A Guide to Understanding Arc Flash Assessment Results

Perhaps you’ve been assigned to make sure your facility is meeting NFPA 70E compliance or you’ve been handed arc flash assessment results and asked to provide your manager a “quick summary” or you’re just curious of the arc flash risk in your facility and want to understand it better. In any case, you’re handed either a large binder or several PDF files of 150+ pages of text, charts, and various lists of data that would take days to plow through. There is a wealth of information included in a properly-prepared arc flash assessment, when there’s time to mine it. But if you’re in a hurry, there’s a much faster way to get to the heart of the assessment, if you know what to look for.Arc Flash Assessment Binders

First, most assessments will include an Executive Summary, which is (usually found) toward the beginning of the assessment and should provide a concise summary of the most interesting findings as well as identify the greatest risks. This will give you a good idea of what to look for in the Arc flash Results tables.

The next most significant section to skip to is the Arc Flash Results table, typically several tabs into the assessment. You will find several pages of tabled results that look something like this:

Arc Flash Results


What to look for in the arc flash assessment results table:

  1. Bus Names with the highest Incident Energy rating below 40 cal/cm2
    • These are buses that workers can safely be exposed to while energized, provided they have properly-rated Personal Protective Equipment (PPE) equal to or greater than the Incident Energy rating of that equipment.
    • Your maintenance department should be aware of these ratings and ensure they have the proper PPE when being exposed to energized parts within this equipment. If not, plant leadership should be made aware, as they are ultimately responsible for providing workers properly-rated PPE for all live work (per OSHA 29 CFR 1910).
  2. Bus Names where the Incident Energy is greater than 40 cal/cm2
    • This means this particular piece of equipment is classified as “Dangerous” when equipment is open while energized, and no amount of Personal Protective Equipment (PPE) will sufficiently protect workers within the Arc Flash Boundary, should an arc fault occur.
    • Steps should be taken to avoid working on this equipment live, or apply engineering controls to reduce incident energy levels and/or live exposure.
  3. The Lower the Trip/Delay time, the lower the Incident Energy
    • The Trip/Delay time is the amount of time it takes the current protection to extinguish or clear an arcing fault. This is typically accomplished through upstream fuses or protective relays tripping an upstream disconnecting device.
    • The high Incident Energy values (in red) correspond to the higher Trip/Delay times, with 2.0 seconds typically being the maximum amount of clearing time most modeling software allows for energy calculations.
      • Where you see Trip times close to or equal to 2 seconds, the current protection is not sufficient for extinguishing the available arcing fault current, and that protection should be reviewed and adjusted accordingly.
  4. Arc Flash Boundaries are critical to keeping unprotected personnel safe
    • Arc flash Boundaries are calculated to ensure that no Incident Energy greater than 1.2 cal/cm2 will be felt outside that distance from the exposed energized equipment. 1.2 cal/cm2 is equivalent to a second-degree burn, so you certainly want to be outside that defined boundary.
    • The higher the Incident Energy levels, the longer the Arc Flash Boundaries must be (in blue). These boundaries only apply when energized parts within the equipment are exposed (i.e. doors are open or cover panels off). It is critical for unprotected personnel to be made aware of where these boundaries are, through signs, barricades, and other related measures.
    • It’s important to know exactly where equipment with higher Arc flash Boundaries are located, in order to determine how personnel or production could be impacted if live work is needed on that equipment. Proper planning could limit or avoid these impacts.
  5. The best way to manage high Incident Energy levels is to reduce them through engineering controls
    • There are often several available methods to reduce the overall Incident Energy levels of your facility’s equipment. They can range from very simple (adjusting protective settings on a relay) to more complex (upgrading your protective scheme).
    • It’s usually best to reach out to an electrical service contractor (like Southwest Electric Co) and ask for recommendations on how to reduce Incident Energy levels for your higher-risk equipment. This will not only provide greater safety to your employees but will also reduce risk to equipment and productivity.
At Southwest Electric Co, we have assisted many customers over the years with their arc flash assessments and incident energy mitigation. We routinely walk through facilities, review one-lines and arc flash assessment results, and then sit down to discuss the various options available.  Feel free to reach out through our website (www.swelectric.com) or your local SWE representative – we are happy to help.