The main companies who make the blankets have extensive data and should be able to help.

Estex/ThermEquip
Oberon

Both have substantial data and should be able to help with possible structures. Most vaults just use lag bolts in the walls installed for this purpose. ConEd (NYC) & Detroit Edison have both done extensive work on this and are willing to share.

These blankets (if you read the standard) are exposed to a real design test. The arc is ejected at the amperage specified. The actual arc electrodes are pointed at the fabric so it is a worst case on arc energy and close to worst case on blast.

See the videos of the tests that the companies show.

There is a theoretical "arc blast" calculation publically available but it is vastly overly conservative. Google for it. In addition the joint NFPA/IEEE study did measure arc blasts. I'm assuming that it will be part of the data set when they release the new IEEE 1584 standard this year or next. I don't know of anything else in public domain.

Here is an article I wrote in my arc flash newsletter with all the citations I have found or been introduced to.

Arc Blast Burn Up The Myth

[font=Times New Roman][/font]Q: (Do you know) of any studies / documented history conducted on the effects of arc flash events and the medical impact from the pressure associated with the blast? For example, we have a client who has a 100 cal suit available for use. While it would "protect" from the 100 cal arc flash, the impact to the body from a close proximity blast could be severe.
A: There are really no full studies. IEEE 1584 now has additional data and this should be published in 2012 or 2013 but for now the data is VERY limited. Only one paper below has data and that data has been shown to be based on a microphone which washed out at about 1 PSI. I know of NO deaths from pressure wave. Many knockdowns. All the deaths I know of are from thermal burns from clothing ignition or lung burns from breathing in the hot gasses (usually from the clothing fire or subsequent fires from burning oil [oil filled cables and transformers]). The pressures could be big but it isn't anecdotally very convincing. I do believe pressure waves can be an issue but it isn't half as bad as we are currently led to believe by some salespeople. I have done about 100,000 arcs and we can knock a mannequin down but we don't usually destroy them (I did break an arm off once by putting it across a disconnect). We are not against equipment that eliminates the arc blast, but last month I learned of a group of workers who refused to wear 100 cal suits for a potential 100 cal event because the equipment company trained them and pushed this idea that the blast will kill you and the suit will "leave a nice corpse". This has NO evidence to support it and leads people to doubt the real science. I challenge the equipment manufacturers to donate - as some have - to IEEE 1584 so real science can be done on this and help to answer a very complicated question without fear and trembling, but with knowledge and integrity. Again, always ask what they are selling. ") "Ignorance isn't what you don't know, it's what you know'; wrong, Yogi Berra.

Here are the papers I have on the subject:
[1] R. H. Lee, "Pressures developed by arcs," IEEE Transactions on Industry Applications, vol. IA-23, no. 4, pp. 760-764, July/Aug. 1987.

[2] D. Sweeting and A. D. Stokes, "Energy transfers within arcing faults in electrical systems," Proceedings of the 8th International Conference on Electric Fuses and their Applications (ICEFA), Clermont-Ferrand, pp. 169-178, Sept. 10-12, 2007.

[3] K. Crawford, D. Clark, and R. Doughty, "Motor terminal box explosions due to faults," IEEE Transactions on Industry Applications , vol. 29, no. 1, pp. 72-81, Jan./Feb. 1993.

[4] M. Drouet and F. Nadeau, "Pressure waves due to arcing faults in a substation," IEEE Transactions on Power Apparatus and Systems, vol. 98, no. 5, pp. 1632-1635, 1979.

[5] J. E. Bowen, M. W. Wactor, G. H. Miller, and M. Capelli-Schellpfeffer, "Catch the wave," IEEE Industry Applications Magazine, vol. 10, no. 4, pp. 59-67, July/Aug. 2004.

[6] F. Lutz and G. Pietsch, "The calculation of overpressure in metal-enclosed switchgear due to internal arcing," IEEE Transactions on Power Apparatus and Systems, vol. 101, no. 11, pp. 4230-4236, Nov. 1982.

[7] P. Chévrier, M. Barrault, and C. Fiévet, "Hydrodynamic model for electrical arc modelling," IEEE Transactions on Power Delivery, vol. 11, no. 4, pp. 1824-1829, Oct. 1996.

[8] G. Friberg and G. J. Pietsch, "Calculation of pressure rise due to arcing faults," IEEE Transactions on Power Delivery, vol. 14, no. 2, pp. 365-370, Apr. 1999.

[9] G. E. Heberlein, Jr., J. A. Higgins, and R. A. Epperly, "Report on enclosure internal arcing tests," IEEE Industry Applications Magazine, vol. 2, no. 3, pp. 35-42, May/June 1996.

[10] A. Bellemare and M. Fortin, "Internal arcing fault tests of pad mounted distribution switchgears," Proceedings of the 2005/2006 IEEE PES Transmission and Distribution Conference and Exhibition, Dallas, Texas, pp. 1016-1019, May 21-24, 2006.

[11] R. L. Doughty, T. E. Neal, T. A. Dear, and A. H. Bingham, "Testing update on protective clothing & equipment for electric arc exposure," IEEE Industry Applications Society Magazine, vol. 5, no. 1, pp. 37-49, Jan./Feb. 1999.

[12] Neal, T. E. and R. F. Parry, "Shrapnel, pressure, and noise," IEEE Industry Applications Society Magazine, vol. 11, no. 3, pp. 49-53, May/June 2005.

[13] R. A. Jones, etal., "Staged tests increase awareness of arc-flash hazards in electrical equipment," IEEE Transactions on Industry Applications, vol. 36, no. 2, pp. 659-667, Mar./Apr. 2000.

No, and this one is a big problem:


The above theoretical calculation, similar to Lee's incident energy calculation, has a lot of issues because it is highly theoretical and misses some details of real world arcs. But at the current time it's the only calculation that I'm aware of.

This is purely conjecture on my part but it appears that the "40 cal/cm^2 limit" comes from using the Lee equation and comparing it to published military data on the effects of concussive pressure waves. So if the Lee equation is wrong, then the 40 cal/cm^2 limit is wrong as well.

It is my understanding from some anecdotal published information from ABB that the pressure wave has a bit of delay, and doesn't last very long. So for very fast interrupting times, it could be bypassed entirely. After a few cycles even though the incident energy continues to grow linearly, the arc blast is essentially done. A high level presentation about the joint IEEE/NFPA study suggests that for a wide variety of conditions arc blast is essentially a constant.

I can see both points here. There would be an initial blast front of course, but whether it is a single wave or a sustained level would depend on the volume of copper being consumed by the plasma and converted to gaseous form. Even if there was a constant pressure after the initial blast, might it be at a lower amplitude than the initial blast pressure? Since after the threshhold is crossed, the current draw would be based on the resistance of the plasma arc rather than the short circuit that initiated the arc flash incident. Does that make sense?

The blast is not a function of the calories. It is a function of fault current. So 40 cal cut off is arbitrary. The cut off should be based on fault current and the box most likely.

Larry has it right from the ABB info. The blast comes from the gasses being compressed to near absolute zero to become plasma and then expanding to 5000-10,000F to eject. This pressure wave comes mainly from the initial compression/expansion thus why Dr. We-Jen Lee saw a negative pressure followed by a positive pressure. There is no good way to measure the blast used to date. My suggestion which was used on the last run was to use a pendelum and measure deflection. It was less than expected on the configurations looked at.

I have seen NO deaths to date from "arc blast" so I'm fairly sure it is overblown. I'm sure we will find one here and there but most of the injuries are thermal and clothing ignition related until flame resistant clothing came into common use.

The companies making the blankets have had no failures I know of to date so I still suggest contacting them to get info on installation. Testing is better than theory on this front.

Actually, copper is less of a problem. Aluminum, being combustible, is a much bigger problem. Regardless, the majority of the pressure wave is caused by heating of the air inside the enclosure according to a paper on a CFD model that I have. The metals themselves don't make a significant contribution, although any chemical reactions that involves the metals does contribute a small amount to the pressure wave (but the amount is minor and temperature dependent). The very high volumetric expansion of metals when they convert from solid to gas sounds impressive and surprisingly at least from what I've read doesn't have much to do with what is happening. If this wasn't the case, then there would be a lot of physical problems to overcome in the design of interrupters especially vacuum interrupters accommodating the expansion of metal vapors at the contact surfaces.

The basic description that I've read from Hugh's pile of papers among others seems to suggest that you first go through a period of adiabatic expansion and the pressure rises very rapidly with the expansion of the gases. Once the gases heat up and expand though the blast is over with because there is no longer any material to continue to expand.

AL is really only a problem if you are wearing ignitable clothing. Minor areas of burn come from molten AL or CU. Most of the worker burn comes from the arc itself or from clothing ignition.

Most of the CU or AL does NOT vaporize. Most is molten droplets and therefore NOT contributing to the blast. The pressure is MOSTLY from the expansion (contra many of the papers which were theoretical). The ONLY real data from arc flash supports the best answer from physics (most blast is from gas expansion from creation of arc roots). More energy does not relate to more blast. More current and box effects relate to more blast. More copper burned (longer clearing time) does not relate to more blast.

More energy (more current) does mean a higher blast pressure. Does that correlate to more current means a higher volumetric expansion of metal to gas as well? Or does the same amount of metal expansion occur, just at a higher radiated force?

Hugh stated that metal to vapor conversion has nothing to do with it, and I'd agree from what I've read and from a very practical point of view. The arc blast is primarily coming from heating up the gasses that already exist inside the enclosure...in other words in most cases, AIR. The only slight caveat that I threw in there is that the rapid oxidation of the metals...chemical reactions...contributes SLIGHTLY but this is again a largely theoretical argument that is probably not even noticeable with actual real world data.

Since all that is happening is rapidly expanding gas from thermal radiation, we can go back to a very simple equation which is that if we hold the volume constant, the pressure is proportional to the temperature. We've got a short time to work with to heat the gas as quickly as possible before it blows the doors off and quickly mixes with the gas in the rest of the room. So P is proportional to T and the higher we can get T before it is released, the higher P gets, but the shorter the time interval over which the pressure wave will be.

Actually I was just curious as to whether there has been a direct correlation made of the amplitude of arcing current to the volume of metal converted to gas.