No Arc Flash Analysis for <75HP Motors

[polareng]

I've read (at the Martin Technical website,

http://www.martinarcflash.com/labeling.htm)

that they do not consider Arc Flash Analysis of motors 75hp or less contribution. I cannot find any information other than their website to substantiate why or what standard they are following that would exclude smaller motor contribution. Is this published somewhere in NFPA 70E that I am missing or IEEE or somewhere else?

 

[tem1234]

In IEEE 1584, they say not to consider motor < 50 hp

 

[JMatt]

Usually motor circuit contribution (even for larger low-voltage motors) is extremely small and won't significantly increase your arc energy.

tem1234 is correct about IEEE 1584.

 

[polareng]

So it sounds like they applied a safety factor to IEEE's recommendation of 50hp limit and went with 75hp.

Thanks tem1234 for pointing out the standard.

 

[JMatt]

They didn't apply a "safety" factor to conservatively apply the standard. Their method is actually less stringent than the standard. If you have to model a system including motors 50HP and less, you'll consider more arcing fault current than if you raise the limit to 75HP.

Using those guidelines, Martin Technical's studies do NOT meet IEEE 1584 standards. It's probably not a big deal. After all, arc-flash hazard analysis is hardly an exact science right now, but you could ruffle their feathers if you wanted to.

 

[polareng]

Per IEEE 1584, "The study must take into account all sources, including utilities, standby and power generators,
and large motors—those 37 kW (50hp) and larger that contribute energy to short circuits."

My understanding is that Martin Technical is considering contributions of 75hp or "greater," not less than, but I think I understand what you are saying even though I don't think it was what I asked...or something like that. And, that a 75hp motor will contribute higher SCA than a 50hp motor.

 

[dpc]

So what are you asking?

David Castor

http://www.cvoes.com

 

[polareng]

I just wanted to know where this was all coming from.

 

[JimS08]

I saw the thread pop-up with our name in it (Martin Technical) regarding the arc flash analysis so I went to this forum to see what it was about.

At one point we ended up switching our practices from 50HP and higher to 75HP and higher based on some calculations that one of our top engineers was doing and showing that the 50HP was not enough to make any material difference. Shortly thereafter we found a flaw in the calculation method and have gone back to 50HP and we need to get this changed on our web site.

So, 50HP and above does need to be considered. Also, one question that has come up is that if you have a pool of motors, do you take the sum of the HP of the pool of motors or do you just look at each individual motor. Our top arc flash engineer (who has done more than 300 analysis and sat on the IEEE committee that developed the arc flash calculations) says that you do not look at the energy from the pool but ony from each individual motor.

I hope this was helpful and thank you for bringing an error to our attention.

 

[polareng]

That's great! Thanks for you response. That is good information.

 

[alehman]

I wonder if it would be desirable to run the calculation with and without motors and take the worst case.

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