The Arc Flash Numbers You Need to Act On

Stay Informed

You’ve heard of arc flash. You’ve even heard some numbers. And while it’s easy to remember that an arc flash is three times hotter than the surface of the sun, it’s not so clear why that matters, or what your facility should do about it.

Sure, the sun is insanely hot, but it’s also really far away. It’s easy to believe that the threat of arc flash is that far away – but it’s not. For facilities that haven’t done their due diligence, that threat is already inside the building.

Let’s leave aside the industry estimates of costs (like hospitalization, litigation, productivity losses, equipment damage, fines) and look at numbers that help you understand arc flash. Incident energy, fault current, and clearing time may seem technical at first, but a basic picture will help you understand the risk of arc flash and evaluate proposals that mitigate arc flash risk for your facility.

Statistics Don’t Save Lives, Good Decisions Do

If you’re wondering how serious of a problem arc flash is, you might look up how many incidents happen in a year. You can find averages including 7,000 burn injuries and 400 fatalities. Estimates in studies range from 3,500 to 30,000 arc flash incidents a year.

From a national, statistical point of view, that’s not a common occurrence. Maybe that makes it sound less concerning. From a safety point of view, though, we want it to be as uncommon as possible. Arc flash is deadly and debilitating. Searing heat, explosive pressure, piercing shrapnel, blinding light, deafening sound waves, toxic gases – we want zero of that in the workplace. Understanding and good engineering help us all keep arc flashes uncommon.

In this article, we’ll explore incident energy and the factors that feed it. With those basics under your belt, you can take a deeper dive with our webinar series, Pulling the Curtain Back on Arc Flash.

Incident Energy

The incident energy is a calculation of the energy that could be released by an arc flash event. An arc flash is different than an electrical shock, although both happen when power goes where it’s not supposed to. Shock happens when power passes through the body. Arc flash happens when power jumps through ionized air and back to the system, or something ungrounded, or to ground. Because the resulting arc travels through the air, which is more resistant, a tremendous amount of thermal energy is released. When an arc flash study looks at the system, it calculates the incident energy to model how severe an event would be.

Technically, the incident energy calculated by the arc flash equations is thermal heat. But that heat is so hot (remember: three times the surface of the sun), that it causes all the other hazards, including blast pressure, concussive sound, and shrapnel.

Calculating the incident energy is the main goal of arc flash study. When you understand how much energy can be released, you can plan to mitigate the danger that energy poses. One way to do that is with personal protective equipment.

However, PPE isn’t a guarantee, and it doesn’t do anything to reduce the severity of the energy released – it just helps meet the risk. An engineered solution does more than meet the risk, it reduces the risk by finding ways to reduce the energy. That depends on understanding the variables that feed the incident energy. Two of the most important variables there are the available fault current and the time it takes to clear the fault.

Understanding the Variables   

The available fault current is the measure of how much current could jump out into that arc. If you’ve ever shorted a 9-volt across your tongue for a laugh, you know the available fault current there is relatively low. To be clear, that tingle in your tongue is a shock, not an arc flash, but the available fault current is still low, and therefore so is the incident energy.

On the other hand, the main switchgear for a large facility has much more current flowing through it. The available fault current is high, and so is the incident energy. The available fault current can be different depending on what part of the system you’re looking at.

The fault clearing time is how long it takes for the system’s protection equipment to trip and stop feeding energy to the incident. In the case of a short in your home, a circuit breaker would trip, stopping that flow of energy. A faster fault clearing time means less of the available fault current can feed the incident energy.

So why wouldn’t you want all systems to trip immediately? The short answer is that the system still has to do the work it’s designed to do, without tripping constantly. And each part of the system exists in context: for instance, you want protective devices closer to the fault (think of that as “downstream”) to trip faster, and devices closer to your main feeders (more “upstream”) to trip slightly slower, so a fault can be cleared locally, instead of shutting down the entire building. 

A Quick Reminder

Before we talk through how those numbers inform your decisions, it’s important to note that calculating incident energy isn’t just a technical concern. Codes regulating arc flash safety can have the force of law. OSHA takes willful violations seriously. Lack of compliance with codes (such NFPA 70E) can lead to fines, liability, and forced shutdowns.

An arc flash does not just happen and go away. It impacts your business, and it permanently impacts the families and lives of those involved.

What do you do with those numbers?

Now that you understand the basics of how available fault current and clearing time relate to the incident energy, and how that informs arc flash risk, the question is how to use those numbers to make informed decisions. For instance, to mitigate risk, you can reduce the available fault current or you can reduce the fault clearing time.

The first step, of course, is making sure you get those numbers. Arc flash studies must be done every five years, and whenever significant upgrades are done. With the incident energies known, you can make sure you:

  • Label equipment correctly
  • Set appropriate arc flash boundaries
  • Update safety procedures and safety training
  • Provide appropriate PPE and the training to use it correctly

You’ll also want to make sure all your electrical system information is up to date and available. One-line diagrams should be clear and correct. Protective device settings should be accurately recorded. Maintenance practices should be updated, especially for protective devices, and a clear schedule for testing equipment for proper grounding, and testing the grounding system itself, is put in place.

Ensuring that your fault protection systems work properly is paramount. Remember, if the fault clearing time is longer, the incident energy increases. Consider developing an arc flash safety plan to be communicated across your organization.

Engineering a Safer Solution

With your arc flash study complete, you can make sure that the right PPE and procedures are in place to meet the risk of that incident energy. But the better you understand your system, the safer you can make it. An arc flash study also gives you the information you need to improve assets, processes, and efficiency. Sometimes the most effective way to reduce incident energy, and therefore risk, is to engineer a new solution for that system.

Hear directly from our arc flash safety experts in our webinar series

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