Incident energy is a measurement of energy, usually heat, striking a surface. If you were sitting near a campfire, the warmth you would feel would come from the radiant heat of the fire, and it could be measured in terms of incident energy.
In most cases, the incident energy depends on three elements:
The intensity of the source of energy
The distance between the source and the surface where the energy is measured
The duration of the exposure
If one of those elements changes, the incident energy will also change. For instance, if you sat right next to the campfire, you would receive more incident energy than if you sat farther away.
Incident Energy and Arc Flash
An arc is an electrical fault in which electric current jumps through the air, like lightning. This process releases a lot of energy as heat, and an arc with a continuing source of power can build on itself, getting stronger and stronger. This kind of runaway arc can become explosively dangerous in the blink of an eye. This phenomenon is called an arc flash.
An arc flash generates extreme levels of radiant heat, intense light, and usually a blast of pressure from expanding air and other material. The arc that becomes an arc flash can be started by a dropped tool, a damaged conductor, or even just some dust or debris that shakes loose as a cabinet door is opened, so they are difficult to predict. Instead, the power of a flash must be estimated, and then protective steps can be taken to limit the damage an arc flash might cause.
When estimating the destructive power of an arc flash, one of the most important considerations is the incident energy that a worker might be exposed to. Calculating the expected incident energy for different distances and situations will help to determine appropriate protective boundaries and personal protective equipment (PPE) that should be used to protect workers.
How is the incident energy for an arc flash determined? Again, the three basic elements need to be considered:
Intensity - how powerful is the arc itself? This can be determined with the system voltage and the maximum available fault current.
Distance - how far from the arc will a worker be? Often, this "working distance" is estimated as 18 inches.
Duration - how long will the arc last? If enough power is available, an arc may last a very long time, but protective devices can identify the beginnings of an arc and quickly shut off power.
The Institute of Electrical and Electronics Engineers (IEEE) has produced a standard, IEEE 1584, that describes a method of calculating the expected incident energy of a hypothetical arc flash, using these details.
Injuries from Incident Energy
Incident energy measures the effective heat of an arc flash, but extreme and instantaneous events like arc flashes can't be usefully measured with the same units we use to talk about the weather. Instead, incident energy is typically measured in calories per square centimeter (cal/cm²). IEEE 1584 includes an example to help users understand what these units really mean:
If a butane lighter is held 1 cm away from a person’s finger for one second and the finger is in the blue flame, a square centimeter area of the finger will be exposed to about 5.0 J/cm² or 1.2 cal/cm².
If unprotected skin is exposed to an incident energy of 1.2 cal/cm², a second-degree burn is expected. Second-degree burns are serious, but can still be treated, so this incident energy value is often used as a benchmark for arc flash safety. Higher incident energy can cause third-degree (incurable) burns. For comparison, a very bad arc flash can easily generate incident energy in the range of 40 cal/cm²!
That sudden burst of heat can indirectly cause other kinds of injuries, as well. It can vaporize metal, which expands dramatically to create a blast of pressure like a bomb going off. This pressure wave can be strong enough to burst workers' eardrums, collapse their lungs, or throw them several yards. The pressure wave will also carry broken fragments of the equipment, and molten droplets of metal, creating a spray of hot shrapnel. The extreme heat can also cause materials to burn or chemically react in unexpected ways; combined with the vaporized components of the equipment, this can create a cloud of toxic smoke and gases.
Arc Flash Protection
The best protection from an arc flash is to only work on de-energized equipment. When there's no electrical power, no arc can form, and there's no risk of an arc flash. When powering down and locking out the equipment would introduce more hazards, or is practically impossible, then some other safe work practices are needed to protect workers.
Equipment that can detect an arc and shut down power to the affected area can limit the duration of an arc. This effectively limits the incident energy, because decreasing the duration or the intensity of an arc will proportionally decrease its incident energy. However, an arc flash lasting only a few milliseconds can still be devastating. Additionally, if the equipment was left powered for safety reasons, then powering it down in an emergency may introduce problems elsewhere.
There's another factor to consider, too: distance. As the distance from the energy source increases, the incident energy decreases drastically. (In technical terms, the incident energy is inversely proportional to the square of the distance.) An arc flash can easily cause second-degree burns in a fraction of a second, because workers are often very close to the equipment when the flash occurs. This is why the National Fire Protection Association (NFPA) has recommended protective boundaries as an important step to protect workers. Where a worker can interact with equipment remotely, they may be able to stay outside this boundary, and the distance can offer significant protection from an arc. Unfortunately, there are many cases where working remotely is infeasible, and a worker has to approach equipment to perform the necessary work.
The last line of defense against the incident energy of an arc flash is PPE. Arc-rated PPE has been tested to block or absorb heat energy in an arc flash situation. The arc rating expresses a protective value in cal/cm², so a suit rated for 8 cal/cm² can block up to that much heat, if used properly. Where the incident energy is less than the arc rating of the PPE being used, a worker may be effectively protected from the heat of an arc flash. Arc-rated PPE is not necessarily tested to protect against other kinds of hazards, though, such as physical trauma or toxic gases.
These protective steps are typically printed on arc flash warning labels applied on the equipment. This ensures that workers have the information they need, where and when they need it. But all of this information is based on an understanding of incident energy, so workers need to know what that means.