What is arc flash?

Jon Travis

2025 / 03 / 13

22548

If you’re looking for the answer to the question “what is arc flash?”...

Then you are in the right place.

In this article I’ll be going through everything you need to know about the arc flash hazard.

Here is what we will cover:

1.   What is an arc flash?
2.   What is an arc flash hazard?
3.   Is it arc flash or arc flash hazard?
4.   How is arc flash measured (incident energy)?
5.   What incident energy level is dangerous?
6.   What causes an arc flash?
7.   The arc flash explosion
8.   How hot is an arc flash?
9.   The difference between arc blast vs arc flash
10.   What are the effects of an arc blast?
11.   How to protect yourself from arc flash
12.   The importance of arc flash training

Ready?

Let’s get started!

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What is an arc flash?

An arc flash is an uncontrolled release of energy caused by an electric arc which exposes any person within its reach to a multitude of hazards such as:

extreme radiant heat (upwards of 20,000 ⁰Fahrenheit),
a plasma ball (or fireball),
molten shrapnel projectiles,
blinding light (ultraviolet and infrared),
a concussive blast,
a deafening sound (enough to rupture eardrums) and
dangerous vaporous gasses.

Pretty scary stuff.

That’s why an arc flash hazard can’t be treated lightly.

What is an arc flash hazard?

This is the definition that’s given to us in both CSAZ462 and NFPA70E:

Arc flash hazard - a source of possible injury or damage to health associated with the release of energy caused by an electric arc.

Breaking this down, I’d like to draw your attention to the word “source”.

The first thing you need to think about is where is the hazard coming from? What is the source?

In the case of arc flash it’s often the things that electrical workers are working on the most…

Motor control centers, breaker panels, switchgear, panel boards and more.

When you are working on these types of equipment, you need to be thinking “this is a possible arc flash hazard.”

This definition is good, but still leaves us with a lot of unanswered questions.

Keep reading and we will try to answer all of them you might have!

Is it arc flash or arc flash hazard?

Before we go any further, just a quick clarification…

To me, there isn’t much difference between the terms arc flash and arc flash hazard. I gave you a few slight differences above, but for practical purposes they are pretty much the same thing.

If you are reading this article, then you are probably here because you have an interest in protecting yourself (or others) from workplace hazards…

An arc flash (or the arc flash hazard) is one of them.

So, for the purposes of this article, I’m going to use the two terms interchangeably… probably whichever one sounds better at the time.

How is arc flash measured (Incident Energy)?

Arc flash is measured by the amount of heat energy an electric arc explosion produces.

This measurement is expressed in calories per square centimeter (cal/cm²) and is referred to as the incident energy.

One important thing to note is that many of the associated hazards listed above (blast pressure, fire ball, deafening sound) are not expressed in the incident energy calculation.

But there is good reasoning for this.

Heat energy is the primary cause of fatality, so it is the primary focus of the standards and regulations.

Almost everything in the electrical safety standards is based on the incident energy values (with regards to arc flash that is).

Here is the definition for incident energy:

Incident energy — the amount of thermal energy, impressed on a surface a certain distance from the source, generated during an electrical arc event.

Source: CSAZ462-21

What incident energy level is dangerous?

If I asked you to choose which one of these incident energies is high enough to cause bodily harm…

which one would you pick?

1.2 cal/cm²
4.0 cal/cm²
8.0 cal/cm²
25 cal/cm²
40 cal/cm²
100 cal/cm²

Not sure?

Well, the truth is, all of them are dangerous…

Even at levels of 1.2 cal/cm² a person would receive at least 2nd degree burns on any part of their body not protected by arc-rated PPE.

Not to mention, they will likely be on fire, unable to see and have an extremely loud ringing in their ears.

Not a good situation to be in…

What causes an arc flash?

While human error or carelessness of a worker account for most incidents, an arc flash can be the result of a wide variety of causes such as static electricity, loose connections, or even a rodent in the vicinity that compromises the distance between energized components.

What causes an arc flash can be broken into two groups.

First are objects which are foreign to the electrical system that get too close to exposed energized conductors and initiate a short circuit. The second is when the equipment itself either malfunctions, is misused or is installed incorrectly.

Some examples of what causes an arc flash are:

a mouse, snake or another animal which is using the electrical equipment as a home;
an uninsulated screwdriver (which has been misdirected);
water infiltration;
corrosion;
equipment fingers or stabs that are bent or warped;
human fingers or other body parts;
a voltage detector or multimeter (possibly incorrect rating, damaged or frayed leads);
leaving tools behind (inside the equipment) after maintenance work.

These are what I would call the “catalyst”…

but why does it explode?

The arc flash explosion

Once the arc flash starts (so the moment just after something has created the electric arc) you would think that your electrical systems would be smart enough to detect that something is wrong and shut down.

But in reality, the system has no idea something is wrong.

And it’s way too slow to react appropriately.

In fact, it thinks that this electric arc is a motor that’s trying to turn on… so what does any good electric system do when someone starts a motor? It sends current to that place in the system to make sure it turns on.

Then it thinks “But wait… this motor must be bigger than it expected, it’s not turning on… I know, I’ll send even MORE current.”

Nope, still not on, so it sends MORE and MORE and BAM!

So… what just happened.

Once the electric arc starts, the system sends thousands and thousands of amps of current to the place in the system where the arc occurred.

The equipment is not robust enough to handle that much current. So in a fraction of a second, that little piece of copper metal has gone from room temperature to be superheated.

Super-heating anything that fast is never going to be good.

But how hot does it get anyway?

How hot is an arc flash?

Arc flash temperature readings can range anywhere from 5,000 to 20,000 ⁰Fahrenheit (or 2,760 to 11,093 ⁰Celsius respectively).

To put that in perspective, copper melts at 1,983 ⁰Fahrenheit and vaporizes at 4,703 ⁰Fahrenheit.

During an arc flash event, there is so much current being sent to the point of an incident that most of the metal inside the electrical cabinet will vaporize and cause a massive explosion, which includes a fireball, heatwave, deafening sound, blinding light, and concussive blast.

And that brings me to my next topic…

Arc blast.

The difference between arc flash and arc blast

The difference between arc flash and arc blast is that arc flash refers to the specifically to the heat energy (incident energy) while arc blast is focused on the sudden release of pressure within an enclosure during an arc flash.

Arc blast is caused by the instantaneous heating of the air surrounding the conductors.

When air heats it expands.

So the arc blast is a result of the arc flash.

Get it?

There is still a lot of confusion surrounding the arc blast and if you keep reading I’ll help you get a better understanding of the arc blast phenomena.

Here is the definition of arc blast:

Arc blast - arc blast pressure is primarily a function of electrical fault current and electrical equipment enclosure failure and the subsequent sudden release of pressure developed within the enclosure by the instantaneous heating, expansion and containment of the air surrounding an arc flash.

Physical trauma can result from being struck by an enclosure door, however research indicates that blast pressure is negligible if enclosure doors are open.

Source: CSAZ462-21

What are the effects of an arc blast?

Historically, it was proposed that the arc blast created by the sudden vaporization of copper (or other metals inside an electrical cabinet) produced a life-threatening blast pressure.

It turns out that this is not the case.

Especially when the enclosure doors are open.

Although copper does expand to around 67,000 times its volume when subjected to the extreme heat of an arc flash event, there is so little of it that is close enough to the arc that very little of it ends up being vaporized.

One thing to keep in mind is that when the enclosure doors are closed it is possible to build up enough pressure to blow the doors off and cause an injury by hitting someone with a door or other broken parts.

All-in-all blast pressures are negligible for open door enclosures and when the doors are closed just make sure you are not in the line of fire.

How to protect yourself from arc flash

Obviously, the best way to protect yourself is to stay away from potential arc flash hazards.

But, for now, electrical workers don’t always have that option.

Even if the plan is to de-energize the system first, they still need to enter a potentially dangerous situation to prove the absence of voltage.

So the best protection today is Personal Protective Equipment (PPE) or some may call an Arc Flash Suit.

Arc rated PPE to be specific…

You need to make sure that every part of your body is covered and protected with some form of arc-rated PPE.

This is a huge topic on its own, so I’ve dedicated an entire post to this topic on its own!

You can read all about Arc Flash PPE — Learn more on our PPE Blog!.

Enjoy!

The importance of arc flash training

Making sure that you fully understand the hazard is important.

And training is a must.

But not all training is created equal.

You want to make sure that you’re getting the proper arc flash training…

But how can you tell the difference?

Proper arc flash training is going to focus on the “how” more than the “what”.

It’s no use showing electrical workers a bunch of videos of equipment blowing up and going over definitions from the standards.

They already know arc flash is bad (unless they’ve been living under a risk).

You need to go over how to determine when you could be at risk, and then how to make sure you stay safe for the entire job!

Arc flash training that focuses on identifying and mitigating risk for jobs that your electrical workers are performing on a day-to-day basis is invaluable.

If you want to learn more, you can visit this page.

And that is just the tip of the iceberg. We have other resources to help you learn even more about Arc Flash and related topics. I just mentioned training, which is arguably one of the most important pieces of the puzzle. We also dive into:

Arc Flash Labels - What is an arc flash label, what does it need to have on it, and how do you read it? We get into that and more, here.

Arc Flash Boundary – We look at how a safe distance is communicated, where it comes from, what it means, and who’s responsible for it. Learn more here.

Arc Flash PPE – Still a bit confused about what arc flash PPE categories to wear?

Arc Flash Values & Levels – Feeling like your Arc Flash levels are too high or need a bit more info?

…We also just published a brand-new article on “Arc Flash Training in 2025”. We think it gives a good overview of the online training space today.