Hi,
I am interested in finding out, if other users apply any sort of factor to reducing incident energy when the door is closed, on a non-type tested switchboard ie not arc fault contained? A client of mine has done this in their own internal document and referenced "the NFPA tables reflect that some protection is afforded by the metal switchboard enclosure given that it sits between the arc fault and the operator."

I don't have a copy of the NFPA document (I only work on LV equipment) and don't have a contact in the client's organisation that can answer this either.

What do you think? Can it be justified?

Thanks!

See "Report on Enclosure Internal Arcing Tests", Heberlein, Higgins, and Epperly, IEEE IAS Magazine, May/June 1996, pp. 35-42, among others. CIGRE also did a lot of research on modelling for the purposes of developing arc resistant gear but the upshot is that on non-venting gear, the pressure in every test they did blew the enclosure doors off within 1 cycle. Furthermore there does not appear to be any sort of cal/cm^2 or kVA rating below which this does NOT occur. It doesn't happen every time but so far a definitive calculation has been elusive.

Doors open/doors closed is only applicable in so much as it is a convoluted way to say that the worker is outside of the restricted boundary and thus unlikely to cause an arc. Once we have an arc IF the doors stay on then the incident energy is in all likelihood never going to get above 1.2 cal/cm^2 since it is thermal radiation. That is the magic of arc resistant gear. But once the doors are off, whether they are open at the time or they are blown off, there is no difference in terms of incident energy.

Here is the one I meant to give you:

"Exposed to the Arc Flash Hazard", Lang and Jones, Paper #ESW 2014-18.

From the conclusions:

"These investigations indicate that failure of the enclosures seems correlated to the amount of arc energy delivered in the first 8-16ms. Failures occurred with arc energies as low as 33 kWs and incident energy calculations as low as 0.3 cal/cm . It is not safe to assume that equipment
inherently provides protection against an arc flash hazard..."

Note that in an arcing fault the pressure builds linearly within the enclosure, related directly to arc power. CIGRE tests previously showed that the enclosure vented within 1 cycle. The same thing happened in the Lang & Jones paper. Larger enclosures (from the Heberlen paper) result in a slower pressure rise (because of a larger volume of air to be heated) but he also that except for very short duration arcs (under 1 cycle). Either way the limit in terms of pressure is in how much pressure the enclosure can withstand. Changes in the arcing time or the arcing power, especially when it occurs for longer than 1 cycle, have almost nothing at all to do with enclosure failures. Hence the reason that kW and cal/cm^2 bear no relationship.

However one trend that is clear here...you can't even say that below a certain kVA, kW, cal/cm^2, or any other similar threshold guarantees that the enclosure won't rupture.

Based on this then there you can't realistically decrease the incident energy rating due to doors closed because you won't get there unless they are arc resistant in which case the incident energy is automatically <1.2 cal/cm^2.

Thanks Paul!