Hotter Than the Surface of the Sun—Arc Flash Incidents and Completing an Arc Flash Analysis in 3 Steps
By Mehdi Banaeian, PE
Whether you are working on a job site or the owner of a property, an arc flash incident is a danger that can and should be avoided if all involved properties are following the proper safety procedures. Tools like an arc flash analysis identify areas of risk, which can help workers avoid incidents.
What Is An Arc Flash?
An arc flash is an electrical discharge that uses the air as a conductor, exploding from a high voltage source to another conductor or ground nearby. The temperature can reach as high as 35,000 degrees Fahrenheit—hotter than the surface of the sun. This can melt nearby metals and damage the electric circuits, as well as cause severe damage to the human body. An arc flash occurs during a fault, or short circuit condition, which passes through the arc gap.
Causes of an arc flash can include overloading/overheating of circuits, damaged circuit breakers, over-fusing, faulty electrical panels, and wiring in a non-workman-like manner.
The Arc Flash Analysis
An arc flash analysis identifies areas of low risk, as well as those at high risk of an arc flash incident. The analysis also assesses the potential severity of injury at a given distance and locates the protection boundary at which there is a severe danger of second-degree burns. This information can be used by companies to effectively train employees on the hazards associated with their job responsibilities and to identify the Personal Protective Equipment (PPE) necessary to keep workers safe in an arc flash incident.
It is an OSHA mandate that employers in electric power generation, transmission, distribution, and related fields complete an arc flash risk assessment. National Fire Protection Association (NFPA) guideline 70E requires this, as well. The NFPA 70E Standard requires an arc flash analysis be performed for facilities that have electrical equipment such as motor control centers, panelboards, switchboards, enclosed circuit breakers, and fused disconnect switches, among others.
The results of an arc flash study help workers understand the potential hazards associated with these electrical locations and what personal protective equipment (PPE) is needed to work safely. In general, the following three (3) steps detail what is required for the completion of a thorough arc flash analysis.
STEP ONE: ACQUIRE EXACT AND MISSING DATA AND UPDATE CURRENT DATA
An arc flash analysis starts with acquiring exact data for the installed equipment and gathering all of the available drawings that the owner may have, including Single Line Diagram, Schematic Diagrams, Electrical Load List, Cable Schedule, and the ETAP model. If the documentation does not exist, they will need to be developed in the field, requiring a detailed field survey. A site walk-down will need to be performed to gather data from the field by taking photos from all transformers, switchgears, and MCCs and all protective devices, including relays, circuit breaker trip units, and fuses to verify any missing information in order to update or generate a Single Line Diagram and an Electrical Transient Analyzer Program (ETAP) model. All existing drawings must be checked against the data gathered from the field, including SLD, Schematic diagrams, and the ETAP model and updated, if necessary. This step is designed to make sure all the ETAP model and all drawings reflect accurate data for the installed equipment. Then, loading information and study cases must be verified for accuracy in ETAP. Available spare inputs/outputs on the existing relays must be verified, as well as the available spaces on the boards, in case relay replacement is required.
STEP TWO: RUN LOAD FLOW, SHORT CIRCUIT, AND RELAY COORDINATION IN ETAP
Once the data has been loaded into ETAP, the load flow will be run to make sure all equipment is loaded within their nominal current and voltage ratings (particularly when new loads added). Then, a short circuit analysis is performed to ensure all protective devices have duty ratings higher than the maximum short circuit fault (bolted fault) and can clear the fault without any damage. Short circuit withstanding of all equipment such as buses or cables will then be reviewed to make sure they will withstand a short circuit current until the fault is cleared by the relevant protective device.
Once that is completed, a coordination study of the protective devices can begin. Relay coordination is an important aspect in the protection system design, as coordination schemes must guarantee fast, selective, and reliable relay operation to isolate the power system faulted sections in order to minimize equipment damage and limit the extent and duration of service interruptions. In some cases, adjustments to the existing settings may be required to resolve coordination issues. After that, the results of both the short circuit and relay coordination studies will be used as inputs for the arc flash study.
STEP THREE: ARC FLASH EVALUATION
NFPA 70E requires facility owners to perform an arc flash risk assessment prior to allowing a worker or contractor to perform a task on energized equipment. The arc flash risk assessment identifies the presence and location of potential hazards and provides recommendations for PPE, boundaries for limited and restricted approaches, recommendations for flash protection, and safe work practices. The “as is” condition of the plant’s equipment must be evaluated. Then, recommendations will be provided to lower the incident energy levels, including optimizing relay coordination, replacing old relays with modern relay, implementing maintenance mode switches, or adding fiber optic sensors combined with the relay’s overcurrent function. The arc flash study includes the following:
- Arc Flash Risk Assessment (Hazard Analysis)
- Arc Flash Labels
- Arc Flash/Electrical Safety Training
Arc Flash Risk Assessment
An Arc Flash Risk Assessment or Arc Flash Hazard Study/Analysis is a calculation performed by a registered Professional Engineer to determine the thermal incident energy found at each location, which determines the various arc flash boundaries and what personal protective equipment (PPE) must be used in approaching each boundary. As part of the study, the engineer should also provide recommendations to reduce the incident energy/arc flash hazard category. An Arc Flash Risk Assessment should only be performed by experienced and qualified electrical engineers familiar with power quality, short circuit studies, NFPA 70E, and Institute of Electrical and Electronics Engineers (IEEE) 1584.
Personal Protective Equipment (PPE)
Electrical Personal Protective Equipment (PPE) is an important line of defense in arc flash safety. Arc flash electrical PPE requirements should feature head-to-toe protection against serious electrical hazards, so that the operator is able to perform the job in the safest, most efficient, and professional manner possible. Appropriate arc flash electrical PPE requirements include fire and heat-resistant gear and insulated tools to protect the entire body from the dangers of arc flash, including the skin, face, and ears. The PPE guideline for arc flash incidents is based on the NFPA 70E guideline requirement of the affected party not receiving more than a second-degree burn during an incident.
Arc Flash Labels
According to arc flash incident guidelines, equipment that is likely to require examination, adjustment, servicing, or maintenance while energized shall be field marked with a label containing the following:
- The nominal systems voltage,
- the arc flash boundary,
- and at least one of the following:
- the available incident energy and the corresponding working distance, or the arc flash PPE category in NFPA Table 130.7(C)(15)(A)(b) or NFPA Table 130.7(C)(15)(B) for the equipment, but not both;
- the minimum arc rating of clothing;
- Site-specific level of PPE.
Arc Flash/Electrical Safety Training
Arc flash hazard and safety training compliance is based on requirements by OSHA and NFPA 70E standards for worker safety protection. In addition to core electrical safety training, arc flash training teaches those subject to arc flash hazards how to recognize the hazards, avoid accidents, read the arc flash hazard labels, and to use and care for personal protective equipment.
Arc flash incidents are a danger of electrical work, but they can be avoided if the proper safety guidelines are followed. A thorough arc flash analysis identifies areas of risk and gives you the tools to educate and train others on the hazards associated with their job responsibilities and how to avoid injury during an arc flash incident.