Arc faults cause a blast of light and heat and also a blast of air pressure. Both are potentially deadly threats.
Of all the potential electrical hazards a workplace may pose, the most terrifying and violent is the arc fault. This phenomenon unleashes both an arc flash and an arc blast and is responsible for an estimated average of 30,000 accidents and 400 fatalities in the U.S. each year. Anyone who works on or near energized equipment is probably familiar with arc flashes and should know how to use arc flash protection. But what is an arc blast, and how is it different? Understanding the dangers of each phenomenon is critical.
What is an arc flash?
When an arc fault causes electricity to escape an electrician system, the electricity travels through the air until it reaches another conductor or the ground. The intense light and heat generated from this high-voltage short circuit is known as an arc flash.
The amount of energy released is only slightly less than that of a bolt of lightning, with temperatures approaching nearly 35,000°F, four times hotter than the surface of the sun. The heat is enough to instantly vaporize metals such as copper and steel, ignite clothing and severely burn the skin, eyes and lungs. The closer a worker is to an arc flash when it occurs, the greater the risk of severe injury or death.
What is an arc blast?
Just as a lightning bolt superheats the surrounding air and creates a pressure wave (which eventually transforms into thunder), an arc fault typically generates both an arc flash and a pressure wave, aka arc blast.
The intensity of heat from the arc flash determines the force of the arc blast, but these blast waves can throw an adult to the ground, collapse lungs, damage ears and send molten metal and shrapnel flying at speeds of 700 miles per hour. Workers as far as 10 feet away can suffer arc blast injuries including burns, shrapnel wounds, eye damage and hearing loss.
What causes an arc fault?
Most of the time, an arc fault occurs when an electrician is performing repairs on a piece of equipment. Arc faults can be triggered by many causes, including:
Physical contact between an energized part and a body part or a dropped, uninsulated tool
Live work on damaged or improperly maintained equipment, including equipment with damaged or deteriorating insulation
Improperly installed breakers or switches
Worn or loose connections
Damaged tools or testing equipment
The PPE you’ll need to mitigate the risk of serious injury from an arc flash is dictated by how close you’ll be to exposed energized equipment. The National Fire Protection Association (NFPA) describes three boundaries around exposed energized equipment that determine what level of protection and what level of knowledge and training is needed. The exact distances change based on the voltage of the equipment and must be calculated before work is performed.
Arc flash boundary (outer boundary)
The arc flash boundary is the farthest boundary from the exposed equipment. If an arc flash occurred, a worker at this distance who is not wearing Category 1 PPE would receive second-degree burns. Working any closer to the equipment would result in third-degree burns.
Limited approach boundary
Within this boundary, a worker faces the risk of shock. At this distance, barriers and warnings should be placed to protect workers.
Restricted approach boundary (inner boundary)
Within this boundary, a worker faces an increased risk of shock. Only qualified workers are permitted within this boundary. That person must have the appropriate arc flash training and knowledge and use Category 4 arc-rated PPE. In some cases, an energized work permit may be required.
Arc flash PPE categories: What arc flash PPE do you need?
NFPA 70E, the NFPA standard for electrical safety in the workplace, spells out the PPE needed for four different levels of hazard risk, determined by the incident energy level. Higher hazard risk levels call for clothing and protective gear with a higher arc rating. Arc ratings reflect the incident energy the materials in the PPE protect against, measured in calories per centimeter squared.
Arc flash PPE Category 1: Minimum arc rating of 4 cal/cm2
A Category 1 hazard risk requires PPE with a minimum arc rating of 4 cal/cm2. This PPE is typically adequate for work performed within the arc flash boundary. Equipment includes:
Arc-rated long-sleeve shirt and pants
Hard hat with arc-rated face shield
Leather boots or shoes
Ear plugs for arc blast protection
Arc flash PPE Category 2: Minimum arc rating of 8 cal/cm2
A Category 2 hazard risk requires PPE with a minimum arc rating of 8 cal/cm2. Category 2 PPE is nearly the same as the Category 1 PPE (an arc-related jacket is also recommended)
but has a higher arc rating to protect against higher temperatures, and therefore an increased burn risk. This PPE level is often appropriate for work within the limited approach boundary.
Arc flash PPE Category 3: Minimum arc rating of 25 cal/cm2
A Category 3 hazard risk requires PPE with a minimum arc rating of 25 cal/cm2. This category is often associated with work within the restricted approach boundary. Category 3 PPE includes:
An arc flash suit including arc-rated pants or coveralls, arc-rated jacket and arc-rated suit hood
Insulated rubber gloves with leather protectors
Arc-rated eye protection
Leather boots or shoes
PPE Category 4: Minimum arc rating of 40 cal/cm2
A Category 4 risk requires PPE with a minimum arc rating of 40 cal/cm2. Category 4 PPE includes the same PPE as Category 3 PPE but with a higher arc rating.
Understanding the dangers associated with arc flash and arc blast and knowing the approach boundaries and the PPE needed to protect against both is essential for working safely on or near energized electrical equipment. It could even save a life.