An arc flash is the sudden, explosive release of electrical energy through the air. This discharge occurs when an electric current deviates from its intended path, such as between conductors or to a ground source, creating a sustained electrical arc. The primary danger of this phenomenon is the intense thermal energy and the resulting pressure wave, not the electric shock itself. Understanding the extreme temperatures generated is crucial for electrical safety.
The Extreme Heat of the Arc Plasma
The temperatures generated at the core of an electric arc are among the highest found on Earth outside of specialized laboratory settings. The plasma at the center of an arc flash can reach or exceed 35,000 degrees Fahrenheit (approximately 19,400 degrees Celsius). This heat is capable of vaporizing metal instantaneously.
To put this temperature into perspective, the surface of the sun is estimated to be around 10,000 degrees Fahrenheit. An arc flash generates heat more than three times hotter than the sun’s surface. This intense heat is generated by the rapid ionization of the surrounding air and vaporized metal, creating a superheated, highly conductive plasma.
The electrical energy released rapidly converts solid metal conductors, such as copper, into a gaseous state. When copper vaporizes, it expands in volume by a factor of 67,000, which contributes significantly to the explosive force. The arc plasma sustains itself until upstream protective devices interrupt the fault current and extinguish the arc.
Mechanisms of Thermal Injury from Arc Flashes
The intense thermal energy transfers to a person through three distinct physical mechanisms. The most significant component, especially at a distance, is radiant heat (light and infrared radiation). This radiant energy can cause severe burns on exposed skin and ignite clothing before the pressure wave or superheated air arrives.
Convective heat transfer occurs as the superheated air and hot gases created by the vaporized materials flow outward from the arc source. This mass of high-temperature gas rapidly transfers thermal energy to the skin and clothing. The intense heat can cause third-degree burns within fractions of a second.
A third mechanism of injury is direct contact with the arc plasma or the molten metal expelled from the equipment. The explosive force propels droplets of vaporized and liquefied conductor material, which can strike nearby individuals. These molten metal droplets, often exceeding 1,800 degrees Fahrenheit, cause deep, localized burns upon impact. The threshold for a second-degree burn on bare skin is commonly cited as an exposure of just 1.2 calories per centimeter squared per second.
Variables That Determine Exposure Severity
The actual thermal dose received by a person is quantified using Incident Energy, measured in calories per centimeter squared (cal/cm²). Incident Energy represents the amount of thermal energy that strikes a surface at a specific working distance. This metric is the standard used in electrical safety to assess exposure.
The distance from the arc source is the most influential factor in determining the received thermal dose. Incident Energy decreases dramatically with distance, following the inverse square law. Moving a small distance farther away from the arc source can reduce the exposure to heat energy by a factor of four or more.
The level of the available fault current in the electrical system directly influences the total energy released during the event. Higher fault currents result in a more energetic and higher-temperature arc flash. System voltage also makes it easier for an arc to sustain itself once initiated.
The duration of the arc is the final variable, determined by how quickly the protective device, such as a circuit breaker or fuse, can clear the electrical fault. A faster clearing time significantly reduces the total energy released. Minimizing the arcing time is essential to lower the calculated Incident Energy and reduce the risk of severe burn injury.