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 Post subject: Non Specific incident energy
PostPosted: Fri Jul 15, 2016 1:31 pm 

Joined: Wed May 06, 2009 12:12 pm
Posts: 26
Location: Tennessee
I have started working at a manufacturing plant where the past practice is to put <1.2 cal on their arc flash labels. The thought being that any run less than several hundred feet fed from bus duct with LPS-RK1 fuses will be less than 1.2 cals. This turns out to be true based on my calculations. Even with the maximum motor contributions the calculations hold.

Does anyone consider this to be in compliance with 70E? Does anyone else do this?


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 Post subject: Re: Non Specific incident energy
PostPosted: Sat Jul 16, 2016 6:06 am 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
BISAM wrote:
I have started working at a manufacturing plant where the past practice is to put <1.2 cal on their arc flash labels. The thought being that any run less than several hundred feet fed from bus duct with LPS-RK1 fuses will be less than 1.2 cals. This turns out to be true based on my calculations. Even with the maximum motor contributions the calculations hold.

Does anyone consider this to be in compliance with 70E? Does anyone else do this?


Yes. First off you need to be in compliance at least in the U.S. with NEC (NFPA 70). NFPA 70E is totally optional, and NEC defines when you must label. 70E is only optional in the sense by the way that OSHA does not have any specific requirements for utilization equipment (they do for distribution equipment, see 30 CFR 1910.269). Instead arc flash falls under the general duty clause and using 70E would be an affirmative defense. An alternative for some operations (depends on the scope of the documents) would be IEEE C2 (NESC) for instance where NFPA 70E would not "apply".

That being the case, NEC requires a label but does not specify content. The labels could just as well read "Warning! Arc flash hazard may exist." This warning label of course gives no guidance at all yet complies fully with NEC.

Furthermore, an arc flash hazard can exist pretty much anywhere that a spark hazard exists. In OSHA's records an IT worker received a serious burn on the person's hands from handling a defective cord that plugs into a laptop. It was one of those typical IEC-style plugs that every laptop or laptop power supply uses. The incident energy clearly was much less than 1.2 cal/cm2. 70E clearly does not intend to protect against ALL arc flash hazards. Yet this example demonstrates that one exists even for 120 VAC circuits protected by a 15 A circuit breaker or fuse, even if the transformer is very small. Doing NOTHING would be fully in compliance with 70E because the standard is based on ensuring that a fatality is unlikely. It does not prevent hospitalization or even a fatality.

Furthermore, specifically in the case of 120 VAC equipment, NO equation currently published provides accurate calculations predicting the incident energy from 120 VAC circuits. 70E and NESC both provide a table based approach and IEEE 1584 provides a statement for SOME circuits that are 208 VAC or less that indicates when some of these cases are under 1.2 cal/cm2 or under 4 cal/cm2. Since this is not a calculation it would be correct to label it as "<=1.2 cal/cm2" or "<4 cal/cm2" since the only thing you have is a range of incident energy, not a specific value. Technically you could also just label it "1.2 cal/cm2" or "4 cal/cm2" knowing that this is a "conservative" result (overestimated) as well.

Further the amount of equipment grows exponentially as you follow the system from the sources on down the "tree" to the loads. From a very practical point of view there are some "rules" that have been developed to somewhat simplify this process. These are boundary-energy methods. The use of these methods determines that incident energy is below a threshold (1.2 cal/cm2 for some plants, 4 or 8 for others). Knowing this allows you to quickly do the field assessment and to outright ignore all the calculations.

Further and along the same line labels have a finite life. In an indoor setting in good conditions they might last at least until either the next major plant modification or 5 years, whichever comes first. Outdoors even with a good thermal printer and a good UV overlay, they might last 2-3 years. If there are corrosive conditions or abrasion from sand, 1 year is pushing it. And if you are not inspecting ALL equipment when it first gets labelled (following either heuristic or more engineered boundary energy rules), and even if a detailed study was done, adding new labels or replacing old ones will be a task that is not daily but will happen on an infrequent basis. It has been my personal experience that all printers, even thermal ones, are a total pain. It takes me probably 3 or 4 minutes to send a label to a printer (including logging in, running the software program, locating the correct location, etc.) but it will take me 10-20 minutes and sometimes longer to get the printer working again if it has not been in use for a while. This is for EVERY printer except for possibly one that it directly attached to the PC (no network issues) in which case it might only take 10 minutes or more. And this is when I'm in the office. In a 24/7/365 operation, a label might be needed when I'm not there. I could probably train a foreman or planner for equipment that is in the model but you get the picture. An alternative is to take your clothing system (say 1.2, 10, and 40 cal/cm2), a set of voltage levels, and equipment types to generate a list of perhaps 30 or 40 labels. The "real" incident energy might be say 3.43 cal/cm2 but it gets rounded up to 10 cal/cm2 and the arc flash boundary also gets rounded up. So only a handful of labels are required instead of hundreds. This eliminates all the label handling issues except for stocking which can be set up with a local print shop.

For all these reasons it is not only common but I would be very surprised to see anyone with a plant that doesn't have "generic" labels like the ones you described. 70E says only that the label must be labelled with either a PPE level (if the table method is followed) OR an incident energy. It doesn't get into how the incident energy is calculated because 70E itself does not specify how to do this. In fact it avoids the question altogether. Depending on the revision there are at least 8 different methods documented in the Annex and there are many more calculations out there than are given in Annex D, some of which probably should be included. Among those that are not included but should be, the tables in NESC were removed in the most recent version but contain equipment-specific recommendations lacking in IEEE 1584 for cases such as <250 VAC, Doan's DC equation is given but Ammerman's more accurate DC equation is referenced but not given, Wilkins simplified and time domain-based equations are more accurate than IEEE !584, ArcPro is strongly suggested by OSHA in 30 CFR 1910.269 for equipment above 15 kV, EPRI has published calculations for arcs longer than 6 inches, and energy boundary methods can frequently result in a safe estimate that requires no calculation at all or minimal calculation effort.


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