Arc Flash - Examples

Examples

In general, arc flash incidents which ignite clothing are highly improbable on systems operating at less than 208 volts phase to phase (120 V to ground) when fed by less than a 125 kVA transformer, as 120 volts does not provide sufficient potential to cause an arc flash hazard. Most 480 V electrical services have sufficient capacity to cause an arc flash hazard. Medium-voltage equipment (above 600 V) is higher energy and therefore a higher potential for an arc flash hazard.

As an example of the energy released in an arc flash incident, consider a single phase-to-phase fault on a 480 V system with 20,000 amps of fault current. The resulting power is 9.6 MW. If the fault lasts for 10 cycles at 60 Hz, the resulting energy would be 154 kilojoules. For comparison, TNT releases 2175 J/g or more when detonated (a conventional value of 4,184 J/g is used for TNT equivalent). Thus, this fault energy is equivalent to 380 grams (approximately 0.8 pounds) of TNT. The character of an arc flash blast is quite different from a chemical explosion (more heat and light, less mechanical shock), but the resulting devastation is comparable. The rapidly expanding superheated vapor produced by the arc can cause serious injury or damage, and the intense UV, visible, and IR light produced by the arc can temporarily and sometimes even permanently blind or cause eye damage to people.

There are four different arc flash type events to be assessed when designing safety programs:

• Open Air Arc Flashes
• Ejected Arc Flashes
• Equipment Focused Arc Flashes (Arc-in-a-box)
• Tracking Arc Flashes