What is arc flash?


blog author iconJon Travis
date icon2021 / 11 / 30
blog views icon1333
What is arc flash?

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. How is arc flash measured?
  3. What causes an arc flash?
  4. The arc flash explosion
  5. Arc blast vs arc flash
  6. How to protect yourself from arc flash
  7. Ready?

Let’s get started!

 

Download free definitive guide to arc flash studies

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 degrees Celsius),  
  • 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.

Is it arc flash or arc flash hazard?

Before we o any further, just a quick clarification…

To me, there isn’t much difference between the terms arc flash and arc flash hazard.

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.

Arc Flash Definition

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

Source: CSAZ462-21

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

It’s 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!

How is arc flash measured?

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).

Incident Energy Definition

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?

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.

Arc blast definition

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.

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.

Get the right training and prevent arc flash injuries

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 PPE.

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 here.

Enjoy!

Now It’s Your Turn

energized electrical work

Hopefully, you now have a better understanding of what an arc flash really is and you’re able to respect how dangerous one can be.

Let me know what you think by leaving a quick comment below right now.

Looking To Learn More About Arc Flash Studies?

Download our free Definitive Guide to Arc Flash Studies to help get a more detailed understanding of the potential arc flash hazard at your facility.

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