Bare facts of arc flash mitigation
Electrical engineers must understand the codes, standards, and design requirements when engineering for arc flash mitigation.
An arc flash is an explosion of light, heat, and energy caused by the shorting of electrical connections, which poses as a serious hazard when working on electrical equipment. An arc flash occurrence can happen as a result of personnel mistakes or as a result of equipment or connection failure.
Many factors go into mitigating the danger that electrical workers face; engineers must thoughtfully design systems to reduce and mitigate these incidents in the field. Owners, facility managers, engineers, and contractors can all contribute to providing a safer place for personnel to work. The process all starts with safe equipment and engineering design, followed by proper installation of equipment and proper training and maintenance procedures.
Maintenance mitigation and design mitigation will be covered in this article, and an overview of the national codes and standards that pertain to arc flash. This holistic approach is intended to keep the best interests of all parties at hand while making personnel safety paramount.
Codes and standards
Before diving into the implementation techniques, it's important to know what governs arc flash regulations. There are four publications that play a large role in arc flash regulations including NFPA, NESC, OSHA, and IEEE.
NFPA 70: National Electrical Code contains a section about arc flash hazard warnings, which states that it is required for electrical equipment to be field- or factory-marked to inform personnel of the risk in working on each individual piece of equipment. The NFPA 70 code specifically refers to its counterpart, NFPA 70E: Standard for Electrical Safety in the Workplace, to outline more detailed requirements for arc flash safety. NFPA 70E outlines many of the arc flash requirements in more detail including arc flash risk assessments, arc flash boundaries, arc flash personal protective equipment (PPE), and electrical equipment labeling.
National Electrical Safety Code (NESC) also outlines arc flash regulations including arc flash hazard analysis to be completed before maintenance is performed on live equipment within certain voltage thresholds above exposure levels greater than 2 cal/cm2.
OSHA also recently added new regulations on arc flash safety. This evolution in regulations defines requirements for arc flash studies being performed. OSHA also outlines the use of PPE when working on and around exposed energized parts.
Lastly, the IEEE Standard 1584 outlines the specification of scope and deliverable requirements for an arc flash hazard calculation. This guide is extremely important in outlining the steps, from beginning an arc flash study and collecting data to making assumptions on unknown data, executing a study, and completing the equipment-labeling process.
These guidelines are the foundation for designing, installing, and maintaining electrical equipment safely while mitigating risk and exposure to the dangers of arc flash incidents.
Arc flash studies, labeling
The prime way to mitigate arc flash incidents is to ensure the facility has recently conducted a study, labeled equipment, noted zones, and trained facility staff in maintenance procedures. NFPA 70E outlines when a study must be conducted and for which types of equipment. Once the proper arc flash study has been performed, the overall system can be analyzed and the incident energy levels at each piece of equipment can be reviewed. At that time, it's important to determine whether modifications need to be made to the system to reduce dangerous incident energy levels.
The next step after any potential system modifications are made is to properly label each piece of equipment to identify its hazard level. The four levels of arch flash hazard range from one (least hazardous) to four (requiring personnel to wear full arc flash suits). Refer to Table 1 for a full description of each arc flash level.
Although it is important to properly label equipment, there also should be arc flash hazard warnings marked on the floor of any areas of danger. It's important to understand that distance greatly reduces the incident energy of arc fault, but knowing and understanding these boundaries will help to keep personnel who are not working on equipment safe while informing and educating them on the limits of arc flash areas.
Two additional maintenance factors that are crucial to be considered are the tightening of equipment conductor connections and the review of old and deteriorating conductors. Over time, conductors can become loose as a result of vibrations and thermal changes. Conductors that are not properly torqued are a prime hazard for arc flash occurrences. They should be regularly inspected and torqued to assure proper connection and to reduce arcing.
The second and potentially more serious factor concerns the degradation of conductor insulation. While torqueing a conductor can be completed in a routine and cost-effective manner, replacing old conductors can be costly and difficult. This poses an obvious challenge and can result in conductors being left in operation longer than their anticipated lifespan. Insulation provides a false sense of security when cracking and degradation can begin to expose energized parts of the conductor.
Arc flash design considerations
Another tactic in mitigating arc flash incidents involves implementing the right initial design or modifying the existing systems to help reduce incident energy levels. This entails fully understanding the electrical system and running an arc flash analysis study. In doing so, it's also important to take into consideration selective coordination of the electrical system. While having a fully coordinated electrical system is important, this must be weighed with adjusting the system to mitigate and extinguish arc flashes in an expedient manner. When working to adjust breaker settings for a coordinated system, creating delays in adjustable tripping will prolong arc flash incidents and expose personnel to increased risk. The decision must be made based on the level of critical operation of the equipment. When it is decided that neither selectivity nor arc flash mitigation can be compromised, there are means to dial in instantaneous breaker time-trip settings while personnel are operating on equipment.
Another design scheme to be used in reducing arc flash incidents involves analyzing and reducing available short-circuit currents. There are a few methods that can be employed to achieve this, including installing a line reactor, installing an isolation transformer, and increasing conductor length. While all three of these options come at a price, they should be carefully weighed as they can greatly affect the available short-circuit current and the respective arc flash incident energy levels.
Employing high-resistance grounding is yet another design choice that impacts the arc flash incident energy levels of a facility. Implementing a current-limiting resistor into the electrical distribution system can effectively increase the resistance and limit the available fault current. While this system has its advantages and disadvantages, the main benefit of this type of grounding is to lessen the effects of arc flash by adding resistance in your ground path.
Note that while a line-to-ground fault can benefit greatly from having resistance at the ground, this configuration does nothing to limit the incident energy of a line-to-line fault. Additionally, it also must be considered that this type of installation may be prohibited in some cases, so further research may be required to implement this strategy.
Arc flash mitigation components
Many engineering design techniques can be employed to help reduce the arc flash hazard. A few of the equipment options include remote racking, remote operators, zone-selective interlocking (ZSI), maintenance switches, arc-fault relays, and arc flash protective equipment.
Various affordable, robot-like products are available that remove rotary-style circuit breakers via remote control, placing the operator outside of the arc flash boundary. Remote racking is one concept that increases the worker's distance from arc flash areas, thereby reducing the available arc flash incident energy levels and improving personnel safety. This method can be reviewed in conjunction with specifying equipment. It's important to know these options are available when specifying switchgear that can interface with remote racking systems to help facility personnel practice safe racking methods.
Another method to mitigate arc flash incident energy is to install a remote circuit breaker operator. This solution can be most effective when existing equipment is realized to be above safe arc flash levels. In some cases, due to high levels of incident energy, it can be necessary to have the utility company go to a facility to power down main equipment. With a remote circuit breaker, the facility will be able to safely de-energize equipment without the need to call in special personnel.
ZSI is another method to consider as it relates to arc flash mitigation. While the primary principle of it is to provide a coordinated electrical system, ZSI can be a valid design option when reducing arc flash incident energy levels. Given this method relies on additional control circuitry, it should be closely analyzed prior to employment. This method can be analyzed when completing an arc flash analysis and reviewing the overcurrent device trip settings.
Specifying the correct overcurrent protection is an important factor when implementing a safe system, which can be accomplished by first completing an arc flash study on the facility at hand. Following that, adjusting the trip settings found on modern overcurrent-protection circuit breakers can be an effective method in mitigating the amount of arc flash incident energy. Carefully reviewing and analyzing the breaker trip settings as it relates to the arc flash study is an important component in setting the power system up to perform optimally.
Typically, in arc flash safety, it is ideal to have the breaker trip at the soonest possible moment in order to mitigate the amount of arc flash incident energy; however, the problem with setting all of the breakers down to the quickest instantaneous trip setting is that it can result in an uncoordinated electrical system and compromise the safety of the electrical system. For instance, having an uncoordinated emergency electrical system within a hospital or medical facility could greatly compromise patient safety if upstream breakers were to trip. These options need to be carefully weighed and discussed with the facility owner and personnel prior to implementing.
Another option relating to trip settings that can be considered is the employment of a maintenance switch, which can provide a safe and effective way to switch between a time-delay breaker that favors coordination and an instantaneous trip setting that provides maximum protection against arc flash levels. This maintenance switch allows personnel to switch to the instantaneous trip setting while working on equipment, providing the most protection. Once maintenance is complete and normal operating conditions resume, they then can switch back to the coordinated time-delay trip setting.
An arc flash relay is another device that can help mitigate incident energy in both medium- and low-voltage switchgear. There are arc flash relays available on the market that measure both light flash and current to detect arc flash occurrences and help mitigate the incident energy. The benefit of detecting both light output from an arc flash while also measuring current, ensures safe operation and works to eliminate nuisance tripping. Arc flash relays can be a great solution for both new and retrofit-type installations.
Finally, arc-resistant switchgear is an option in the defense against arc flash. Arc-resistant switchgear is specifically designed to help redirect potential energy blasts away from personnel and out duct channels built into the switchgear. If you have ever worked at a facility that has experienced an arc flash, you'll know the damage that can result to not only the equipment itself, but also the adjacent equipment. Arc-resistant switchgear helps to channel the blast away from both personnel and adjacent equipment.
Mitigation training strategies
One important concept when designing for arc flash mitigation is connecting with the owner and creating a plan for arc flash mitigation safety. It's important to educate on mitigation techniques and outline the safety issues that relate to it. If the engineering team is designing in a bubble without collaborating with the owner and contractor, it's easy for safety-implementation items to get value-engineered during the budgeting process. It's important to educate the owner and agree on the level of engineering implementation so that it's clearly understood what cost impact certain mitigation techniques can have on the overall budget.
In summary, there are many methods that can be employed to help mitigate the risk of arc flash. While not every single arc flash is avoidable because of equipment failures, proper maintenance, personnel training, proper studies to understand risk, and employing the correct equipment are all means by which the risk of arc flash can be greatly mitigated. Being well-educated and taking the proper training steps can help ensure a safer system for all involved.