Arc Flash Hazard Research Articles

Best Practices for Implementing High-Resistance Grounding in Mine Power Systems

Proper implementation of high-resistance grounding of mine power systems can reduce arc flash and shock hazards by limiting ground fault current while also permitting reliable detection and clearing of the fault. IEEE Standard 142 defines a high resistance grounded system as one with a purposely inserted resistance that limits ground-fault current to levels such that the fault current (usually thought of as less than 10 A) can flow for an extended time without exacerbating damage. However, the per-phase zero-sequence resistance of the system should not exceed the distributed per-phase capacitive reactance of the system; otherwise, the system will be prone to transient overvoltages and relay selectivity problems. Recent research has shown that in practice, the zero-sequence resistance may be considerably larger than the system capacitive reactance, thereby violating the definition of a high-resistance grounded system. This paper outlines procedures for proper sizing of the neutral grounding resistor considering the distributed system capacitance. The paper begins with a discussion of the problems caused by distributed capacitance in high-resistance-grounded mine power systems. Subsequently, procedures for determining system capacitance, sizing the neutral grounding resistor, and establishing relay pickup settings are given. These procedures are straightforward to apply and do not require computer modeling for implementation. Numerical examples of the procedure applied to a high-voltage longwall system and also an underground mine distribution system are provided.

Hazardous-Rated Electrical Equipment and the Arc-Flash Hazard

This paper evaluates the relationship between hazardous-rated electrical equipment, which is suitable for installation in Class I, Division 1 (Zone 1), or Division 2 (Zone 2) locations, and an arc-rated switchgear, which is designed to withstand the effects of an internal arcing fault. Using UL 1203 (Explosion-Proof and Dust-Ignition-Proof Electrical Equipment for Use in Hazardous (Classified) Locations), IEEE C37.20.7 (IEEE Guide for Testing Metal-Enclosed Switchgear Rated Up to 38 kV for Internal Arcing Faults), and the paper titled “Pressure Developed by Arcs,” this paper will explore the applicability of hazardous-rated equipment in the protection of personnel from an arc-flash hazard.

DC Arc Flash Studies for Solar Photovoltaic Systems: Challenges and Recommendations

A dc arc flash hazard exists in solar photovoltaic (PV) power systems, but there is no widely accepted methodology for characterizing the severity of the hazard. Calculation methods have been proposed, and most rely on the nameplate $I$ – $V$ characteristic of the PV modules at standard test conditions to determine the worst case incident energy. This paper proposes to consider other factors in performing a dc arc flash hazard analysis, including possible weather conditions and variations of PV module characteristics from the datasheet ratings. It is recommended to consider two conditions when determining the worst case incident energy from a PV system: 1) the failure of all protective devices to trip within 2 s due to insufficient current and 2) the array output power exceeding the nameplate rating due to technological and environmental factors.

The Effects of System Grounding, Bus Insulation, and Probability On Arc Flash Hazard Reduction—Part 2: Testing

This paper provides a follow-up to the paper The Effects of System Grounding, Bus Insulation and Probability on Arc Flash Hazard Reduction—The Missing Links . In that paper, system grounding and bus insulation were considered a means of reducing the probability of an arc flash incident by approximately two orders of magnitude. In September 2013, additional testing was conducted on a low-voltage motor control center (LVMCC) with the expectation that engineering enhancements such as insulated vertical bus, high resistance grounding, and other techniques could be implemented to further reduce the probability of an arc flash incident. The testing performed was with faults initiated in locations typically found in operating petrochemical facilities, in LVMCCs, with the starter unit door open. This paper discusses the findings of testing on real-world electrical equipment in an effort to further understand and minimize the probability of an arc flash incident. This paper reviews the physics of the arcing fault and how ignition wire, geometry, and the number of anode/cathode pairs all contribute to arcing fault energy and personnel exposure.

Optimal directional digital overcurrent relays coordination and arc-flash hazard assessments in meshed networks

Summary The article addresses the arc-flash hazard assessments combining the optimal coordination of digital overcurrent relays. A novel objective function is adopted to concurrently reduce the arc-flash hazards while adapting the optimal settings of directional overcurrent relays subjects to set of coordination constraints. The flower pollination algorithm (FPA) is applied to attain the target. The proposed FPA-based technique is employed to define three main parameters for the relay settings. The performance and value of the proposed FPA-based methodology are demonstrated on the 8-bus and 15-bus meshed networks with various fault scenarios. Comparisons with the other competitive algorithms and subsequent discussions validate its good performance. Efficacious attributes of the novel objective function are compared and highlighted over the traditional objective function commonly used by the other researchers reported in the literature. Copyright © 2015 John Wiley & Sons, Ltd.

Exposed to the Arc Flash Hazard

It is widely acknowledged that an arc flash hazard may exist when energized conductors are within equipment in an enclosed condition if a person is interacting with the equipment at close proximity in such a manner that could cause an electric arc fault. This paper presents the results of scouting tests of arc flash events within enclosed low-voltage equipment (internal arc faults). This testing was performed to see if the incident energy mitigation efforts of one company were adequate to prevent arc flash exposures to equipment operators should an arc fault occur within the equipment they interact with. Arc faults were initiated within low-voltage disconnect switches, industrial control enclosures, and motor control center (MCC) buckets. Only the possible severity of such events on nearby operators was investigated. No attempt was made to identify the likelihood of such events from operator interaction, nor all of the potential causes of internal arc faults.

Arc Flash Mitigation Using Active High-Speed Switching

One method of mitigating arc flash hazards associated with medium-voltage switchgear is the installation of active high-speed switch (HSS) systems. These systems are designed to detect and quench a burning internal arc in less than one-third of one electrical cycle. The internal arc is extinguished by the HSS's action of redirecting the fault current path from arcing through open air back to the intended current path of the switchgear bus. The new low-impedance current path provided by the HSS operation collapses the voltage at the point of the fault to near zero so that the arc is no longer sustainable. The system's high speed of operation compared to arc quenching via circuit breaker tripping translates directly to lower arc flash incident energy and minimal equipment damage. Another benefit of such active high speed systems could include switchgear compliance to the IEEE C37.20.7 guide for testing arc-resistant metal-enclosed switchgear without any arc by-product venting requirements. This paper explores application considerations of HSS systems relative to other available means of controlling and reducing the hazards of internal arcing faults in medium-voltage switchgear.

Prevention Through Design: Reducing Risks Near Service and Main Disconnects

A common practice is to install a service disconnect or a main disconnect for a feeder system in the same enclosure compartment as the downstream overcurrent protective devices. This can result in potential safety risks of shock and arc flash. This paper will address the advantages and disadvantages of isolating a service disconnect for electrical service for industrial and commercial installations from the perspective of prevention through design. This paper will compare shock and arc-flash hazard for both cases when the service disconnect or a main breaker is isolated and when it is in the same enclosure.

The Journey of an Enterprise in a Process Industry Toward Improved Electrical Workplace Safety

This paper is a case study outlining the experience of a multisite cement producer and their experience in implementing an enterprisewide electrical workplace safety program, including compliance with the National Fire Protection Association 70E-2012 standard. Following a brief primer regarding arc-flash hazards, this paper outlines how the company first established a case for change and buy-in across the enterprise that includes 13 cement plants that are located across the USA and Canada, and then proceeds in describing the processes used to identify the defined work scope, select a supplier partner, and complete arc-flash studies at each site. The advantages of scale in areas, including common methods of data collection, selected software, standardized labels, site training, and energized work permits, are addressed. Finally, the lessons learned during this project and a phased remediation plan at each site, which was developed to reduce or eliminate potentially hazardous conditions, will be discussed. Finally, a few concepts based on “safety by design” will be explored, where there might be an opportunity to design the hazard out or down for new plant projects or expansion.

The Effects of System Grounding, Bus Insulation, and Probability on Arc Flash Hazard Reduction—The Missing Links

This paper provides a discussion on the theory behind reducing the risk and severity of an arc flash incident. In particular, the variables associated with the calculations of energies from an arcing fault are presented in an effort to show the futility of present methods for the determination of incident energy levels in the electrical industry. A number of commonly ignored design concepts that significantly reduces the risk of electrical hazards will be discussed two of which include 1) the system grounding and, 2) solid insulation. This paper will discuss risk and the management of risk as a means of reducing the probability of an incident. It will then show how risk-reduction should be used in the design, construction, operation and maintenance of electrical equipment as means for the safeguarding of employees in the workplace.

Arc-Flash Hazard Calculations in LV and MV DC Systems—Part I: Short-Circuit Calculations

The IEEE 1584 Guide does not address arc-flash hazard in dc systems. The interest in this subject is evidenced by current publications. The National Fire Protection Association (NFPA) 2012 methodology has added a small description of the calculations of arc-flash hazard in dc systems. However, there are serious inaccuracies in the application of the methodology that is advocated in the NFPA. A review of the current literature indicates that there is no comprehensive document on the subject and that some of the assumptions made in the current literature are not accurate, e.g., with respect to the short-circuit current profiles and protective devices in dc systems, which are very fundamental to the accurate calculations of arc-flash hazard in dc systems. This paper is presented in two parts. This part provides equations with practical examples of short-circuit calculations in dc systems. The second part provides the analysis of arc-flash hazard. Accurate calculations of the short-circuit currents and ascertaining their profiles is the first step in the arc-flash hazard analysis.

Arc-Flash Hazard Calculations in LV and MV DC Systems Part—II: Analysis

Based on the results of the short-circuit calculations in Part I of this paper, the calculation of the arc-flash hazard in dc systems is demonstrated. Much like ac systems, the arcing currents are not the same as the short-circuit currents. The National Fire Protection Association 2012 methodology recommends that the arcing current be taken as 50% of the calculated dc short-circuit current and that the arc-flash hazard be calculated for a time duration of 2 s. Both these concepts are much flawed and will give inaccurate results. It is also imperative to appreciate the short-circuit protection in dc systems. Semiconductor devices have very little energy let-through withstand capability and are protected with fast-acting semiconductor fuses or dc circuit breakers that may arrest the prospective short-circuit current and limit the incident energy release to a small value. This paper calculates the arc-flash hazard in specimen cases to demonstrate the methodology of the calculations in low-voltage and medium-voltage dc systems.

NEC Design Compliance, System Protection, and Arc Flash Hazard

The National Electric Code (NEC) specifies limits on settings of transformer overcurrent protective devices that directly affect system protection. However, compliance with NEC design guidelines does not assure adequate system protection. The result can be zones of poor or nonexistent protection and “Extreme Danger” arc flash hazard (AFH) ratings. This paper discusses the existing system protection in a large paper mill and makes recommendations for improving that protection and lowering AFH ratings.

Networking at ESW 2013 [Standards

This issue of the magazine is dedicated to electrical safety. The author has been asked to outline some of the advancements in the standards that have resulted from the IEEE Industry Applications Society (IAS) Electrical Safety Workshop (ESW). Over the years, the ESW has provided this networking opportunity for attendees working on better and safer ways to work for other organizations such as the Canadian Standards Association, the National Fire Protection Association (NFPA), the International Electrotechnical Commission, and government organizations such as the US Department of Energy. The collaborative IEEE and NFPA study on arc-flash hazards not only meets at the workshop but informs the attendees of their progress each year. This is an open invitation to the members of any IEEE standards working group who feel that they would benefit having a meeting in conjunction with the ESW and opening their group's membership to ESW attendees. Please contact us through our Web site: http://sites.ieee.org/ias-esafc/ .

Reducing arc-flash hazards: One company's efforts to systematically improve electrical safety

The risk of arc flashes in power distribution equipment has long been recognized as a serious hazard in the petroleum and chemical industry. This article explores the methods one company used to systematically reduce the risk of personnel injury due to arc flashes on its existing facilities and on a new project. The process includes incorporating procedures and technologies that allow for minimizing the need to work on or near energized equipment, moving essential work tasks to safe distances, and eliminating or reducing the arc-flash potential.

Mitigating Arc-Flash Hazards: A Case Study of a Mill-Wide Project

This article provides a comprehensive account of a mill-wide project to bring a more than 30-year-old market pulp mill into compliance with NFPA 70E requirements and to mitigate arc-flash hazards to reasonable enough levels to allow effective maintenance. The project spanned three years and was completed in 2011. The major activities to be discussed (and through which the project had to progress) include the decision to initiate, data collection or re-creation, and update of single-line diagrams; creation of a database and software model, short-circuit analysis, multiple upgrades, or replacements to address over-dutied equipment; arc-flash analysis, protection/coordination changes to address let-through energy, equipment upgrades or replacements to address let-through energy and mitigate arc-flash hazards; equipment labeling; and training of personnel.

Prioritizing Circuit Breaker and Protective Relay Maintenance Using an Arc Flash Hazard Assessment

In difficult economic times, companies sometimes delay the performance of maintenance on electrical power distribution equipment to save costs. If this happens, some circuit breakers or protective relays will miss their scheduled maintenance and testing. This can create a significant safety risk, if those protective devices are used to manage arc flash hazards. This paper describes a study method to determine which circuit breakers and protective relays in a system are the most critical, so they can be prioritized for continued proper maintenance.

Arc-Flash Energy Reduction Techniques: Zone-Selective Interlocking and Energy-Reducing Maintenance Switching

The safety of personnel that work on electrical power distribution systems has received increasing attention from a number of standards and regulatory organizations in recent years. Organizations such as the IEEE, the Occupational Safety and Health Administration, the National Fire Protection Association, and several other groups are actively engaged in ongoing discussions on improving electrical worker safety, specifically in the area of protection from arc-flash hazards. While standards developed by these organizations discuss improvement opportunities, there are a number of techniques and technologies that are currently available for mitigating arc-flash exposure. This paper will provide a general overview of the key arc-flash reduction techniques and will specifically focus on two of these technologies-zone-selective interlocking and arc energy reduction maintenance switch systems. The benefits, expected performance, and the appropriate application of each of these arc-flash reduction solutions will be presented.

The arc-flash hazards of fire pumps: redesigning protection schemes for electrical installations

In recent years, engineers have been redesigning the protection schemes for electrical installations to reduce the arc-flash hazard associated with switching and maintaining equipment. For electrical fire pumps, however, National Fire Protection Association (NFPA) 20, Standard for the Installation of Stationary Pumps for Fire Protection, prescribes very strict requirements for reliability of the power feed and overcurrent protection for an electric fire pump. The result is that current is allowed to flow for a longer time period to ensure that a fire pump may start without nuisance tripping. These requirements are made without consideration for the associated arc-flash hazard. They do not acknowledge that a redundant fire-pump installation can provide equal or greater system reliability, plus greater operator safety, than is possible with a potentially hazardous reliable power feed to a single fire pump. This article will summarize reliability requirements currently mandated for electric fire pumps, discuss the arc-flash impact of those requirements, and propose language that could be incorporated in future code revisions to reduce safety hazards while preserving operating reliability of system installations.

Traditional Time–Current Curves Are Not Enough, Adding $I^{2}{t}$ Considerations

Overcurrent device selectivity has always been desirable in power distribution systems. Recent changes in the U.S. National Electrical Code (NFPA 70-2005 and subsequent) have increased the demand for near perfect selectivity. Simultaneously, heightened sensitivity to the effect of slow clearing times on arc flash hazard has increased the need for fast and sensitive overcurrent protection, particularly at the larger upper tier overcurrent devices. When the selectivity solution is to oversize or to slow down protection at midlevel feeders above the branch circuit breakers (CBs), arc-flash-incident energy at higher levels in systems can reach unacceptable levels. Optimized protection and selectivity may be achieved by optimizing instantaneous settings and algorithms in upstream protectors. Such selectivity may be accomplished primarily by understanding the current-limiting behavior of the downstream protective device and understanding the thermal capacitance needed to melt upstream fuse elements or intentional damping added to upstream CB trip units. When CBs trip in subcycle times and clearing times are instantaneous, traditional time-current curves (TCCs) are insufficient to correctly demonstrate device operation. The authors will describe two graphical representations based on the <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> let-through of the downstream device and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> requirement to trip the upstream device. The <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">t</i> representations more effectively describe protective device behavior in the instantaneous region below 0.010 s normally excluded in traditional TCCs. Furthermore, a technique, a theoretical basis, and a proposed test protocol that are suitable for a wide range of devices will be presented.