What is it?
Arc flash is the term used to define the release of energy caused by an electrical arc. Temperatures associated with arc flash incidents can reach as high as 35,000 deg F—four times the temperature of the sun’s surface. More disturbing is the fact that hospitalization reports related to arch flash incidents occur five to ten times a day in the U.S. alone. If these are the reported cases with serious injuries, how many more times a day might arc flash occur in which, fortunately, no one is harmed?
Beyond extreme heat, arc flash also presents pressure, auditory, projectile, and inhalation hazards. Pressures associated with some arc flash incidents have been great enough to blow down nearby concrete walls.
How to avoid it
To mitigate the risk of arc flash occurring in your facility, a number of steps can provide a high degree of safety:
* Do all maintenance repair work on de-energized equipment (a requirement of OSHA 1910.333);
* Perform operation and/or racking only with the electrical panel door closed;
* Operate equipment remotely;
* Use arc-venting switchgear (arc flash will typically exit via the weakest part of the structure in which the electrical components are housed—which is typically the front door of the unit). Arc fault venting allows arc flash energy to exit in a specific direction away from where personnel are likely to be;
* Set protection devices, such as differential relays, to react quickly. However, this may not be optimal in all cases as such relays can cause unwanted tripping due to minor overloads;
* Adapt settings during exposure. Establish sensor setup so that when someone comes close to the electrical unit, sensors will adjust to deliver that fastest possible tripping; and
* Use current limiting fuses/circuit breakers, as these can interrupt in as little as one-quarter of a cycle, but may not operate correctly at arc fault levels.
Even with all these safety steps available, there will still be occasions when personnel will be required to interact with machinery in circumstances that don’t fit these parameters. In those cases, other precautions are required.
Regardless of when or how qualified internal or external personnel will interact with electrical equipment, facility management is required to be knowledgeable of Arc flash hazards and provide the proper PPE (personal protective equipment) required to protect workers and communicate the risks.
2008 NEC article 110.16 states that a flash protection notice should be placed on any electrical equipment that requires inspection and maintenance. The notice can include arc flash boundary details, PPE category requirements, and potential shock hazard when the cover is removed. However, current NEC requirements do not state that such specific details be listed on the notice. All that is required is the basic notice of “Danger: Arc Flash Hazard Exists; Appropriate PPE Required.” Specific arc flash details have not been mandated on these notices as these numbers will change over the course of the day based on use of the electrical systems.
OSHA requires an employer to determine the presence of a hazard, require appropriate PPE, communicate decisions around PPE, and ensure on-hand PPE fits personnel. But OSHA’s requirement does not directly address arc flash—it applies to any hazard in the workplace that an employee may face.
NFPA 70E 2004 130.3, on the other hand, specifically requires a flash hazard analysis be performed to protect against arc flash and to determine the arc flash boundary.
Though it does not directly enforce the NFPA standard, OSHA does recognizes it as an accepted industry practice and states that any company following NFPA 70E 2000 edition or later is in compliance with OSHA standards.
The benefits of arc flash safety compliance are fairly obvious:
* reduced risk of employee injury/death;
* safety info for sub-contractors;
* limit equipment damage;
* increase system reliability for better uptime for production; and
* potential reduction in workers’ comp and insurance claims
As such, there’s really no good reason not to do it, though many manufacturing sites tend to put off compliance since it is not required to operate the facility.
To bring your facility in compliance with regulations, you must follow NFPA 70E guidelines and conduct an arc flash study.
If you wish to calculate your arc flash hazard manually, the following information is required:
* up-to-date one line diagram to verify where all power is coming from;
* short circuit study MVA and source MVA ratings, equipment and cable impedance, cable length, and motor contribution; and a
* coordination study to know how fast protective devices will react to level of fault you may experience.
Reference sources are available through IEEE and NFPA to aid in conducting the calculations for analysis and to determine appropriate PPE.
As an added step toward achieving proper compliance, employing a third-party vendor to conduct the analysis will ensure that the calculations are performed correctly, proper documentation is provided, and that your facility is in compliance and personnel are safely protected.