The explosive nature of an electric arc has been documented in a number of case studies. The intense heat from an arc flash causes sudden expansion of air resulting in a blast, pressure waves and the resulting explosions rival that of explosive substances. Energies produced by arc flash explosions have been known to propel large objects such as switchboard doors, bus bars and personnel several feet at very high speeds. As an example of an arc blast, a 50kA arcing fault will provide enough energy to propel a person weighing 70kg (approx. 160 lbs.) and standing 2 feet away from the arc source with an initial acceleration of approximately 50 m/sec (approx 110 mph.), collapsing workers lungs, rupturing eardrums and causing permanent hearing loss. The gases expelled from the blast also carry by-products of the arc with them including droplets of molten metal similar to buckshot. Intense heat and ballistic pressures produced during arcing fault present real hazards that must be addressed before the work is done on energized equipment.
IEEE 1584 provides a method for calculating incident energy values in calories or Joules per square centimeter at working distance. However, the caloric values obtained by applying the IEEE 1584 procedure are not always obvious, and do not completely address the explosive nature of an arc. The aim of this article is to provide a procedure for calculating TNT equivalent of an arc blast as a method of quantifying energy released in explosions.
The kilogram of TNT is used as a unit of energy, approximately equivalent to the energy released in the detonation of this amount of TNT. TNT equivalents have been used in a number of applications for relative comparison purposes giving a sense of destructiveness as compared with ordinary explosives, like TNT. A gram of TNT releases 980-1100 calories upon explosion. This is standardized to 1000 thermo chemical calories, or 1 gram TNT = 4184 J.
IEEE 1584 clause 9.2 states that "system X/R ratio [...] were found to have little or no effect on arc current and incident energy and so they are neglected". Neglecting the X/R ratio and considering equivalent system impedance z = V/Ibf connected in series with arcing resistance, the voltage across arc resistance can be expressed as:
where V is system voltage, Ibf is the available bolted fault (short circuit) current at the point of arc, Iarc is arcing current calculated using IEEE 1584 formula 5.2.1 or formula 5.2.2 for systems rated less than 1000V or above 1000V respectively. Taking into account the resistive nature of an arc, arc flash TNT equivalent in three (3) phase arc can be expressed as:
where tarc, sec - arcing time determined using circuit protective device time-current characteristics.
Energies produced by arc flash explosions are comparable to dynamite explosions. These forces can be significant. They have been known to blow workers tens of meters away from the arc causing falls and injuries that may be more severe than the burns themselves. Arc blast explosive effects should be anticipated together with heating effects when selecting personal protective equipment and justifying work on energized equipment.
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