40 cal/cm^2 Deleted – But Some Confusion Remains.
When the topic of incident energy above 40 calories per square centimeter (cal/cm^2) comes up, the discussion can be quite interesting. People will sometimes refer to the high values in terms of a bomb or some other sensationalized description. Although a higher calculated incident energy can be more hazardous, all is not as it appears to be. Is the large value due to a very strong source or is it simply due to a protective device possibly taking a long time to clear? Each will behave differently.
When performing an arc flash study, if the calculated incident energy exceeds cal/cm^2 at any locations. people often just shake their head and ask, “Now what do we do?” We need to place the equipment into an electrically safe work condition but that in itself poses some risk.
When the 40 cal/cm^2 value is exceeded, it is often treated like an absolute go/no-go threshold and can trigger many different responses and comments that are not always correct. Above 40 cal/cm^2, arc flash labels may have the statement “No PPE Available.” This value also frequently triggers using the signal word “DANGER” on the label. There may be comments made such as, “Above that value, the blast pressure will kill you.” My favorite sensationalized comment that I have heard is, “Above that level, PPE just Continue reading
