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Arc Flash

What is Arc Flash

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There are really only two types of electrical hazards: shock and arc flash. CSA Z462 gives a short definition for arc flash:

  • Arc flash hazard – a dangerous condition associated with the possible release of energy caused by an electrical arc. Arc flash takes two forms:
    • Arc blast refers to the pressure wave created during an arc flash incident. This pressure wave can throw molten metal at you at very high speeds in addition to causing physical harm like concussions.
    • Arc burn refers to the incredibly high temperature in the area around an arc flash. This temperature can be hotter than the surface of the sun and can cause debilitating burn injuries if you aren’t wearing appropriate PPE.

CSA Z462 mentions that under normal operating conditions, most equipment is not likely to cause an arc flash hazard. It then points to Table 4A for examples of activities with potential for causing an arc flash hazard. Because of the severity of arc flash incidents, it is very important that you understand what it is and how it can harm you.

Arc Flash Causes

An electrical arc occurs when current passes through the air from one conductor to another. A lightning strike could be considered an arc from the clouds to the ground. An arc fault occurs when you have unwanted arcing in your electrical system. This could be because of a breakdown in wire resistance, for example, due to heat. Arc faults can also occur during switching: current cannot immediately drop to zero, so some of the current arcs across the gap as a switch is opened. Undesired arc faults can damage your electrical system.

An example of an electric arc

Arc flash is a severe case of arc fault. In high voltage systems, when an arc occurs it usually burns out and destroys the physical conductors, so air is the only conductor left. Air is normally an excellent insulator, but will break down to plasma when the ratio of voltage to arc length is relatively large.

Arc Flash Effects

A high voltage arc in a small space will have a very large electric field and will cause the air to break down in to plasma. This plasma will use up all of the energy available to it before it dissipates. The plasma will create temperatures as hot as 35,000 degrees Fahrenheit (Clark, n.d.) which is hotter than the surface of the sun!
When an arc flash occurs, temperatures can climb so high that it can burn the skin right off of you! Not only that, but these temperatures tend to vaporize any nearby metals (such as the ones used to make the conductors themselves). When copper (a common wire material) is vaporized, it suddenly expands to 67, 000 times its original volume. (Ray A. Jones, 2000)

When an arc flash occurs there is a brilliant flash of light followed by incredible concussive force

Here’s a quick example using one foot of AWG 10 wire. AWG 10 wire has a conductor diameter of 0.1019 inches. (Wire Gauge and Current Limits Including Skin Depth and Strength, n.d.) A one foot long piece of AWG 10 wire has a volume of 1.80271*10-5 cubic feet ((0.10192/4) * 12 inches * 1 cubic foot / 1728 cubic inches).   That seems pretty small. But when it gets vaporized during an arc flash, it suddenly takes up a space of 1.21 cubic feet! Now consider that a cable can be made up of multiple wires: A typical arc flash packs the same explosive power as dynamite (caused by the superheated metal as it expands into vapor). This explosive action is known as arc blast. If the force of the arc blast doesn’t kill you, you are at serious risk for broken bones and organ damage, especially if the explosion knocks you into a hard surface. It’s even more dangerous if you’re working at a high elevation: the arc blast could knock you off a solid foothold and put you at serious risk for fall injury.
Not only does the arc blast cause a pressure wave that can throw you around with a large amount of force, but it will also throw around any metal that didn’t get vaporized during the arc flash. Usually so hot that it becomes molten metal, this can burn through clothing as well as any equipment nearby and can cause serious harm to anything it touches. The explosion and high temperatures could also cause damage to or destroy nearby electrical equipment that is not part of the initial arc flash incident.
Overall, arc flash is a great risk to both people and equipment, and can result in very costly injuries to people or damage to equipment. The risk of an arc flash incident can be reduced by appropriate electrical safety program usage. In order to account for arc flash in an electrical safety program, an arc flash study is required.
As always, thanks for reading!
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Arc Flash Mitigation techniques – PPE

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050308-N-1485H-062 Pacific Ocean (Mar. 8, 2005) - Hull Maintenance Technician 2nd Class Benjamin Bates of Cabot, Ark., practices arc welding pipes together inside the Pipe Shop aboard the conventionally powered aircraft carrier USS Kitty Hawk (CV 63). Hull Technicians are responsible for maintaining Kitty Hawk's hull, fittings, piping system and machinery. Currently under way in the 7th Fleet area of responsibility, Kitty Hawk demonstrates power projection and sea control as the U.S. Navy's only permanently forward-deployed aircraft carrier, operating from Yokosuka, Japan. U.S. Navy photo by Photographer's Mate 3rd Class Lamel J. Hinton (RELEASED)
PPE is the last line of defense when it comes to arc flash mitigation, according to the hierarchy of hazard mitigation. Appropriate PPE selection is important to prevent introducing new risks to workers on job sites.

Safe PPE Selection

In order to select PPE, you first need to have an incident energy study on your workplace. PPE levels are determined based on the incident energy levels of the equipment being worked on, so you can not appropriately select PPE without an incident energy study. CSA-Z462 has a table that matches types of PPE to arc flash incident energy levels. This table is an excellent guide to use when selecting your PPE.

Is this worker wearing all the appropriate PPE for his job?
Is this worker wearing all the appropriate PPE for his job?

I personally do not recommend doing work on anything with an incident energy level higher than 40 cal per square centimeter: when incident energy is that high, you should figure out a way to reduce the incident energy level before assigning any work. All PPE should be appropriately labelled so that you know what is acceptable to wear for the present work conditions. At minimum, when working on a system with an arc flash risk, you should be wearing arc rated long sleeve shirts, pants, and undergarments, insulated leather or rubber gloves and boots, eye, face, and ear protection, and a hard hat.

Other Factors that Influence PPE Selection

Other factors to consider when selecting PPE include the working environment and the amount of time it will take to complete the job. Higher levels of incident energy require more bulky PPE. Bulky arc flash suits can impede a worker’s vision, hearing, and mobility. The large, sealed, bulky clothing is cumbersome and also gets hot quickly, which causes increased levels of fatigue. All of these things can lead to new risks that might actually increase the danger to workers on site, which is the opposite of what arc flash mitigation techniques are supposed to do!
One last thing to be sure of is that your PPE is in good repair. Old, worn out PPE full of holes won’t protect you in the event of an arc flash incident, and should not be worn by anyone.

Job Planning

Sometimes it is necessary to do work in heavy, uncomfortable PPE.

You don’t want to be stuck wearing this for too long if you don’t have to be.

When this is the case, it is important that jobs be planned appropriately to reduce risk involved. If fatigue is an issue, schedule multiple people working on the same job in short shifts as opposed to having one person doing all of the work. The buddy system (having at least two people working on the same job) also has the added benefit of safety: the second person can immediately respond if something goes wrong while the first person is working.
You can also assess the functions of the equipment being worked on to see if you can’t reschedule work for a period when the equipment can be de-energized, avoiding the need for the bigger arc flash suits.
Worker training is very important. The NFPA notes that many workers who suffer electrical injury have insufficient training and do not properly use their PPE. Training personnel to appropriately assess hazards and risks is very important because these hazards and risks influence the level of PPE required for a job. All workers should be properly trained in the use of all PPE required for a job: if you aren’t using the PPE properly, then it won’t protect you. Once again, this goes against our goal of arc flash mitigation!

Closing

When selecting PPE, you need to know the incident energy on the equipment you are working on. You then need to make sure the labels on your PPE are appropriate for the work and that your PPE is in good repair. When doing energized work, first try to reduce the incident energy levels to avoid having to wear the bulkier PPE. If that isn’t possible, have multiple people working in short shifts to avoid fatigue, or reschedule to a time when you can de-energize the system. Make sure that all employees are trained in the selection and use of PPE.
Stay safe, and as always, thanks for reading!
If you like this post but want some actionable advice, tips and information, check out our newsletter. You can sign up here or below.
By signing up you get a free report on what an Electrical Safety Program is, and how to go about building one at your facility.