An arc blast can knock people off of elevated platforms, blow doors off hinges, and throw shrapnel across a room, to which the arc flash PPE provides little to no protection.
As part of the study, the engineer should also provide recommendations to reduce the incident energy / arc flash hazard category, which requires a short circuit study and a protective device coordination study.
An Arc Flash Study or Analysis should only be performed by experienced and qualified electrical engineers knowledgeable in power system engineering, IEEE 1584, NFPA 70E and arc flash studies.
Conducting an arc flash study is a complex process and requires engineers familiar with conducting power analysis studies and arc flash analysis in particular. Properly collecting all the data is the first phase of the project, which is difficult for anyone to do if they are not first familiar with all the potential outcomes and pitfalls of conducting an arc flash analysis. The engineer that conducts the study needs to be proficient in conducting short circuit studies, protective device coordination studies and have a strong understanding of NFPA 70E and IEEE 1584.
Beyond technical qualifications, in-house assessments are something that plant managers or engineers have little time for, often resulting in the project not getting completed or conditions of the electrical equipment changing before completion, making the results void.
The biggest reason not to do the study internally is the cost of getting it wrong. If someone is injured or killed due to an arc flash and the analysis was incorrect and done by someone who is not considered qualified to conduct the study, the liability will rest with the person or group that performed the study.
For larger organizations that have multiple or large facilities and are willing to invest in developing a team to perform the analysis, conducting the studies internally can help save money. For all other organizations, conducting an arc flash analysis internally typically has little or no upside compared to any cost savings.
It also defines incident energy as “the amount of energy impressed on a surface a certain distance from the source, generated during an electrical arc event.”
NFPA 70e requires the calculation and creation of a “flash protection boundary.” This imaginary boundary, which surrounds the potential arc point, specifies what level of personal protective clothing and equipment must be used by qualified workers who enter within that boundary.
A short circuit study is not required to complete an arc flash study, however, short circuit information is required in order to analyze an electrical distribution system to determine if changes can be made to mitigate arc flash hazards. A minor change in an adjustable breaker may make the difference in the result of an arc flash hazard category 4 or a 2. The availability of the short circuit information is a standard output of an arc flash study calculation, however, there is a big difference between having the information available and doing a report. Arc flash mitigation can be completed with short circuit information, but without doing a study. A study is value-added information to help a plant run more efficiently.
Most companies that complete an arc flash study / analysis also choose to get the short circuit study as well in order to take advantage of the information at a reduced cost compared to doing just a short circuit study. Future standards for conducting an arc flash will most likely include a short circuit study in order to help standardize the expected results of an arc flash program.
Over current protective devices are rated, selected and adjusted so only the fault current carrying device nearest the fault opens to isolate a faulted circuit from the system. This permits the rest of the system to remain in operation, providing maximum service continuity.
The study consists of time-current coordination curves that illustrate coordination among the devices shown on the one-line diagram. Note that protective devices are set or adjusted so that pickup currents and operating times are short but sufficient to override system transient overloads such as inrush currents experienced when energizing transformers or starting motors.
A protective device coordination study is not required as part of an arc flash study, however, doing this analysis one can determine if minor revisions in breaker settings or equipment can lead to major reductions of arc flash hazards. No arc flash analysis should be completed without first doing a protective device coordination analysis in order to save money and to remove potential hazards.
1. Perform an Incident Energy Analysis (Arc Flash Analysis) – Performing an arc flash analysis should be done by a professionally licensed electrical engineer familiar with NFPA 70E. This method takes into account the available short circuit current, fault clearing time and other variables to determine the incident energy, which is expressed in cal/cm2. Protective clothing rated for the incident energy levels can then be chosen to match the hazard.
2. Use the Arc Flash PPE Categories Method using Tables 130.7 (C)(15)(A) and (B) – These tables offer a guide to picking the proper PPE Category of clothing based on voltage, task and equipment being used. The use of this table is predicated on first knowing the available short circuit current and fault clearing time of the equipment. The results of these studies must meet the parameters of the chart.
Due to the level of engineering required to use the chart, most companies elect to perform the Incident Energy Analysis as it is more accurate that using the tables.
- System voltage
- Magnitude of the current
- Duration of the arc
It is important to note that the clearing time (or duration of the arc) can significantly affect the intensity of an arc flash whereby lower amperage systems can become more dangerous than higher amperage systems.
It is estimated that 5 to 10 arc flash and blast explosions occur in electrical equipment every day in the United States with 2,000 people each year being admitted to burn centers for severe burns.
OSHA suggests to the employer what to do, but does not define how to do it. The role of NFPA 70E, IEEE and NEC is to provide guidance on how to properly implement the OSHA regulations.
The regulations that govern arc flashes are:
- OSHA Standards 29-CFR, Part 1910. Occupational Safety and Health Standards 1910 subpart S (electrical), Standard number 1910.333 specifically addresses standards for work practices and references NFPA 70E. OSHA 29-CFR 1910.335 (a) (1)(i) requires the use of protective equipment when working where a potential electrical hazard exists and 29 CFR 1910.132(d)(1) requires the employer assess the workplace for hazards and the need for personal protective equipment.
- NFPA 70E. This regulation provides guidance on implementing appropriate work practices that are required to safeguard workers from injury while working on or near exposed electrical conductors or circuit parts that could become energized. Part II 2-1.3.3 regarding Arc Flash Study / Analysis states: “Arc Flash Hazard Analysis shall be done before a person approaches any exposed electrical conductor or circuit part that has not been placed in an electrically safe work condition.” This Arc Flash Hazard Analysis must be done to determine the level of Personal Protection Equipment (PPE) that a worker must use. The arc flash boundary in inches along with the incident energy must be displayed at each location. Each electrical panel must be marked with an ANSI z535 approved Arc Flash Warning Label.
- The National Fire Protection Association (NFPA) Standard 70 – “The National Electrical Code” (NEC) contains requirements for warning labels, including ANSI compliance.
- The Institute of Electronics and Electrical Engineers (IEEE) 1584 – this provides a guide to performing Arc Flash Hazard Study Calculations.
- A facility must provide, and be able to demonstrate, a safety program with defined responsibilities.
- Calculations for the degree of arc flash hazard.
- Correct personal protective equipment (PPE) for workers.
- Electrical safety training for workers regarding the hazards of an arc flash.
- Appropriate tools for safe working.
- Warning labels on equipment. Note that the labels are provided by the equipment owners, not the manufacturers.
- Companies will be cited and fined for not complying with these standards.