you know an arcing fault has occurred in the arcing vault? 

There will normally be some sound and light and overcurrent, similar to a circuit breaker. The upstream device could trip if desired. Otherwise, it would require a disassembly and inspection. 

Question: What is a ROM estimate on how much costs this ArcBlok adds to the costs of the equipment? 

Please contact with your local Schneider Electric representative. 

Question: Does ArcBlok work on top entry MCCs? 

Yes. 

Question: Can the ArcBlok be used to meet requirements of NEC 240.87 for arc energy reduction? 

NEC 240.87 requires the use of one of several methods where the highest continuous current trip setting for which the actual overcurrent device installed in a circuit breaker is rated or can be adjusted to 1,200 amps or higher. ArcBlok supplements this requirement by lowering the incident energy level on the line side of the overcurrent protective device as well. 

Question: All examples showed a main breaker downstream of the ArcBlok. Can it be specified in a main lugs MCC and does it cover an entire incoming section? 

ArcBlok would work on a main lugs panel, however, there would be no reduction in incident energy on the load side without an associated OCPD. 

Question: Is the ArcBlok system retrofittable into existing MCC units? 

Yes, in some cases. Contact your Schneider Electric sales representative for further information. 

Question: Please let us know how much percentage cost to add these to switchgear 

It is available only with MCC now. For price please contact Schneider Electric representative. 

Question: Is the ArcBlok technology currently only available in MCCs of 1200 amps or less? 

Yes. It will be expanded higher amperages in 2020. 

Question: Can one of these new arc block type main breakers be installed into an existing MCC? 

Yes, in some cases. Contact your Schneider Electric sales representative for further information. 

Question: Can you explain again how it allows for the possibility of a “no outage” internal arc event as is noted on the conclusions page 25, item No. 4? 

The let-thru of current of an ArcBlok is a curve similar to a circuit breaker and hence the same coordination techniques can be used. For example, selectivity will be assured if the instantaneous override of the upstream device is above the let-through current of the ArcBlok for the fault considered.  

Question: How do we as engineers calculate the incident energy for arc flash labels? 

IEEE 1584-2018, IEEE C2 (the National Electrical Safety Code), NFPA 70E and other sources provide guidelines for arc-flash analysis for various types of systems. Even in the design stage when final parameters may change, calculations may be performed to help identify locations with potentially high incident energy levels so that mitigation strategies can be defined. As with other electrical calculations, the arc-flash study should be updated as the design evolves. 

Question: Are the Easergy sensors available to apply on other load side connections, such as for feeder breaker/controller load terminals? 

Yes. 

Question: What about exhaust gasses on the other end of the conduits — is there guidance on the minimum size and length to ensure the gasses don’t create a new hazard in the upstream connected equipment? 

The line side is isolated, conduits are installed using conduit hubs and sealed with duct putty to prevent exhaust from the cable vault.  

Question: Can a Model 6 be field installed with the ArcBlok?  

Yes, in some cases. Contact your Schneider Electric sales representative for further information. 

Question: Are these products available for purchase and what is the lead time for delivery?  

Yes. Please contact with your local sales rep. 

Question: How does it compare with Eaton’s arc quench system? 

While it is difficult to compare the two solutions directly as they are completely different technologies, the sealed nature and internal barriers of ArcBlok, which help reduce the likelihood that arc flash events even occur, is a major benefit. After all, while a “quenching” system can respond very quickly to an arc-flash event, the easiest arc-flash to manage is the one that never happens. 

Question: What’s the budget cost for doing the field mod? 

Please contact your Schneider Electric sales representative for further information. 

Question: How does this solution impact selective coordination calculations via SKM? We usually have to download manufacturers data information to model a system. 

The presence of the ArcBlok module should not affect selective coordination evaluations of systems required to meet requirements such as those contained in NEC 700 or 701. ArcBlok affects faults on the line side of the MCC main breaker. Without ArcBlok, those faults are uncontrolled and would be cleared by the upstream overcurrent protective device. At worst, that would still happen with ArcBlok in place. Faults downstream of the main are outside the ArcBlok module and the operation of the main breaker or other devices can be considered as they would normally be. 

Question: Will the SKM module take ArcBlok into account? 

Calculation results would vary depending on specific configuration considered — e.g., line side of main, load side of main, ArcBlok cover on, ArcBlok cover removed, etc. All the major software packages are flexible enough to allow entry of user-defined clearing times that allow for calculation of incident energy levels in any configuration desired. Selectivity with arcing faults is not normally considered. 

Question: Is ArcBlok a one-time use product? 

No. It can be used with the same life expectancy with your Model 6 MCC with proper maintenance. 

Question: What about relabeling arc flash IE/PPE with ArcBlok ? 

One possibility would be to label the ArcBlok module itself with the available incident energy based on the upstream overcurrent protective device (i.e., energy level with the cover removed) and to label the rest of the MCC based on tripping of the main breaker.  

Question: Does Square-D anticipate a similar device for higher voltages — 4160 volts, 12.47 kilovolts? 

It is in research and development stage. 

Question: Could ArcBlok be utilized in retrofit types of projects (assuming Square D)? Is it approved for Canada (it was mentioned UL listed)? 

Yes, in some cases. Contact your SE sales representative for further information. For Canada: Yes. 

Question: Can we see what’s inside of the ArcBlok after an arc flash incident? 

Please contact your Schneider Electric sales representative for further information. 

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