FAQ 2017-05-22T23:57:31+00:00

FAQ

An arc flash is an electrical explosion due to a fault condition or short circuit when either a phase to ground or phase to phase conductor is connected and current flows through the air. Arc flashes cause electrical equipment to explode, resulting in an arc-plasma fireball. Temperatures may exceed 35,000° F (the surface of the sun is 9000° F). These high temperatures cause rapid heating of surrounding air and extreme pressures, resulting in an arc blast. The arc flash / blast will likely vaporize all solid copper conductors, which expand up to 67,000 times their original volume when vaporized. The arc flash / blast produces fire, intense light, pressure waves and throws off flying shrapnel.
There are a variety of reasons why an arc flash can occur, but most of them are human error and preventable. Many arc flashes occur when maintenance workers are manipulating live electrical equipment for testing or repair and accidentally cause a fault or short circuit. Improper tools, improper electrical equipment, corrosion of electrical equipment, improper work techniques and lack of electrical safety training are just some of the events that can lead to a devastating arc flash or arc blast.
It is important to note that wearing the proper personal protective equipment (PPE) can limit burns to second degree burns, but that PPE is last line of defense in risk control. Those who experience an arc flash and are wearing the proper equipment can still be seriously injured or even killed from the force of the arc flash blast.

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.

An Arc Flash Study or Analysis is a calculation performed by professional engineers to determine the incident energy found at each location. This determines the various arc flash boundaries and what personal protective equipment (PPE) must be used when approaching each boundary.

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.

Yes, you may conduct your own arc flash study, however there are many issues to consider.

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.

NFPA 70e defines an arc flash boundary as “a flash protection boundary within which a person could receive a second-degree burn if an electrical arc flash were to occur.”

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.

The NEC® and NFPA 70E require labeling of equipment to warn of potential arc flash hazards. Each panel must be marked with an ANSI approved Arc Flash Warning Label to warn and instruct workers of the arc flash hazard, voltage, arc flash boundary and required PPE (Personal Protective Equipment). Subject to the requirements of the facility and arc flash analysis, labels are attached for each analyzed point of concern.
The short-circuit study is based on a review of one-line drawings by a professional engineer. Maximum available fault current is calculated at each significant point in the system. Each interrupting protective device is then analyzed to determine whether it is appropriately designed and sized to interrupt the circuit in the event of a bolted type of short circuit. Next, the associated equipment must be reviewed to insure that the bus bar is adequately braced to handle the available fault current. Finally, the bolted fault currents are converted into arc fault currents for additional analysis.

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.

A coordination analysis is the examination of the electrical system and available documentation with the goal of ensuring that over current protection devices are properly designed and coordinated.

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.

In order to select the proper PPE (personal protective equipment), incident energy must be known at every point where workers are likely to perform work on electrically energized equipment. There are two methods to determine the proper PPE:

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.

This is the amount of thermal incident energy to which the worker’s face and chest could be exposed at working distance during an electrical arc event. Incident energy is measured in joules per centimeter squared (J/cm2) or calories per centimeter squared (cal/cm2). Holding your hand over the hottest part of the lighter flame for one second is equivalent to one calorie.
The hazard / risk category is specified as a number representing the level of danger, which depends upon the incident energy. The category ratings range from 0 to 4, where category 0 represents little or no risk, and category 4 signifies the greatest risk. Above category 4 (>40 calories/cm2) all equipment is considered too dangerous to work on energized because of the tremendous pressure blast.
The energy released by the arc is a function of:

  • 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.

The incident energy exposure caused by an arc flash can be affected by the system configuration, system fault levels, and exposure time. System fault levels can be reduced by changing the system configuration to reduce available fault current, and by using current limiting devices such as fuses, breakers, and reactors. Using faster acting relays and trip devices can reduce arcing time or exposure time. A protective device coordination study should also be conducted to ensure proper device settings. Instantaneous relays could also improve clearing times, limiting the arc exposure time. Fuse ratings and characteristics should also be evaluated to determine if a smaller and/or faster fuse could be used to help reduce the exposure time.
The degree of injury is directly related to the power of the arc flash, the distance the person is from the arc flash itself, and the protective equipment worn by an individual. Due to the force from the explosion of energy (the blast) and the intense heat, burns, concussions, collapsed lungs, hearing loss, shrapnel injuries, and broken bones are common. Death can and does occur from these injuries, but is mostly associated with the blast.

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 mandates that employers identify electrical hazards, warn their workers about them and provide the proper protective equipment and training related to working around the hazards. These are the only official regulations.

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:

  1. 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.
  2. 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.
  3. The National Fire Protection Association (NFPA) Standard 70 – “The National Electrical Code” (NEC) contains requirements for warning labels, including ANSI compliance.
  4. The Institute of Electronics and Electrical Engineers (IEEE) 1584 – this provides a guide to performing Arc Flash Hazard Study Calculations.
Compliance with OSHA involves adherence to a six-point plan:

  1. A facility must provide, and be able to demonstrate, a safety program with defined responsibilities.
  2. Calculations for the degree of arc flash hazard.
  3. Correct personal protective equipment (PPE) for workers.
  4. Electrical safety training for workers regarding the hazards of an arc flash.
  5. Appropriate tools for safe working.
  6. Warning labels on equipment. Note that the labels are provided by the equipment owners, not the manufacturers.
  7. Companies will be cited and fined for not complying with these standards.

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