arc flash "Dangerous" levels...

[jim dungar]

...the equipment doors/enclosure did not blow open?

The pressure required to 'blow the doors open' is no where near as high as the level require to likely cause death.

From the CDC
http://www.cdc.gov/niosh/docket/archive/pdfs/NIOSH-125/125-ExplosionsandRefugeChambers.pdf"The human body can survive relatively high blast overpressure without experiencing barotrauma. A 5 psi blast overpressure will rupture eardrums in about 1% of subjects, and a 45 psi overpressure will cause eardrum rupture in about 99% of all subjects. The threshold for lung damage occurs at about 15 psi blast overpressure. A 35-45 psi overpressure may cause 1% fatalities, and 55 to 65 psi overpressure may cause 99% fatalities. (Glasstone and Dolan, 1977; TM 5-1300, 1990) "


I believe effects of arc blast, like doors being blown open, needs to be emphasized even at locations with incident energies levels well below 40cal/cm^2.

 

[MD84]

That is interesting. Do you know what range of magnitude pressure waves could be experienced due to an arc blast?

 

[Bugman1400]

This is a interesting discussion. Are there any folks that perform arc-flash calcs that take into account the Stokes and Oppenlander method?

 

[gadfly56]

I agree with a lot of the information here but I do question the open vs closed enclosure topic.

I agree that the risk of causing an incident is reduced when doors and panels are closed. However should an incident occur a closed box could present an increased arc blast due to pressure building to a higher degree before exploding the enclosure and releasing the energy.

Incident energy classified as dangerous will surely blow open the doors to a panel or enclosure in a violent manner.

I am not suggesting opening doors to reduce risk or hazard.

I worked at a company where we operated medium voltage equipment classified as dangerous. We used a third party analysis which ultimately allowed us to operate the equipment. Multiple layers of redundant safety measures were utilized to mitigate the hazard.

Increasing distance from the calculated point will reduce incident energy inversely proportional to the square of the distance.

Although NFPA 70E allowed local operation, one vendor I have worked with devised a string pull mechanism to open breakers remotely that did not utilize remote operators.

I do not have a copy of 70E in front of me but I believe that if one is not manipulating equipment in such a manner to cause an arc flash it would not be required to wear ppe. This does not prohibit the use of ppe. I would say that operation of a breaker qualifies as an operation that could cause an arc flash. What constitutes an operation that could cause an arc flash is subject to interpretation.

Maybe. In free air without restriction, incident blast energy is reduced by the cube of the distance for a point source. Think about it; ideally the pressure wave moves out from the source in a hemispherical fashion. Granted in an electrical room we have walls and contained equipment that can cause reflections, but those secondary effects might actually cancel to zero depending on your exact location in the room. They are just really powerful "sound" waves after all.

 

[GoldDigger]

The blast or pressure wave does not necessarily fall off inversely with the volume of the sphere.
To the extent that it is a wave rather than a static pressure the energy density will only fall off as the area of the spherical surface. And that is proportional to radius squared.
I suspect that the damage done by the near field blast will end up somewhere between the two exponents.

 

[gadfly56]

The blast or pressure wave does not necessarily fall off inversely with the volume of the sphere.
To the extent that it is a wave rather than a static pressure the energy density will only fall off as the area of the spherical surface. And that is proportional to radius squared.
I suspect that the damage done by the near field blast will end up somewhere between the two exponents.

The expanding gases, which are what does the work, fill the spherical volume which goes by the radius cubed. See this interesting presentation from FEMA.

 

[mbk.2k3]

ok this thread is all over the place, let me try to consolidate using an example, and perhaps the experts can correct me if i'm wrong.....

lets say i've performed an arc-flash study using my software tool, and the equipment in question is a medium voltage starter with start/stop buttons on the front and a handle assembly to rack the starter in and out.

the study has given me the incident energy and the flash protection boundary.

here is how I think it practically all works together:

1. Use NFPA70E table 130.7(c)(15)(A)(a) to determine whether or not PPE is required for your task.

e.g. reading a panel meter, requires no PPE. but operation of the contactor to start/stop the motor requires PPE if its a brand new installation, or if there is evidence of impending failure. in addition, racking the starter in or out also requires PPE, regardless of equipment condition.

2. The PPE required to rack the starter in/out would be determined by the arc flash study results. So if the incident energy at the starter is 22.5 cal/cm^2, I'm using Category 3 PPE. This is based on the standard tables available everywhere. This PPE is required regardless of whether the covers are open or closed (right?).

3. The arc-flash study also tells me the flash protection boundary (where the incident energy is limited to 1.2 cal/cm^2). In my case this is 366.6" or 30.55'. Does this mean that anyone within 30' of the starter needs to wear Category 3 PPE when the starter is being racked in/out, regardless of whether the covers are open or closed?

thoughts?

 

[GoldDigger]

My first thought is Yes.
That is my second thought too.
Rather than having them don PPE that they might not even have, I would simply have them clear the area during the operation.

P.S. in follow up to the previous post, I will note that although the blast pressure (assuming no wave nature) falls off as the cube of the distance, the arc flash energy, propagating outward rather than filling space, will fall as the distance squared.
Giving us the 1.2 at 30', yes? That means that the distance for 22.5 must be pretty large (on the order of 6')

 

[MD84]

The thread isn't all over the place. The OPs question has been answered and the conversation has continued in further detail.

The flash protection boundary designates a boundary in which second degree burns could be received. It is a boundary which should only be crossed by a qualified person wearing PPE. This person should also be listed on the JSA and energized electrical work permit if applicable. This does not mean the person needs to be wearing HRC 3 PPE. This person would need to wear PPE based on the incident energy available at the location they will be working.

An example of this could be another worker providing a support role such as witnessing equipment operation. The incident energy at the witness location can be calculated using the inverse square law based on the AFHA.

This is my interpretation. Ultimately a companies standard operating procedures will dictate work practices.

 

[gadfly56]

ok this thread is all over the place, let me try to consolidate using an example, and perhaps the experts can correct me if i'm wrong.....

lets say i've performed an arc-flash study using my software tool, and the equipment in question is a medium voltage starter with start/stop buttons on the front and a handle assembly to rack the starter in and out.

the study has given me the incident energy and the flash protection boundary.

here is how I think it practically all works together:

1. Use NFPA70E table 130.7(c)(15)(A)(a) to determine whether or not PPE is required for your task.

e.g. reading a panel meter, requires no PPE. but operation of the contactor to start/stop the motor requires PPE if its a brand new installation, or if there is evidence of impending failure. in addition, racking the starter in or out also requires PPE, regardless of equipment condition.

2. The PPE required to rack the starter in/out would be determined by the arc flash study results. So if the incident energy at the starter is 22.5 cal/cm^2, I'm using Category 3 PPE. This is based on the standard tables available everywhere. This PPE is required regardless of whether the covers are open or closed (right?).

3. The arc-flash study also tells me the flash protection boundary (where the incident energy is limited to 1.2 cal/cm^2). In my case this is 366.6" or 30.55'. Does this mean that anyone within 30' of the starter needs to wear Category 3 PPE when the starter is being racked in/out, regardless of whether the covers are open or closed?

thoughts?

First, you're right, as one of the offenders, my apologies.

Second, I'm not up on 70E, but I took a quick look, and the Arc Flash Boundary is the distance at which a person is likely to receive a second degree burn (D.7.5). If the arc takes place within the enclosure, how do you get exposed to the flash? You don't. There's also an equation for calculating the flash in a box 20 inches on a side with one face open. These suggest to me that the calculations generally assume covers off. In your case it would probably be the arc-in-a-box unless the energized parts are fully exposed on rack-out.

Are you sure that your boundary calculations aren't in millimeters rather than inches? Thirty feet seems excessive.

 

[jim dungar]

Thirty feet seems excessive.

This is about right, as the equipment is a MV starter. But the incident energy will fall off, so it would be possible to calculate the distance where '8cal' PPE is sufficient.

 

[GoldDigger]

My first thought is Yes.
That is my second thought too.
Rather than having them don PPE that they might not even have, I would simply have them clear the area during the operation.

P.S. in follow up to the previous post, I will note that although the blast pressure (assuming no wave nature) falls off as the cube of the distance, the arc flash energy, propagating outward rather than filling space, will fall as the distance squared.
Giving us the 1.2 at 30', yes? That means that the distance for 22.5 must be pretty large (on the order of 6')

Too late to edit.
My "Yes" was to PPE of an appropriate type for the distance, not for Category 3 PPE at any distance.

 

[MD84]

Lol millimeters.

30' is not excessive.

Arcs do not always stay contained in boxes.

 

[zog]

So let me ask a potential "dumb" follow up question. If I'm in front of a piece of switchgear and operating a breaker or switch with all doors closed, does that affect the level of incident energy compared to if the enclosure was open? I know the calculations don't take into consideration switchgear construction (i.e. metal enclosed vs. metal clad) but common sense would say there is less incident energy if the arc is behind a close cabinet. Thoughts?

Unless it is the newer arc rated switchgear design the gear is not designed, and normally will not, contain the arc flash. In fact, in some cases it can be worse when you consider the pressure build up prior to failure. Consider a firecracker lit in the palm of your open hand compared to a firecracker inside your closed fist. Whats going to do more damage ?(No JPP jokes please)

 

[zog]

It is too bad we cannot calculate arc blast, I believe there are many instances where doors are blown open, and people knocked down, even when the fault is <8cal/cm^2 of incident energy. Likewise, I am sure there are many 'Dangerous' locations, like transformers secondaries, that have relatively passive arcing events that happen to last for a long time.

There is some guidance on this, Ralph Lee's "other paper" was the basis for arc blast calculations. IEEE 1584 has been doing more research over the last few years on pressure wave calculations based on this paper and lab tests (I have actually provided switchgear to them for this research and have videos, not allowed to share them though).

 

[gadfly56]

My first thought is Yes.
That is my second thought too.
Rather than having them don PPE that they might not even have, I would simply have them clear the area during the operation.

P.S. in follow up to the previous post, I will note that although the blast pressure (assuming no wave nature) falls off as the cube of the distance, the arc flash energy, propagating outward rather than filling space, will fall as the distance squared.
Giving us the 1.2 at 30', yes? That means that the distance for 22.5 must be pretty large (on the order of 6')

Consider further, if your assumptions are correct, what that 22.5 is at the working distance, which is taken as 910mm (table D.7.3). mbk gives the incident energy in his example at the starter as 22.5, not 6 feet away. Hence, my confusion.

 

[ATSman]

Arc Flash Video

Arc Flash Video

Great Discussion.
For those that have not seen this old classic video of what could happen
when racking in a breaker:

https://www.dropbox.com/sh/qe9brl0axzz4moe/AADPJnWdMDixafA_auLsqkK7a?dl=0

Click on or copy and paste to browser then in Dropbox click on the file Arc Flash.wmv to view.

 

[MD84]

Good example of work that should be performed de-energized or remotely.

Racking a breaker should be done on a dead bus if performed locally. Remote racking is acceptable if proper procedure is followed.

Operating a high energy breaker should also be performed remotely. Many times it is not possible to close in a breaker on dead bus due to under-voltage protection schemes. I suppose under-voltage protection could be bypassed but it is not practical. Remote operation is preferable.

 

[adamscb]

The way I understand the code is that if an enclosure is rated as "arc flash dangerous", that means you are not allowed to work inside the space while energized. You're good to go if you de-energize the enclosure. Having an enclosure rated as "arc flash dangerous" is an NEC violation, and should be corrected ASAP.

 

[mayanees]

Having an enclosure rated as "arc flash dangerous" is an NEC violation, and should be corrected ASAP.

That would be incorrect.

An arc-flash DANGEROUS rating on a panel is not an NEC violation and does not need to be corrected.

I see some Power Studies where the engineer will make a list of PPE 3, 4 and Dangerous-level ratings at panels, and recommend that the facility budget to reduce the incident energy levels, and that's just not money well spent. There may be a case where it's desirable to reduce the level while interacting with a particular panel, but it's not wise to spend money to reduce incident energy levels throughout a facility.