Arc Flash Accuracy

[trchambe]

I'm debating the accuracy of arc flash hazard analysis. Several I have spoken to speak of incident energy and appropriate protective clothing as definitive values. "If you have a certain clothing rating, it WILL protect against an arc flash of a particular value." However, it seems that it is a continually evolving field of study, and is relatively new to the "mainstream" engineering community. With the amount of approximation and guesswork involved (I am specifically referring to conductor lenght approximations of a few hundred feet for a utility, several tens of feed for an industrial/commercial setting, and standard or default values of %Z of dependant upon transformer sizes), how accurate would you guess that the results truly are, and how confident would you be in recommending a hazard risk category or clothing rating?

I should also mention... I don't mean to suggest that these conductor lenghts, %Z, etc approximations are ballparked. Often times, the appropriate data is unavailable, or would take an unrealistic amount of time to obtain. They are based on what I understand to be generally accepted values... but how accurate are these generally accepted values?

 

[alehman]

Errors on data you can actually measure aren't the half of it. The equations used for these calculations are empirical approximations that continue to evolve. Another big unknown is frequently the fault current available from utility sources. There are also assumptions in to the calculations such as distance from the arc and arc length, enclosure type, etc. I would consider arc flash calculations as only an order-of-magnitude approximation. Doesn't mean you shouldn't do them.

Search this forum for other threads on this subject.

Alan
“The engineer's first problem in any design situation is to discover what the problem really is.” Unk.

 

[dpc]

I agree completely with Alan. Arc-flash calculations are very rough approximations at best. Even the arc rating of the clothing is not a guarantee of protection.

We do the best we can. Any protection is better than none and the calculations can show which areas are at higher risk than others.

David Castor

http://www.cvoes.com

 

[rbulsara]

Ditto.



Rafiq Bulsara

http://www.srengineersct.com

 

[stevenal]

Arc rated clothing will not protect you from injury, it is intended to keep injuries curable.

 

[rbulsara]

That too only "burn" injury. Curable burn injury is the second degree burn, the most painful.

Also clothing does not protect one from injuries from flying tools or shrapnel or the blast pressure, etc.

Rafiq Bulsara

http://www.srengineersct.com