Marcelo wrote:

Interestingly the distance exponent for open air is 2, same as for a small enclosure. Maybe there is an underlying assumption that in open air the working distance is always greater than 610mm. In that case the exponent of 2 will yield less energy as show by the column above for 910mm = D at exponent of 2. that makes sense to me as the energy is dispersed in all directions.

I thought the open air case is obvious.

If we assume that energy is released at a point and radiates outward then the energy flux (cal/cm^2) would be dependent on the area of a sphere equal to the working distance. Thus the exponent would be 2 since it is proportional to 4/3*pi*R^2.

If the enclosure is very small then clearly the shape of the enclosure is immaterial. Similarly for very large enclosures there is no difference. However for "medium" size enclosures especially where there are groups of equipment such as in switchgear or MCC's, the incident energy tends to be "shaped". Unfortunately and this has not been mentioned yet, as the working distance changes, the influence of the enclosure shape also changes. If the working distance increases far enough no matter the enclosure shape, it will act like an "open" condition. Similarly if it is decreased at some point, enclosure shape matters. At this time though these different factors are assumed to be constants and there is no way that I know of to adjust for enclosure shape beyond picking the exponent from the table of equipment provided.