Arc Flash & Blast

Arc Flash and Blast hazards per NFPA 70E

What is ARC FLASH and why is arc flash dangerous?

Electrical Safety Technician providing Arc Flash Risk AssessmentWhen an electric current passes through air between ungrounded conductors or between ungrounded conductors and grounded conductors, the temperatures can reach 35,000°F. Exposure to these extreme temperatures both burns the skin directly and causes ignition of clothing, which adds to the burn injury. The majority of hospital admissions due to electrical accidents are from arc flash burns, not from shocks. Each year more than 2000 people are admitted to burn centers with severe arc flash burns. Arc flashes can and do kill at distances of 3 m (10 ft).

What is ARC BLAST and why is arc blast dangerous?

The tremendous temperatures of the arc cause the explosive expansion of both the surrounding air and the metal in the arc path. For example, copper expands by a factor of 67,000 times when it turns from a solid to a vapor. The danger associated with this expansion is one of high pressures, sound, and shrapnel. The high pressures can easily exceed hundreds or even thousands of pounds per square foot, knocking workers off ladders, rupturing eardrums, and collapsing lungs. The sounds associated with these pressures can exceed 160 dB. Finally, material and molten metal is expelled away from the arc at speeds exceeding 1600 km/hr (700 mph), fast enough for shrapnel to completely penetrate the human body.

Protection against Arc Flash Hazards

Protection against the arc flash hazards described in Section K.3 is intended to protect employees from receiving incurable burns. Although arc-rated garments are available with ratings greater than 40 cal/cm2, the advice given in Informational Note No. 3 to 130.7(A) recognizes the extremely dangerous concussive forces, sound, and shrapnel that occur in arc blast events where the incident energy exceeds 40 cal/cm2. Arc-rated garments with higher ratings can be used by employees when they are testing for the absence of voltage or are performing some type of diagnostic task and are within the arc flash boundary of equipment where a high level of incident energy exists, but they are not intended to provide arc blast protection.

Arc Flash Danger & Damage

Arc terminal temperature is estimated to be in excess of 35,000°F, with the plasma of vaporizing metal having a temperature of 23,000°F. An atomic bomb after 0.3 seconds reaches only 12,600°F and the surface of the sun is only 10,000°F. Vaporizing copper produces a volume increase of 67,000 to 1 while vaporizing water has a volume increase of 1670 to 1. For an arc flash/blast event, a volume expansion of 40,000 to 1 is a conservative estimate. This explosive expansion of vaporized metal and superheated air creates an acoustic and pressure wave.

Ralph H. Lee, in his paper "Pressures Developed by Arcs," IEEE Transactions on Industry Applications, Volume 1A-23, No. 4, pp. 760–764, July/August 1987, provides information that has been quoted by many regarding arc blast pressures, which includes information such as the following. The pressures from arcs are developed from the expansion of boiling metal and the super-heating of air by the arc passing through it. Vaporizing copper expands 67,000 times in volume, just as water expands 1670 times while becoming steam. This expansion accounts for the expulsion of molten metal droplets from the arc, which can be thrust up to distances around 10 ft (3 m). This pressure generates ionized vapor (plasma) outward from the arc for distances that are proportional to the arc power. Fifty-three kilowatts of power will vaporize 0.05 in.3 (0.328 cm3) into 3350 in.3 (54,907 cm3) of vapor, and one cubic in. (16.39 cm3) of copper vaporizes into 1.44 yd3 (1.098 m3). The super-heating of the air is linked to the generation of thunder by lightning. Immediately the hot vapor forming an arc starts to cool, but while hot it combines with oxygen in the air to become an oxide of the metal vapor.
As it cools in the air, it solidifies and becomes minute particles that appear as smoke — copper and iron are black, and aluminum is grey. These particles are poisonous if inhaled; they are quite hot and will cling to any surface they touch. The pressure from a 100 kA arc can reach around 400 lb/ft2 at a distance of 3.3 ft (1 m), which is about ten times the value of wind resistance that walls are generally built to withstand.

Therefore, a 100 kA arc could destroy a conventional wall at a distance of around 40 ft (12 m). A 25 kA arc at a distance of 2 ft (0.6 m) can produce a pressure of around 160 lb/ft2.
The average man’s front body projects around 3 ft2 of body area, while the upper body projects about 2.2 ft2 of body area. The 25 kA arc at 2 ft is sufficient to place a total pressure on the front of the average man’s body of around 480 lb (3 _ 160). The following equation can be derived from the information contained in Ralph H. Lee’s paper cited earlier on pressures developed by arcs and can be used to estimate the arc blast pressure:

P = (11.5 _ Ia)/D0.9
P = pressure (lb/ft2)
Ia = arcing current (kA)
D = distance from the center of the arc (ft)

However, it should be noted that this equation has not been generally adopted and additional testing is required. Arc blasts have been known to propel large objects such as personnel, switchboard doors, and bus bars several feet at high rates of speed. A 50 kA arc fault can provide enough energy to drive a 160 lb person standing 2 ft from the arc around 110 mph (50 m/sec). Arc blasts can provide enough pressure to collapse lungs and rupture eardrums if hearing protection is not used.

The TNT equivalent of an open air arc can be estimated using information contained in Ralph H. Lee’s paper "The Other Electrical Hazard: Electric Arc Blast Burns" IEEE Transactions on Industrial Applications, Vol. 1A-18, No.3, p. 246, May/June 1982. A gram of TNT releases 980 to 1100 calories upon explosion. In defining the ton of TNT, the value was arbitrarily standardized by letting 1000 thermo-chemical calories equal 1 gram TNT which equal 4184 joules. As stated on page 248 of Ralph H. Lee’s paper previously cited, the maximum arc energy of an open air arc is 0.5 times the maximum kVA bolted fault capacity of the system at the point of fault. Therefore, the TNT equivalent in grams of an open air arc can be given by the following equation:

TNT Equivalent = 0.5 _ 1.7321 _ Ibf _ Vsys _ t
4184 (grams)
Ibf = the steady-state bolted fault short-circuit current available at the location of the arc
Vsys = the system line to line voltage
t = total clearing time of the arc current