We found a few locations at our place that are above 40 calories. It sounds like 70E prohibits live work at these areas. What do we do now?

I have yet to find a difinitive statement in the 70E or OSHA prohibiting work above 40 cal/cm2. If you have found it let me know.

Certainly it is ill advised to work above 40 cal, due to the arc blast, but I can find no source for a total restriction.

I recall it is in the 70E handbook but it is an explanation and not part of the actual standard. It is on page 81 near the top and goes

.... "The 70E Technical Committee determined that although it might be possible to protect a person from such extreme thermal exposure, the clothing would be unlikely to protect the worker from the effects of the accompanying pressure wave. If the arc flash analysis indicates an exposure of more than 40 calories per square inch, the task must not be performed until an electrically safe work condition exists. FR clothing with a very high incident energy rating might be needed to perform the steps necessary to establish and electrically safe work condition. However this is the only task that should be accomplished with the equipment energized."

Most people I know adhere to this explanation just in case something goes wrong. It would be difficult to explain to a jury why you did something that was in writing as not a good idea even if it is technically just an explanation and not part of the official 70E standard.

I know it's nit-picking, but I've seen several articles and heard many people say that you CANNOT work if the energy is >40 calories. That is simply not true as far as I can find.

We don't allow work if the energy is >40 cal where I am, and I agree with the decision from a safety / liability standpoint, but it simply is not in the standards.


Big Boom, the answer to your question (if you choose to embrace the 40 cal restriction) is to stop all live work where the analysis shows >40 cal, and start looking at things you can do to lower the energy. The simplest is relay changes. Lower the settings on the protective relays (change breakers / fuses) upstream, if possible, to reduce the time / energy.
Add differentials.
Increase the working distance, use hot sticks, etc.
There are also several products that will trip the breakers faster in the event of a fault. Time and Distance are the easiest factors to control.

Work Above 40 Calories / Square Centimeter

It is probably a long way off, but some of this confusion will go away with the next major revision to IEEE 1584. I see part of the problem as the way incident energy is presently calculated. 40 Cal with lower energy per time that has a longer clearing time is not going to have the same blast as 40 Cal with higher energy per time and a short clearing time. i.e. 40 Cal accumulating over 2 seconds would not produce the same blast pressure as 40 Cal over 10 or 20 cycles. The "blast issue" will hopefully be resolved sometime and then we will know what incident energy level is truly dangerous from the blast perspective rather than having 40 Cal getting thrown around.

I agree with K. Jackson and WDeanN - I don't recall the "40 Cal Limit" actually being part of a standard other than the explanation and everyone seems to refer to it - due to liabilty in many cases.

Above 40 cal

One of the previous posts refers to the verbiage within the 70E that states you should achieve an electrically safe work condition if the calculated incident energy is above 40 cal/cm2. If your program is touting itself as "70E Compliant" then that should be the rule you follow. Sure, there are 100 calorie suits out there, and they will probably do the job, but ifyou're going to say you are complying, you should comply. What we hear is that there will be some resolution in the 2008 Revision, but we have to wait I guess.

That's just the problem, Dave. The language you referred to is not in the 70E. It is a recommendation in the handbook, but is not part of the official language of the 70E. People keep referring to this as a mandate, and it is not. So I can work on something above 40 cal/cm2 energized and still be compliant, if I wear appropriate PPE.

Over 40 cal/cm2

I found this in a PP presentation but can not verify the sources but it makes sense . Looks like re-engineer or make it dead .

Work Above 40 cal/cm2
Label equipment warning that no PPE is available
Refer to safety procedures
Must use engineered solution
Current limiting fuses
Arc Resistant switchgear
Remote racking
Remote operation through Smart Equipment
Zone Interlocking
IP20 shrouding ( Ingress Protection )
Re-coordinating protective devices
Increasing distance
Enabling instantaneous function
De-energizing before operation

also found this

Practical methods in reducing the dangerous arc flash hazard areas in large industrial facilities
Hodder, M.; Vilcheck, W.; Croyle, F.; McCue, D.
Pulp and Paper Industry Technical Confe, 2005. Conference Record of 2005 Annual
Volume , Issue , 23-23 June 2005 Page(s):191 - 198
Digital Object Identifier 10.1109/PAPCON.2005.1502065
Summary:Industrial and commercial facilities have recognized that arc flash prevention is a part of a complete safety program. Quantification of the arc flash hazard level and labeling procedures are a major portion of this effort. When calculating incident energy, the engineer must deal with two main issues in addition to the burn hazard-blast pressure effect on the human body and worker comfort or mobility with multilayer flash suits and associated flame resistant protective equipment.[color="Red"]For work tasks where calculated incident energy levels are above 40 cal/cm2 it is desirable to reduce the exposure to the worker to reduce the burn injury. This can be achieved with either a change to the work method or by engineering design[/color]This paper will discuss various methods that have been used to reduce the incident energy levels from above 40 cal/cm2 to levels below 40 cal/cm 2. Specific solutions implemented at a large chemical manufacturing facility are presented. The solutions include equipment upgrades, overcurrent protection modifications, changes to work methods, and worker training which increase the effectiveness of an already robust safety program. In addition, design changes that could be considered to keep incident energy below 40 cal/cm2 for expansions or additions to the power system are discussed

Well put (Or pasted) capt Jim, all of this nonsense of is it allowed or is it not sickens me. How many of those plants that allow work >40cal have ever been in a burn center, ever debreeded someone they love 3 times a day for 3 months?

Shut it down or engineer a way to reduce the hazard. Arc Flash mitigation is what I do and it is easier than everyone thinks, just a matter of the right solution for the problem, everone looks to a bigger flash suit or a commercially available product, when some simple engineering and a few modifications can solve the problem.

Everyone needs to remmember, arc flash safety is in its infancy, it took the US industry 29 years to start paying attention to the 70E, give the market a few years to find some solutions, things I have designed are alerady showing up more and more in planst across the US and there are smarter people than I working on solutions also, give it time, until then, turn it off if >40cal!

Easy decision... Don't! My position is always engineer from the "Witness stand back". Put yourself into the picture. On the stand with a plaintiff's lawyer crossing you on your decision to engage her dead husband in hot work without properly or unknowingly correctly assessing the risk factors. At the end of the day someone will be held accountable. This is the legal community's stock in trade.

Good point, a utility did a study on the average cost of a serious survivable arc flash accident and came up with $17.4M, mostly medical and legal costs.

Others have pointed out that there is a ton of companies offering 70E training or consultanting (many of which have no idea what they are doing) dont think the lawyers are not jumping on the bandwagon too.

I have attached a simple drawing of a common switchgear at our plant. I did this after reviewing the arc flash study that was done. (this is a BBC S/G) It shows the PPE categories and, according to the study, where these categories change. There are a few questions that arise when trying to decide how to label the panels. On either end, where the power comes in, the bus is a "category 1" up to the fuses and after the fuses it changes to "dangerous" until the main breakers. On the ends there are two panels that can be removed, top and bottom, there is a fiberglass(?) partition between where the fuses are and where the switch is. This is not a solid partition, it has gaps around the edges to allow for cables. Is this partition adequate for opening the top panel with just Cat.1 PPE? ( I don't think it is...) . I've said this before - I want the guys working on this stuff to be safe, I don't know for what reason they might need to open these panels while energized but just in case I want it to be labeled correctly.... this configuration is the same throughout the plant, whether it's a GE or BBC S/G, does this configuration even seem correct? My boss doesn't understand why it's Cat.1 and then goes to dangerous...? We are going to look at another plants Arc flash study to compare, but wanted to get you guys thoughts, they're usually pretty good.

Your results are pretty typical for Industrial plants. The CAT 1 is probally right, lower available fault currents and INST trips on the 50/51 relays on your MV breakers feeding these switches.

Your CAT III areas are a simple fix, we are doing a lot of these lately, retofit you Mains and Tie with Quick trip or similar. We have several breakers in our shop undergoig this upgrade right now, we do alot of ABB/BBC breakers here because we used to be ABB and stock all of the breaker parts.

Your "Dangerous" areas (Actually is just the part of your redline on the seconday side of the transformer I am guessing) is a harded fix. We did a bunch of these in the spring, your replace your fuses in your MV switch with a mini VCB and sense current on the seconday side of the transformer with anything from a simple 50/51 relay to a arc flash relay (Light sensing). But then again, why would you need to go in this area to do anything? Good switching procedures and SWP's can eliminate this area of concern.

thanks a lot Zog,

this all goes back to labeling the panels correctly and finding if and where we would put viewing panes for IR scans.

No problem. Just noticed you are a nuke sub guy, Me too, A1W, SSN-701 and MARF staff. EMC(SS)

SSN-673 Sturgeon class USS Flying Fish, fast attack out of Norfolk, VA. Very interesting experience! So that's how you got into your career, huh?

Best thing I ever did, and couldnt wait to get out. But yep, I stayed out of nuclear for awhile and did training and NETA testing on power systems, thanks to GW nukes are making a comeback so I got back into that industry.

Been asked many times what I think about Nuclear power, I really think it can be good if we manage it properly and the more we work with the waste teh more likely it'll be that we'll find something to do to it or with it.

I worked in an oil refinery up until last year, now am finishing my EE degree, and I thought I couldn't wait to get out of the Navy, sheesh!

Oh, so do you think that the partition in the S/G would allow labeling the top panel as Cat. 1? (it already has been but I am tasked with figuring it all out again)

Not unless it is arc rated gear.

It certainly does not look like it's arc rated, it's about 1/16 - 1/8 inch thick and looks like fiberglass (as seen through the window). Does that sound like anything you've seen before?