One of our other engineers asked what was the allowable openening in a bezel to maintain a 0 HRC for an electrical enclosure. I told him I didn't think you were allowed to put holes in the bezel and still have the HRC at 0 when you opened the door to access the breakers. I would think that putting a 4" openning in a bezel would act as an oriface and restrict the pressure of the incident enercy, but increase the velocity of the blast effect

The door has an effect (reduction) in incident energy. BUT there is no engineering/modeling data to quantify the impact of the covers. Except for arc resistant gear (in which the above modification would require retesting/certification), the net effect of covers on enclosures is IGNORED. Thus there is no difference whether the enclosure is open or not using methodologies listed in the annex of 70E, assuming you are using the engineered approach.

Using the tabular approach, the tables were developed using a qualitative approach by the 70E Technical Committee. Nothing has been published explaining their justification for lowering the H/RC categories when reductions were taken so we are left with taking guesses. Thus there is no possible way to estimate how big an opening has to be before it may change the rating between "doors closed" and "doors open", when this matters, unless you are able to get the 70E Technical Committee to evaluate it for you (basically, duplicating the methodology).

I put forth an alternative method to achieving "reductions" when using the engineering approach as part of a risk assessment by calculating the likelihood of an arc flash occurring.

I was mulling this over again and I came to some conclusions (fairly quickly after reading your last post - My attention was just diverted, and I neglected to post my thoughts), and the only guarantee pertaining to protection that an enclosure affords is that the enclosure acts as protection against shock.

Second, unless an enclosure is specifically rated for arc flash - including the blow-out panels and venting the incident energy. the only real degree of protection against an arc flash event thermal effect would depend solely on how far the actual incident is from the boundaries of the enclosure. Close proximity to the sheet metal would mean that the metal would be vaporized, and no protection afforded. The distance from the sheet metal of the enclosure is inversely proportional to the likelihood the metal will be vaporized. (I know that is blatantly obvious)

Its degree of protection against the effects of arc blast pressure just depends on how far you are away from the enclosure and your orientation to the door (or blow out exhaust - whichever is applicable), because that is the part most likely to be coming off the enclosure at a high rate of speed.