Electrical Explosion - Arc Flash Blast

In a typical electrical explosion, the tremendous temperatures of the arc cause the explosive expansion of both the surrounding air and the metal in the explosion path. For example, copper expands by a factor of 67,000 times when it turns from a solid to a vapour. The incredible dangers (resulting in physical injury and also death) result directly from the high pressures, intense sound, and penetrating shrapnel that emanates from the explosion. The intense pressure of a blast can easily exceed hundreds or even thousands of pounds per square foot, knocking workers off their ladders. An Electrical Explosion can also easily rupture eardrums and collapse lungs. The sounds associated with such a blast can exceed 160 dB. Finally, explosive material and molten metal is thrown out from the explosion zone at speeds exceeding 700 miles per hour. This is certainly fast enough for shrapnel to absolutely penetrate the body.

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Simply put, an arc flash is an electrical explosion produced by ionized air and vaporized metal during a fault. Numerous toxic substances are produced due to the electrical explosion, which can be inhaled. This, in addition to inhaling the intense heat, may increase the chance of death.

Physical Injuries May Include:

Shock Wave
Blinding light
Shrapnel
Intense heat
Contact with energized components
Toxic Smoke
Molten Metal
Noise Levels
Hearing Loss
Falling
Concussion

Electrical Explosion Occurrance

Each and every day in North America, between five and 10 electrical explosions occur in commercial, industrial and institutional electrical system equipment and machinery. Any electrical worker who is exposed to such electrical explosions is at significant risk for serious injury or even death.

Arc flash, or explosion, is described by the National Fire Protection Association as “a dangerous condition associated with the release of energy caused by an electric arc.” A sufficient reduction of the insulation or the isolation distance between two energized components is the primary cause of an explosion.

Serious damage to equipment is a likely outcome in an explosion. Sometimes, affected equipment such as a phase to phase circuit breaker is so badly damaged that replacement is the only option. And, of course, the human body is equally capable of being destroyed or irreversibly damaged, with no replacement option.

Components of Electrical Explosions

Two basic components of electrical explosions:

Heat Radiation (quantified as “Incident Energy Level”)
Pressure Wave (also known as “Arc Blast”)

Electrical Explosion Heat
Electric arc temperatures are considered to be approximately 4 times hotter than the sun’s surface. The temperature at arc terminals can reach 35,000°F (for reference, the surface temperature of the sun is ~9,000°F, and the temperature of a wood fire is ~900°F)

Electrical Explosion Heat Radiation
Heat radiation exposure is a function of:

Distance to arc
Available fault current
Fault clearing time
Equipment type
Gap between conductors (determined by equipment)
Vaporizes metals
Ignites clothing
200 degrees F - skin not curable (the point at which cell death occurs)
Possible to become fatally burned or seriously injured when working at a distance of 10 feet or more from an explosion.

Electrical Explosion - Personnel Reaction

Some potential effects on personnel due to Heat Radiation:

External burns, potentially very severe
Internal burns, such as to the lungs due to ingestion of vaporized metal and superheated air
Health effects due to inhalation of toxic gases and heavy smoke due to the burning of paint, insulators, and other components
Partial or total loss of sight
Disability
Death

Electrical Explosion - Pressure Wave

Electrical blast (or explosion) is the result of the rapid expansion of air caused by an electric arc:

Peaks in the first ½ cycle of fault (~ 9msec)
Pressure levels of 2,160 pounds per square foot (psf) in the immediate vicinity of the explosion have been detected.
Caused by superheating of air and vaporizing of conductors (air expands to roughly 1670 times and copper expands to roughly 67,000 times its volume – i.e. 1 in3 becomes 1.4 yd3)
Function of arc fault gap and available fault current

During an explosion, molten metal droplets travel as much as 10 feet or more and faster than 700 mph! Expansion produces an explosion that results in:

Molten metal
Fragmented metal
High temperatures
Pressure on the body

Electrical Explosion - Potential Effects on Personnel:

Injury due to blast
Collapsed eardrums leading to partial or near-total loss of hearing and possibly tinnitus – Sound levels of 141.5 decibels at 2 feet from the blast blast have been detected.
Collapsed lungs
Injuries due to shrapnel being ejected from equipment
One positive benefit: can lessen the effects of Heat Radiation due to personnel being thrown away from equipment, but this can also lead to other injuries