Arc Flash over 40 cal/cm2

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sarnal

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I have some equipment that is listed as "dangerous" and over 40 cal/cm2. My understanding is that NFPA 70E does not explicitly prohibit work at locations with energy levels above 40 cal/cm2. Plan to lock out but still need to verify the absence of voltage. What PPE is required for LDL testing?
 

mayanees

Senior Member
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Westminster, MD
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Electrical Engineer and Master Electrician
I have some equipment that is listed as "dangerous" and over 40 cal/cm2. My understanding is that NFPA 70E does not explicitly prohibit work at locations with energy levels above 40 cal/cm2. Plan to lock out but still need to verify the absence of voltage. What PPE is required for LDL testing?
There's no prohibition for arc flash levels higher than 40 cals, just suit up to the calculated incident energy level. If there's no arc flash label then you have to do the 70E categories method, and if it meets the criteria (max fault current and ocpd opening time) then a PPE category can be determined up to category 4, which is 40 calories. The RED DANGER labels were fallout form older editions of 70E where an informational note (IN) stated additional care must be taken when above 40 cals, and the software makers did templates for labels in RED DANGER. The IN was removed. If you have to do a calculation get all the necessary info and I'll do the calc for you. Hopefully it fits in the 70E category method 130.7(C)(15). Or if they listed the incident energy level on the label you're good to go.
 

paulengr

Senior Member
There's no prohibition for arc flash levels higher than 40 cals, just suit up to the calculated incident energy level. If there's no arc flash label then you have to do the 70E categories method, and if it meets the criteria (max fault current and ocpd opening time) then a PPE category can be determined up to category 4, which is 40 calories. The RED DANGER labels were fallout form older editions of 70E where an informational note (IN) stated additional care must be taken when above 40 cals, and the software makers did templates for labels in RED DANGER. The IN was removed. If you have to do a calculation get all the necessary info and I'll do the calc for you. Hopefully it fits in the 70E category method 130.7(C)(15). Or if they listed the incident energy level on the label you're good to go.

Did not answer the question.

The answer is two fold. First since you did the calculation method you are required to do the second half, the risk assessment. In the table based method first you look at the task then if based on the table there is a risk you determine PPE based on the hazard. This is fundamental here. Read the definition of arc flash hazard.

Also I highly suggest looking at the annexes of OSHA 1910.269. That section is for power distribution but their arc flash procedures are much more clear than 70E. 70E tells you to do a risk assessment but doesn’t tell you how. It talks about evaluating equipment condition but doesn’t say how. OSHA does spell those out clearly in simple language.

If you are not doing something that could cause an arc, there is a possibility that the equipment can spontaneously explode but there is also a possibility that you can be struck and killed by a meteor. In 70E terms we say there is no arc flash hazard if it isn’t likely. Just walking by for instance is not a hazard and the 70E committee has used this example many times.

What we are looking for is comparable risk...that what you are doing is no worse than your average day doing any other task in an industrial plant.

So first look at equipment condition. Does it have fluid leaking out of it? Burn marks? Rusted out doors? Anything else that triggers your “spidey sense” suggesting you may be about to have a bad day? With these situations there are very few actions you can do which aren’t risky. It’s best to just treat this as arc flash hazard no matter what is being done since even normal tasks can go bad quickly, like trying to open or close a damaged breaker...often leads to arcing and lots of damage. This is abnormal operation.

Second what are you doing? Are you going to open the door and manually disconnect energized conductors? Test for live wires by shorting the bus together with your screwdriver wrapped in electrical tape “for safety” and see if a breaker trips? Is there some chance you could fall in or drop something into a live panel or did you remove the tip covers on your meter so you might accidentally bridge the conductors? Any action that is likely to cause an arc is one where we have to consider the hazard. But many activities don’t have this risk. Things like operating a disconnect, cutout, or breaker. Reading of working with a display panel or operating control buttons. Opening and closing doors if cables are properly secured so they can’t get pinched (10 feet of loose rolled up leads does not count). Using a multimeter with the probe covers under 300 V where the shock rule is no contact, or with gloves at higher voltages. Taking pictures with an IR camera, UV camera, ultrasound readings, or other no contact tools.

So forget about the 40 cal non-rule. At some point you simply do not have adequate PPE or there is no reason to take the risk. There is 150 cal arc flash suits but why not just be reasonable and stop with 10 cal shirts, pants, and a face shield and gloves?

In later informational notes 70E states that the incident energy can be lower or higher than what was calculated if the equipment isn’t operating normally. So with abnormal operation you don’t operate. Go to the next higher upstream equipment and operate it there to disconnect and isolate things. Just don’t mess with clearly malfunctioning equipment while energized.

Second case is where you are doing normal operation such as opening or closing disconnects...things where the “human factor” doesn’t apply. There is a chance something could happen but it is well below normal risks. This is where many people get off into the weeds by not understanding the definition of arc flash hazard in 70E. So no PPE required.

Third case is it’s working properly but for whatever reason you have to do something that could cause an arc such as racking in a breaker or landing a wire in a live panel or testing for voltage where you can cause an arc accidentally. This is where you must wear arc flash PPE to the required level. If you don’t have that level of PPE you can’t do the task. So substitute something else like going upstream or using some other method.

I keep talking about upstream because typically with properly designed equipment the wiring or bus on the secondary side of a transformer is only protected from shorts eventually by the primary side. Arc flash is really high until downstream of the breaker or fuse after the transformer. So it is also usually quite low upstream of the transformer. The worst situation is when you have an MDP tied directly to the secondary side. Since it’s all open the whole panel gets rated same as the transformer secondary bushings. Opening the breaker or disconnect on the primary side usually gets the job done with vastly lower PPE requirements.
 
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