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Engineers calculate arc flash exposure based on where the person is expected to be when performing work on electrical equipment, this is known as the working distance.

NFPA 70E 2024 defines Working Distance as "The distance between a person’s face and chest area and a prospective arc source."

When performing an arc flash evaluation, also known as an incident energy analysis, NFPA 70E 2024 130.5(G) states "The incident energy exposure level shall be based on the working distance of the employee’s face and chest areas from a prospective arc source for the specific task to be performed."

The further back you can get your face and chest from a source, the further the "working distance" and the safer you will be from an arc flash.

Take a look at the example label below, the incident energy calculated for this piece of equipment is 6.1 cal/cm^2 at 18" (18" is the working distance on this label):

18" is a very common working distance as it is the approximate length from the ends of your hands to your face and chest. What's important to note is that this label does not indicate the arc flash hazard if you are closer than 18", this would be the case if you were to move your face closer to the equipment to get a better look... Doing so would expose you to higher arc flash values than what is indicated on the label. If you are wearing an arc flash face shield and balaclava with a system rating to 8 cal/cm^2 you would want to reconsider positioning your face any closer than 18". Alternatively, if you were to move further back, the exposure would be less.

Arc flash calculations can be performed at any working distance. Standard engineering software defaults to multiple working distances, such as:

  • Low voltage (600 V and below) MCC and panelboards - 18"
  • Low voltage (600 V and below) switchgear - 24"
  • Medium voltage (above 600 V) switchgear - 36"

Rozel typically uses a standard working distance of 18" for almost all types of equipment regardless if it is a 15A fused disconnect switch, an MCC, a Panelboard, a low voltage switchgear or a medium voltage switchgear. The reason is to avoid confusion. We have seen some of the most qualified electrical workers in the world make mistakes when reading a label, understanding what you need to wear is one thing but coupling it with where you need to stand is another. We understand that the standard working distances change based on physical characteristics of equipment and standard depths. However, we've also seen employees literally crawl into equipment to perform work. In this particular situation, you would want to perform additional calculations to understand exposure (by the way this was in a nuclear facility with some of the most stringent safety requirements in the country). We prefer to err on the side of caution.