Arc Flash
Electrical arcing occurs when electrical current passes through air instead of a conductor. It is usually caused by a flashover or when conducting material temporarily creates an alternate path for electrical current to flow. The high resistance of air causes large amounts of energy to be converted into heat. This heat has associated effects such as infrared radiation, an initial pressure wave, molten metal and metal vapor. Together these effects can create a hazardous working environment that can lead to injury or death.
Arc flash hazard as defined by the NFPA 70E: “A source of possible injury or damage to health associated with the release of energy caused by an electric arc.” Broken down into its components, we get:
- Arc: current flowing through air, caused either by human error or faulty equipment
- Flash: release of energy including a bright flash of light
- Hazard: the danger posed to personnel in the presence of the arcing event
Equipment is designed and used to separate personnel from energized conductors and circuit parts through application of barriers or insulation. Well-maintained equipment has a very low chance of spontaneously causing electric arcs. As such, arc flash incidents are rare.
Air has very poor electrical conductance, so it is not easy for current to pass through it. In fact, in most electrical systems it is used as the main form of insulation. If through voltage spikes or falling equipment a temporary flashover does occur, the arc flash will be sustained in three-phase systems. This sustained arcing current leads to hazardous release of energy in various ways:
- Radiation: a flash of light due to high temperature (up to 20,000 °C) with high thermal energy released.
- Metal vapor: at the striking point the metal (copper or aluminum) of electrical conductors quickly melts and evaporates.
- Blast wave: metal vapor expands in an explosive fashion causing a high-pressure wave. The wave turns any loose parts into projectiles and results in sound levels up to 165 dBA.
Shock Hazard vs Arc Flash Hazard
One of the main reasons that arc flash receives less attention than shock hazard (at least in Europe) is that it poses a very small to no hazard for a majority of electrical distribution. For a distribution board with a nominal current smaller than 100 ampere it is unlikely that an arc flash releases a hazardous amount of energy. For main distribution the energy can reach catastrophic proportions.
This is a large difference with shock hazard, where the hazard depends on the voltage. An electric shock from a subdistribution board is just as dangerous as one from main distribution. Because most work activities happen at smaller subdistribution this will end up topping the list of a risk assessment.
A general solution for arc flash risk is hard to find or has limited effectiveness. Requiring standard PPE for example could lead to having protection that for 90% of the tasks is not necessary and for 5% is completely inadequate. To correctly deal with arc flash risk means focusing on tasks that may be infrequent but are exposed to high hazard.
