Your questions answered: Electrical Systems: Arc Flash Reduction and NEC 240.87 Requirements
James A. Smith, senior offer specialist at Schneider Electric, and Antony Parsons, PhD, PE, senior staff engineer at Schneider Electric, tackled unanswered questions from the May 6, 2015, webcast on arc flash reduction and NEC 240.87 requirements.
Q: When there is protective covering on a piece of switchgear, such as panels-in-place, does the intent or letter of the NEC require the operator of a breaker to wear PPE for arc flash protection? For example, if you operate a large 480-V breaker from the front of the panel by pushing the button to open the breaker, is PPE required by the NEC?
Antony Parsons: The short answer: it depends. The longer answer: this is a question about safe work practices. These are not addressed by the NEC, but instead by NFPA 70E: the Standard for Electrical Safety in the Workplace. NFPA 70E-2015, Section 130.2(A)(4) states that normal operation of electric equipment shall be permitted where all of the following conditions are satisfied:
- The equipment is properly installed.
- The equipment is properly maintained.
- The equipment doors are closed and secured.
- All equipment covers are in place and secured.
- There is no evidence of impeding failure.
Q: Table 130.7(C)(15)(A)(a) has similar criteria for the task of "normal operation of a circuit breaker, switch, contactor, or starter," and says that arc flash PPE is not required if all of the conditions are met.The catch is that all five conditions must be satisfied or arc flash PPE is required. In our experience, many sites do not implement an effective electrical maintenance program, and even among those that do, equipment may fail one or more of the other criteria. Do arc flash maintenance switches cause concerns with selective coordination?
James Smith: Either maintenance switches or instantaneous trip functions set below the trip levels of downstream devices can compromise coordination. This highlights the need to conduct an arc flash study and coordination study to better understand where those instances occur, and to plan accordingly.
Q: Given the inherent current limitation of fuses, does the NEC recognize this type of protection as an "approved equivalent means?"
Parsons: Just to be clear, the NEC does not specifically recognize any "equivalent means," but instead provides this provision as a way to potentially include other technologies or solutions. The final call on whether a solution is "approved" or not presumably rests with the local authority having jurisdiction. Regarding the fuses in particular, current-limiting fuses can be very effective at helping control arc flash hazard levels, particularly when they operate in their current-limiting region. But in systems with relatively low available fault current, the fuse clearing time may be significantly longer and arc flash energy levels downstream would rise as well. The best practice would be to perform an arc flash analysis to investigate the potential impact of the fuse under consideration to ensure that an appropriate level of arc energy reduction is achieved, similar to our recommendation for the potential application of a breaker within stantaneous trip as an "approved equivalent means."
Q: Why would you consider arc resistant switchgear as not being an equivalent means for arc flash reduction? Does Schneider Electric make arc resistant switchgear?
Smith: The reason that we do not consider arc resistant gear to be an "approved equivalent means" in this context is that, unlike the other options listed in NEC 240.87, it does not reduce the level of arc flash incident energy present in the system. For the other listed solutions, this is accomplished by reducing the duration of the arcing fault through faster breaker operating time or by extinguishing the fault. Arc resistant equipment contains the arcing energy, but does nothing to reduce the energy. Yes, Schneider Electric makes arc resistant MV switchgear to 2B and 2C ratings, and LV UL-1558 switchgear and UL-845 MCC to 2B ratings per the IEEE C37.20.7 testing standard, and it can still be an effective part of an overall arc flash mitigation program.
Q: Any thought on why the NEC and NFPA 70E do not clearly define all electrical equipment that has a potential flash hazard to be labeled? (Example: disconnect switch at 200 A feeding an HVAC unitrequiring PPE Category 3).
Parsons: While the NEC and NFPA 70E standards both have arc flash labeling requirements, the specific requirements in NEC 110.16 and NFPA 70E 130.5(D) are quite different. However, both standards have one element in common: the requirement that electrical equipment "...likely to require examination, adjustment, servicing, or maintenance while energized..." should be labeled. The list of equipment that may require labeling (switchboards, panelboards, etc.) is inclusive (e.g., "...equipment such as..."), so both standards provide general guidance rather than an exhaustive list of equipment requiring labeling.
Q: Why is ground fault protection not considered an arc energy reduction method while instantaneous trip is in an LSIG trip unit?
Smith: The ground fault protection only monitors phase-to-ground faults and does not address phase-to-phase or 3-phase faults. Because instantaneous protection covers phase-to-phase as well as phase-to-ground faults, it covers the entire spectrum of potential arc flash incidents. Because ground-fault protection or high-resistance grounding systems do not directly address phase-to-phase or 3-phase faults, they are not considered to be equivalent solutions.
Q: Some utility companies are providing maximum fault current levels per infinite bus calculations vs. actual data based on utility source impedance. Can the higher fault current based on infinite bus result in a lower PPE requirement vs. the actual fault current based on source impedance data? Should the high and low arcing fault tolerance band be adjusted when using infinite bus value?
Parsons: While the infinite source assumption (i.e., that the fault current delivered to a given facility is limited only by the impedance of the service transformer) is conservative for short-circuit analysis, it may not be for arc flash analysis. If the higher fault current associated with the infinite source assumption results in overcurrent protective devices tripping more quickly, this can result in lower arc flash incident energy levels being reported downstream. Our typical practice is to vary the utility source impedance (e.g., using assumed "high" and "low" utility cases) to investigate the effect of the utility fault contribution on the arc flash analysis results. There could be other ways to handle it as well. Whatever the case, the best practice would be to go beyond simply running a single analysis scenario based on the infinite bus assumption.
Q: Do you have an opinion on insulated vs. uninsulated bus in distribution equipment?
Smith: Non-insulated bus has an open-air clearance according to the UL standards that acts as an insulator between the phases. During normal operation and commissioning practices, this has been proven to be an adequate "insulation" for decades. Physically insulating the bus does decrease the chance of an arc flash incident occurring, and therefore reduces the risk of an occurrence. Insulated bus and arc resistant switchgear are two examples of solutions that reduce the risk to the worker without necessarily reducing the hazard (i.e., the available level of incident energy from the arc flash event).
Q: Is there a feeling in the community that these protective measures override the fundamental initial safety option of always working electrical equipment de-energized?
Parsons: Certainly not! Or at least there shouldn't be, although, sometimes I do worry that this is the case. The fundamental protection strategy in NFPA 70E is that equipment should be placed in an electrically safe work condition before any work is done, and no arc energy reduction means, no matter how effective, should change that.
Q: Are electrical distribution systems that use fusible switches better at handling potential arc flash events and delivering better selective coordination results?
Smith: Selectively coordinated power systems that effectively address arc flash hazards can be designed with either fuses or circuit breakers. While current-limiting fuses do an excellent job of limiting arc flash energy levels when they operate in their current-limiting region, lower fault currents could result in longer clearing times and increased energy levels downstream. This is a particular concern in larger circuits,such as those governed by NEC 240.87 (which is applicable to circuit breakers sized 1,200 A and above). Just as with circuit breakers, the application of fuses must be evaluated through performing arc flash and coordination studies to ensure that the design intent will be met.