What is Arc Flash?

Arc Flash is the result of a rapid release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or a ground. During an arc fault, the air is the conductor. Arc faults are generally limited to systems where the bus voltage is in excess of 120 volts. Lower voltage levels normally will not sustain an arc. An arc fault is similar to the arc obtained during electric welding, and the fault has to be manually started by something creating the path of conduction or a failure, such as a breakdown in insulation.

The cause of the short normally burns away during the initial flash, and the arc fault is then sustained by the establishment of a highly-conductive plasma. The plasma will conduct as much energy as is available, and is only limited by the impedance of the arc. This massive energy discharge burns the bus bars, vaporizing the copper, and thus causing an explosive volumetric increase, (the arc blast,) conservatively estimated as an expansion of 40,000 to 1. This fiery explosion devastates everything in its path, creating deadly shrapnel as it dissipates.

The arc fault current is usually much less than the available bolted fault current and below the rating of circuit breakers. Unless these devices have been selected to handle the arc fault condition, they will not trip, and the full force of an arc flash will occur. The electrical equation for energy is volts x current x time. The transition from arc fault to arc flash takes a finite time, increasing in intensity as the pressure wave develops. The challenge is to sense the arc fault current and shut off the voltage in a timely manner before it develops into a serious arc flash condition.

Arc Flash Hazard

The process requires an evaluation of your facility to identify the hazards that exist and to establish the protective equipment needed for your employees and contractors. The responsibility for anyone who works on your plant’s electrical system remains with YOU, regardless of your employer.