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PowerPedia:Arc flash

Lasted edited by Andrew Munsey, updated on June 14, 2016 at 10:05 pm.

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Arc flash, is a result of arcing current causing a three phase short circuit in electrical distribution equipment and a big blast made up of conductive plasma. This conductive plasma is called the arc flash and is dangerous to electrical workers.

Introduction

"The IEEE 1584 standard (an international standard) provides a method to calculate arc flash currents based on testing conducted on electrical boxes given specific configurations and controls. The following equations are given in the IEEE (Institute of Electronics and Electrical Engineers) standard for arcing current:

For applications with a system voltage under 1000V: log Ia = K + 0.662 log Ibf + 0.0966 V + 0.000526 G + 0.5588 V (log Ibf) – 0.00304 G (log Ibf) [equation 1]

For applications with a system voltage of 1000V and higher: log Ia = 0.00402 + 0.983 log Ibf [equation 2]

Deriving the equations for standard voltages and applications we note that arcing current (Ia) is typically much less than three phase fault current (Ibf) for 480V systems, even lower at 240V and below, and somewhat lower than bolted fault current for systems 1000V and higher. Arcing fault occurs first and then feeds the bolted f!ault which follows. In 2004, a paper entitled Improved Method for Arc Flash Hazard Anaysis plasmaThere was an error working with the wiki: Code[1] was written. In this paper an anomaly is noted at voltages greater than 783V with a gap of 32mm, the arcing current exceeds bolted fault current when bolted fault current equals 100kA. This is based on the equation 1 above. Also, given the IEEE 1584 equations above arcing currents higher than bolted fault currents exist for bolted fault currents below 1.724kA (equation 2). No explanation of this is given except to say the anomalies within the IEEE 1584 equations should be avoided, since it appears as if Ohm’s Law has been broken." There was an error working with the wiki: Code[2]

Regulations

OSHA is the enforcing government body in the United States requiring electrical safety programs addressing arc flash hazards and concerns. Canada also has strict policies. Since the IEEE 1584 standard is internationally accepted a lot of countries are following standards for addressing arc flash hazards. The most common policy that companies adopt will be to place a bright label on electrical distribution equipment with an indication of an arc flash hazard and also instructions for the electrician to work safety, such as protective clothing requirements. The National Fire Protection Agency also provides guidance in their standard the NFPA 70E for addressing arc flash hazards in the United States.

Arc flash properties

Image:Arcflashblast.gif

This is a photo of an arc flash incident on electrical equipment. An electrician is exposed to the blast.

Intensive heat can rise up to 20,000 degrees Celsius. Deafening sounds, second degree burns and higher, death, being propelled in the air, broken bones are all results of the arc flash blast that occurs during such an event. Protective clothing can protect fairly safety up to 40 calories per square centimeter of incident energy, however, values above that protective clothing can only protect against the heat aspect in terms of it's fire rating however, tears in clothing from the pressure wave cannot be accounted for above this 40 calorie limit.

The table below outlines the different levels of protection protective gear can offer.

Maximum Incident Energy (calories square centimeter)Protective Clothing Level

1.20

41

82

253

404

Arc flash videos

Real live arc flash incidents have been caught on film. The following internet links provide real videos of the arc flash phenomenon. It is clear that these blasts have a lot of heat and energy.

Arc Flash Incident #1

Arc Flash Incident #2

Arc Flash Incident #3

Arc Flash Incident #4

External References

There was an error working with the wiki: Code[3] Wilkins, Allison and Lang, Improved Method for Arc Flash Hazard Analysis, IEEE 2004.

There was an error working with the wiki: Code[4] Michelle Murphy and Stephen Goulet,The link between the Arc Flash Phenomenon, Emerging Plasma Sciences and Exotic Energie, IEEE 2015.

See also

Directory:Plasma

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