Arc Flash Inconsistency Question
Arc Flash Inconsistency Question
(OP)
Here is my system data:
V = 12.47kV
Isc = 8.338kA
Ia = 8.12kA
D = 914mm (36")
t = 1.1 seconds
G = 152mm (~6")
Using the IEEE 1584 equation in 70E I get an incident energy of 11.3 cal/sq cm
Using the theoretical equation [D.8.4] in 70E
I get an incident energy of 70 cal/sq cm
How can they be so different? I ask because my Gap is not 6", it is 8" and 70E states that the 1584 equations cannot be used with gaps larger than 152mm. So I must use the theoretical equations and state that the arc flash hazard is much much greater than it seems it should be. Using the 1584 equation you can see that I.E. vs. Gap is a linear plot. Is it a terrible assumption to think that it is also linear past 6 inches? I know I can't use the 1584 equations because they haven't been tested with larger gaps, but I have a hard time labeling the hazard as 70 cal/sq cm when it seems that the theoretical equations are grossly conservative. I am going to get the clearing time down to a little over 5 cylces, which according to 1584 equation is about .8 cal/sq cm. The theoretical equations gives over 5 cal/sq cm. Is the descrepancy between the two methods really this big or am I making a mistake somewhere?
Thanks,
Travis
V = 12.47kV
Isc = 8.338kA
Ia = 8.12kA
D = 914mm (36")
t = 1.1 seconds
G = 152mm (~6")
Using the IEEE 1584 equation in 70E I get an incident energy of 11.3 cal/sq cm
Using the theoretical equation [D.8.4] in 70E
I get an incident energy of 70 cal/sq cm
How can they be so different? I ask because my Gap is not 6", it is 8" and 70E states that the 1584 equations cannot be used with gaps larger than 152mm. So I must use the theoretical equations and state that the arc flash hazard is much much greater than it seems it should be. Using the 1584 equation you can see that I.E. vs. Gap is a linear plot. Is it a terrible assumption to think that it is also linear past 6 inches? I know I can't use the 1584 equations because they haven't been tested with larger gaps, but I have a hard time labeling the hazard as 70 cal/sq cm when it seems that the theoretical equations are grossly conservative. I am going to get the clearing time down to a little over 5 cylces, which according to 1584 equation is about .8 cal/sq cm. The theoretical equations gives over 5 cal/sq cm. Is the descrepancy between the two methods really this big or am I making a mistake somewhere?
Thanks,
Travis






RE: Arc Flash Inconsistency Question
RE: Arc Flash Inconsistency Question
RE: Arc Flash Inconsistency Question
http://www.epc-website.com/
Your numbers are correct - whether either value makes any sense is another question.
RE: Arc Flash Inconsistency Question
On Easypower, when doing arc flash calculations, which do you use? 1584 or 70E? and why?
RE: Arc Flash Inconsistency Question
And I put "based on IEEE 1584-2002" on all the labels as well.
I do think it's more accurate at 480V. It's probably somewhat low for medium-voltage, but the Lee equations are probably way too high.
RE: Arc Flash Inconsistency Question
RE: Arc Flash Inconsistency Question
RE: Arc Flash Inconsistency Question
Thanks Again
Travis
RE: Arc Flash Inconsistency Question
Arc resistance is a big part of these calculations and impedes quick clearing times one would normally expect of a fault in these situations (that is another gripe I have with the forumula as it exists).
But everything is emperical, and not much linear extrapolation is possible.
I would use IEEE 1584, and play with the default spark gaps length, but also see what is happening to the arc current as the gap is increased.
RE: Arc Flash Inconsistency Question
RE: Arc Flash Inconsistency Question
As long as you have a reasonable basis for your PPE requirements and are consistent in application, I don't think OSHA will be a concern.
Of course plaintiff's lawyers are another matter...
RE: Arc Flash Inconsistency Question
Travis
RE: Arc Flash Inconsistency Question
RE: Arc Flash Inconsistency Question
RE: Arc Flash Inconsistency Question