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  • David Peterson

Why is "Arc Flash" such a Big Deal?

In the world of electronics, safety is always a big deal - for good reason.


Most people have gotten some small shock from an electric fence, or from a hand slip while plugging something into a wall outlet.


But that makes you think: if it just hurts or zaps for a second, why is electrical safety such a big deal in every industrial facility?


It's true - most electrical mishaps with residential voltage do not result in fatalities.


However, when it comes to industrial voltages, safety is the subject of training, classes, and expensive protective equipment.

It must be MASSIVE voltages used in industry right? That's the only explanation for such safety precautions.


Well, not exactly.


Industrial voltages are only about 4x larger than those that come out of your wall outlet.


In the big scheme of electronics, the difference between 120vAC and 480vAC is not that drastic, and it doesn't seem like it should lead to such dangerous situations.


The secret: it's not all about voltage.


And, contrary to popular opinion, it's not all about current either...


Instead, let's talk about energy.


The explosiveness of the situation lies in understanding how much potential energy is ready to be unleashed by an arc resulting from two pieces of metal that got just a biiiit too close together.


Several different standards exist for determining the exact amount of output energy (specifically NFPA and IEEE), but they both result in the following units: Energy/area.


That means that the energy potential, either in Joules or Calories that can be present over every square inch or square centemter of the person working on the equipment.


Therefore, the distance from the location to the operator is also a fundamental factor in the calculation.

As for the specific calculations, those details are best left up to an engineering team, but it does exist in the NFPA 70E for curious minds.


The three main factors that are involved in the arc-flash calculation:

  1. The short-circuit current. This is NOT the sizing of the smallest fuse feeding the branch, this is the total capacity of the circuit to provide current if given a short from Line to Neutral. More current = more energy potential.

  2. The time-delay of the circuit protection elements - fuses and breakers. They might be quick, but not instant. Being mechanical devices, they have a delay in response, it cannot be instant. Longer arc duration = more energy potential.

  3. The distance from the arc source to the operator. Obviously, you would try to limit exposure in all cases, but if maintenance must be performed, you need to supply the right energy calculation. Greater distance = less energy potential.

Combining those three elements (along with conversion factors and some exponents) yields an energy potential in J/cm^2 or cal/cm^2.


When someone must perform maintenance inside a control cabinet, they should ALWAYS examine it for warning labels applied in accordance with NFPA 70 section 110.16 before opening the door.

Different energy levels will require different protective equipment and minimum safe distances allowed before touching anything.


What if you don't see a sticker or warning?

Well, best advice - assume it can kill you!


Approach with caution and examine every wire and conductor inside. Carefully measure voltages to ensure that every circuit is either dead, or safe enough to handle properly.


When it comes to safety, don't assume that the other person did their job correctly - your safety is your responsibility.


When you assume a high risk, like when opening a potentially lethal cabinet, your senses and awareness should be operating in top condition.



When you understand what can kill you, and what to watch out for, it allows you approach situations and take charge faster with less risk to you and those around you.


That's what we like to see - efficient and safe.

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