Tammy Gammon

Instantaneous arcing-fault models developed for building system analysis

An arcing fault is a dangerous form of short circuit that may have a low current magnitude. In the case of such faults, the magnitude of the current is limited by the resistance of the arc and may also be limited by the impedance of a ground path. This lower level fault current is often insufficient to immediately trip phase overcurrent devices, resulting in the escalation of the arcing fault, increased system damage, tremendous release of energy, and threat to human life. Despite modern advances in system protection and the adoption of National Electrical Code Section 230-95, people continue to be injured or killed from arcing faults, initiated by accidental physical contact or through a glow-to-arc transition. The initial phase of an arcing-fault research project was to review the historical evolution of arc modeling for low-voltage systems. A summary of the electrical aspects and the physics involved in arcing faults appeared in previous work. Todays better analytical tools facilitated the development of new instantaneous arc models with current-dependent arc voltages, which better represent the arcing phenomenon than the assumed arc voltage associated with previous instantaneous arc models. The arc currents in a typical medium-size building system are determined and harmonic analysis is performed.

Dc arc models and incident energy calculations

There are many industrial applications of large-scale dc power systems, but only a limited amount of scientific literature addresses the modeling of dc arcs. Since the early dc-arc research focused on the arc as an illuminant, most of the early data was obtained from low-current dc systems. More recent publications provide a better understanding of the high-current dc arc. The dc-arc models reviewed in this paper cover a wide range of arcing situations and test conditions. Even with the test variations, a comparison of dc-arc resistance equations shows a fair degree of consistency in the formulations. A method for estimating incident energy for a dc arcing fault is developed based on a nonlinear arc resistance. Additional dc-arc testing is needed so that more accurate incident-energy models can be developed for dc arcs.

Arcing-fault models for low-voltage power systems

An arcing fault is a dangerous form of short circuit that may have a low current magnitude. In the case of such faults, the magnitude of the current is limited by the resistance of the arc and may also be limited by the impedance of a ground path. This lower level fault current is often insufficient to immediately trip overcurrent devices, resulting in the escalation of the arcing fault, increased system damage, tremendous release of energy, and threat to human life. Despite modern advances in system protection, people are critically injured or killed each year when they are in the vicinity of an arcing fault that is either accidentally physically initiated or initiated through a glow-to-arc transition. The initial phase of an ongoing arcing-fault research project was to review the pioneering work, dating back to the 1920s. The historical evolution of arc modeling for low-voltage systems and a summary of the electrical aspects and the physics involved in arcing faults were reviewed in a companion paper (1999). After a comprehensive literature search was completed, todays better analytical tools facilitated the development of new arc models with current-dependent arc voltages. A current-dependent arc voltage better represents the arcing phenomenon than the assumed arc voltage associated with previous instantaneous arc models.

IEEE\/NFPA Collaboration on Arc Flash Phenomena Research Project

Annually more than 2000 workers are admitted to hospital burn centers for extensive injuries caused by arc flash accidents. Arc flash incidents occur when unintended electric current flows through air, superheating the air, and causes an explosion. Arcing faults can be unintentionally initiated when workers drop a tool or wire, which provides a temporary path between two energized phases or phase and ground. Arcing faults are sometimes initiated when cheap meters, lacking adequate insulation for the available energy levels, explode during troubleshooting. Arcing faults can occur during switching events, and sometimes without any user intervention when insulation or isolation between electrical conductors is not sufficient to withstand the applied voltage. Environmental contaminants such as metallic dusts, vermin, and forgotten articles or tools can lead to the development of arcing faults. Arcing faults are known to develop in aging equipment, especially when not properly maintained and inspected. Recognizing the significant threat posed by arc flash hazards, IEEE and NFPA have joined forces on an initiative to support research and additional testing to increase the understanding of the arc flash phenomena. Several areas of the arc flash phenomena need further research and testing validation to provide relevant information that can be used for developing safety strategies to protect workers. The identified areas include but are not limited to: (a) Heat and Thermal Effects, (b) Blast Pressure, (c) Sound and (d) Light Hazards. The test results of this project will provide information to help more accurately predict the hazards associated with high energy arcing faults, thereby improving electrical safety standards and providing practical safeguards for employees in the work place. The proposed research and testing plan

Analyzing the IEEE 1584-2002 data set and developing simple incident energy equations for low-voltage systems (< 1 kV)

The release of IEEE Standard 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations, is formal recognition of the danger of arcing faults in electrical systems. The standard includes an extensive data set used for developing the 1584-2002 arc-flash calculator which predicts three-phase arc current and incident energy for appropriate selection of overcurrent protective devices and personal protective equipment, respectively. As an additional benefit, the published data set can further enhance the understanding of the electrical characteristics of arcing faults in industrial power systems. The 1584 data set has been both quantitatively and qualitatively analyzed to assess the relationships between arc current, arc voltage, system voltage, arc power, and incident energy, as well as other variables such as gap widths and the effect of equipment enclosures. Simple relationships for estimating three-phase arc current, arc power, and incident energy on low-voltage (< 1 kV) systems are presented.

The IEEE 1584-2002 Arc Modeling Debate and Simple Incident Energy Equations for Low-Voltage Systems

The data set and equations presented in the IEEE Standard 1584-2002, IEEE Guide for performing arc-flash hazard calculations, were examined in previous work by the authors. Simple estimations for predicting arc current and incident energy were also presented. This work furthers the understanding of arcing by comparing theoretical circuit results with the low-voltage data published in IEEE 1584-2002. The applicability of popular approaches to arc modeling is discussed. Also, simple formulas for predicting the incident energy released by three-phase arcing faults in low-voltage (<1 kV) systems are validated and compared with results based on IEEE 1584-2002 equations. The role of arc duration in predicting incident energy is addressed

“Arc flash” hazards, incident energy, PPE ratings and thermal burn injury - A deeper look

Tremendous resources are being invested in arc flash studies and personal protective equipment (PPE) to protect workers from “arc flash” hazards. In the flurry to comply with OSHA regulations and NFPA 70 and 70E standards, the real understanding of the arc hazard and incident energy may be lagging behind. The term “arc flash” does not adequately convey the range of potential arc hazards-light, pressure, and heat transmission, as well as others. The term “arc flash” also fails to emphasize that arc flash injuries primarily arise from thermal burns and that the risk of a potentially severe or fatal arc burn is often present when performing electrical work. Worker risk assessment and the appropriate PPE are represented as definitive quantities in “cal/cm 2;” however, the quantitative potential heat exposure and heat protection afforded by PPE are usually less precise than what concrete numerical values imply. Basic concepts of incident energy, PPE ratings, and burn injury are also explored in this paper to help identify factors influencing the burn hazards posed by arcing faults in electrical power systems.

Addressing arc flash problems in low voltage switchboards: A case study in arc fault protection

Marine switchboards are manufactured to specifications similar to industrial switchboards but sustain arcing faults more frequently. Thus, the marine environment can serve to accelerate aging, showing what industrial switchboards will experience over time. U.S. Navy data are used to show that faulty connections are the primary cause of arcing faults in marine switchboards. Various approaches to arc detection and to the prevention of arcing failures will be examined. The approaches which were integrated into an automatic arc fault protection system for the Navy will be discussed. The historical effectiveness of arc fault protection systems on Navy ships will be discussed. The authors believe that the success achieved in this harsh marine environment is statistically significant and holds lessons for the deployment of arc fault protective systems in critical land-based power distribution systems.

Current-Limiting Fuses: New NFPA 70-2017 Section 240.67, Arc Modeling, and an Assessment Based on the IEEE 1584-2002

When operating in current-limiting mode, current-limiting fuses can effectively mitigate the heat and pressure hazards associated with an arc event because they limit the current in the first one-quarter cycle and interrupt current flow in less than 8.3 ms. The effectiveness of current-limiting fuses is reinforced by their inclusion in NFPA 70E-2015: Note to Table 130.7(C)(15)(A)(b), Annex D.4.6, and Annex O.2.4(4). The incident energy levels associated with current-limiting fuses can be determined from the IEEE Standard 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations. IEEE 1584-2002 presents a method for directly determining incident energy without first determining arc current or arc duration. The fuse equations were derived from 600-V arc tests involving a range of current-limiting fuse sizes. These data are reviewed to assess the performance of current-limiting fuses in reducing arc energy in accordance with NFPA 70-2017 Section 240.67. The IEEE 1584-2002 equations are used to assess the performance of these fuses in comparison with an arc energy reduction maintenance switch and the combination of both technologies. The viability of predicting arc current and incident energy for installations involving current-limiting fuses and implementing NEC Section 240.67 using general arc models is discussed.

A Review of Commonly Used DC Arc Models

The dc arc hazard is a great concern to industry. Quantitative arc hazard assessments are performed on dc systems to determine a nearby workers potential incident energy exposure during an arcing event. Four viable dc assessment methods are reviewed in this paper. The most widely used model for predicting dc incident energy is based on Lees theoretical arc model; the electrical arc power is determined from the maximum power transfer theorem, and the arc is depicted as a spherical radiant source with uniform heat transmission in all directions. Like Lees model, Ammermans model assumes complete conversion of electrical arc energy into thermal energy, but arc power is determined from an iterative technique constrained by arc power and circuit characteristics. Ammerman incorporates multiplying factors which account for the higher incident energies associated with arcing in enclosures. Based on dc arc testing, the applicability of an existing software package has been extended to dc systems through multiplying factors, and equations for dc rail and transit systems have also been developed. Model derivation is examined in this paper for suitability to arcing in general and dc specifically. Model performance is assessed using the available limited data (ac or dc). Example calculations are provided.