Arc flash and arc blast

An arc flash is one of the most violent events in industry. In a fraction of a second, an electrical fault through air releases heat, light, pressure, sound and molten metal. This lesson explains what an arc is, what happens physically during an arc flash, what the injuries are, and which factors make one arc far more dangerous than another. It deliberately stays at an awareness level: the engineering calculation of incident energy is the job of a qualified engineer using IEEE 1584, but every qualified worker needs to understand the inputs well enough to know when a label may be wrong.

What an arc is

An electric arc is a sustained flow of current through ionized gas, called plasma, between two conductors or between a conductor and ground. Air is normally an excellent insulator, but once something bridges the gap, even briefly, the current heats the air and vaporizes metal. The vapor and plasma are conductive, so the arc sustains itself as long as the voltage can drive current across the gap and no protective device interrupts it.

Arcs are usually started by one of a small number of events:

• A tool, probe, wire, screw or washer dropped or slipped across phases or between a phase and ground.
• A worker's movement bringing a conductor too close to another during tightening, removal of a cover or pulling of a cable.
• Insulation failure from age, heat, moisture, contamination or mechanical damage.
• Loose connections that overheat, char and eventually arc.
• Racking or operating equipment that fails mechanically, such as a breaker stab misaligning with the bus.
• Animals, such as rodents, snakes and birds, bridging conductors.
• Testing errors: a meter set to amps across a voltage source, test leads with damaged insulation, or an instrument with too low a category rating.

The common thread is that most of these involve someone working on or near the equipment. Equipment that is closed and in a normal operating condition rarely fails catastrophically on its own; equipment that is open and being worked on is far more likely to.

What happens in an arc flash

Arc temperatures can reach about 35,000 °F (about 20,000 °C). Copper vaporizing in the arc expands to roughly 67,000 times its solid volume, producing a pressure wave that can knock a worker off a ladder, rupture eardrums, collapse lungs and throw enclosure doors and covers across a room. The arc emits intense ultraviolet and infrared light that can damage eyes, and sound levels can exceed 140 dB. Molten metal droplets and fragments are thrown outward at high speed, and the arc products include toxic vapors of copper, aluminum and insulation materials.

Most of the energy reaches the worker as radiant and convective heat. The quantity used to measure it is incident energy, the amount of thermal energy per unit area on a surface at a given distance from the arc. It is expressed in calories per square centimeter (cal/cm²) or joules per square centimeter (J/cm²), where 1 cal/cm² equals about 4.184 J/cm².

Burns from arc flash are often far worse than their area suggests, because ordinary clothing ignites and continues burning after the arc is over. Synthetic fibers such as polyester and nylon melt and stick to the skin, deepening the burn. That is why arc-rated clothing must not be worn over meltable underlayers and why everyday clothing under arc-rated PPE must be made of non-melting fibers.

Arc blast

The arc blast is the mechanical side of the event. A pressure wave expands from the arc in milliseconds, and in an enclosure it is directed out of any opening, which is exactly where the worker stands. Doors and covers that are not latched, or that are held by only a few screws, can be torn off. Workers have been thrown several meters, and the sudden pressure can cause concussion, lung injury and ruptured eardrums. The blast also launches molten metal and fragments of bus, insulators and components.

Arc-rated clothing is tested for its thermal performance, not for blast protection. NFPA 70E and CSA Z462 both note that at high incident energies the blast pressure can be a serious hazard even when the thermal energy is within the rating of the PPE. Many employers therefore prohibit energized work where incident energy exceeds 40 cal/cm², even though the standard no longer states a fixed upper limit, and require the equipment to be de-energized instead.

Hearing protection is required whenever a person is inside the arc flash boundary, whether or not they are actively working, because the sound pressure alone can cause permanent hearing loss.

The factors that determine incident energy

IEEE 1584-2018, IEEE Guide for Performing Arc-Flash Hazard Calculations, is the method used in North America to calculate arcing current and incident energy for three-phase AC systems from 208 V to 15 kV. A qualified engineer performs the calculation, usually with software, as part of an arc flash study. The worker does not need to perform it, but should understand what drives the result:

• Available bolted fault current at the equipment: the higher the fault current, the higher the arcing current and the faster the energy is released. IEEE 1584 is valid for bolted fault currents from about 500 A to 106 kA at low voltage.
• Arcing current and clearing time: the time it takes the upstream protective device to detect the arcing current and interrupt it. Incident energy is roughly proportional to arc duration, so clearing time is often the most important factor.
• System voltage: higher voltage sustains longer arcs across wider gaps.
• Gap between conductors: wider gaps at higher voltages; IEEE 1584 provides typical values by equipment class.
• Electrode configuration: the 2018 edition models five configurations, such as vertical conductors in a box (VCB), vertical conductors terminated in an insulating barrier (VCBB) and horizontal conductors in a box (HCB). Horizontal conductors in a box direct energy straight out of the enclosure and can produce much higher incident energy.
• Enclosure size: a smaller enclosure concentrates energy toward the opening.
• Working distance: the distance from the arc source to the worker's face and chest. Incident energy falls rapidly with distance.

Likelihood: when does an arc actually happen?

Not every task near energized equipment carries the same likelihood of initiating an arc. NFPA 70E Table 130.5(C) lists tasks and equipment conditions and indicates whether an arc flash is likely. Operating a properly installed, maintained and closed circuit breaker in normal operating condition is generally not considered likely to cause an arc flash. Removing bolted covers, racking breakers in or out, working on exposed energized parts or performing tasks on equipment showing signs of impending failure are considered likely. The 2024 edition renamed the equipment condition column operating condition and added a note in 130.5(B) that closed doors do not necessarily provide protection unless the equipment is arc-resistant.

Normal operating condition has a specific meaning. All of the following must be true: the equipment is properly installed, it is properly maintained, it is used according to instructions included in its listing and labeling and by the manufacturer, the doors are closed and secured, all covers are in place and secured, and there is no evidence of impending failure. A missing screw, a door held shut with tape, a burning smell or a buzzing sound means the equipment is not in normal operating condition.

Arc flash in DC systems

DC arcs behave differently from AC arcs. An AC arc passes through current zero 120 times a second, which helps extinguish it; a DC arc has no current zero and can be harder to extinguish once established. Battery systems can supply very high fault currents, and protective devices on DC circuits may not clear as quickly as their AC equivalents. IEEE 1584 does not cover DC, so DC incident energy is estimated using other methods, such as those in the informative annexes of CSA Z462 and NFPA 70E, which are deliberately conservative. Lesson 10 returns to batteries, UPS and photovoltaic systems.