What is Arc Flash

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)

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.
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