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Know The Facts

An arc flash is a low frequency, high consequence event which is caused when electric current flows through an air gap between conductors, or between conductors and earth. The explosive release of energy can continue to develop until something in the system either trips, melts or vaporizes. All of this happens can happen in a fraction of a second.

Electrical arcs produce some of the highest temperatures known to occur on earth, up to 35,000°F (19,426 °C). This is 4 times the temperature of the surface of the sun which is about 9000°F (4982°C).

The intense heat from an arc causes a sudden expansion of air. This results in a blast with very high air pressure.

Copper expands at a rate of ~67,000:1 when vaporized. Comparatively, water expands at only 1,670:1 when converted to steam.

Five to ten arc explosions occur in electrical equipment every day in the United States (CapSchell, Inc.)

Fatal burns can occur even when the victim is several feet from the arc. Serious burns are common at a distance of 10 feet.

Distance is your friend! If you have no need to be in a switch gear room or substation, don’t be. If you can operate the equipment remotely, do so.

Debunk The Myths

Arc Flash Labeling = NFPA 70E Compliance

NFPA 70E requires that equipment that is likely to require examination or maintenance while energized be field marked to notify qualified persons of the potential risk. However, the standard also requires that sites have an electrical safety program in place, up to date one-lines, work permits, and must provide training for employees. The labeling of equipment alone doesn't satisfy all of these requirements.

More Labeling = Better Safety

The idea of adding additional labels to equipment to provide for different scenarios is very tempting. However there are a few issues that arise when we consider marking with multiple lables:

  • Unless isolated or barrier protected, two adjacent pieces of equipment should not have separate arc flash label information. For example, a fault in one bucket of an MCC could propogate to an adjacent bucket.
  • Multiple lables increase the risk of reading the wrong one for a given situation. This can result in electricians making decisions based off of incorrect information.
  • !STICKER OVERLOAD! - When every door of an MCC is marked with an AFH label, let alone multiple labels, this can desensitize us to the actual hazard being warned against. If everything is important, then nothing is important.

I Can Just Use Current Limiting Fuses.

While current limiting fuses are usually a very good tool in reducing the incident energy associated with an arc flash event, there are situations where they are ineffective. Proper planning and system knowledge is needed to know if they will adequately operate under a fault condition. As a general rule, current limiting fuses that are rated 600A or less do an excellent job at reducing incident energy to well below the 1.2 cal/cm^2 (5 J/cm^2) threshold.

Shouldn't I Use The Category Method? It's Easier/Cheaper.

The NFPA allows for either using a task based category method, or using a detailed analysis to establish an arc flash label system. A misconception is that using the category method is cheaper, easier, and allows for the most conservative approach.

In actuality, NFPA 70e stipulates that you must know the available short circuit current and the fault clearing times of the protective devices in order to determine if the category method is approriate or not. In addition to this, the category method requires that the specific equipment and task be listed in the availble tables, and that the equipment be operated in its designed parameters.

The above required information is also collected during a detailed incident energy analysis. If this information must be collected for either method, it is much more cost effective to perform a full incident energy analysis. You can then train personnel on the actual hazard and the approriate PPE.