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ekstra   ara
 Post subject: Calculation for new piece of machinery
PostPosted: Wed Feb 22, 2012 5:36 am 

Joined: Wed Jan 26, 2011 12:04 pm
Posts: 32
We had a comprehensive study done in 2010. We just added a new piece of 208V machinery, in parallel to many that were included in the study. The upstream 208V panel was calculated with an incident energy of 0.5 cal/cm2 at 18", and the new downstream equipment is installed at essentially the same distance (80') as the parallel equipment. The parallel equipment was all calculated to have incident energies less than 1.2 cal/cm2 at 18". Is it a straighforward exercise to assume that the new equipment can use the same label as its physically similar equipment? Thanks for any feedback.


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PostPosted: Wed Feb 22, 2012 8:08 am 
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Joined: Wed Jun 04, 2008 9:17 am
Posts: 428
Location: Spartanburg, South Carolina
It would be if the new equipment is served from the same panel, has the same breaker type & rating, the same wire size & type, the same conduit type, and no other changes have been made to significantly change the available fault at the source.


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PostPosted: Wed Feb 22, 2012 6:37 pm 
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Joined: Tue Oct 26, 2010 9:08 am
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Location: North Carolina
Almost! If there is a significant motor contribution, it may increase the incident energy of not only the new equipment but also the old equipment may need to be relabeled as well!

Based on the small description you gave this is probably unlikely. However it's a judgment call and you will have to judge for yourself. Usually after the first one or two that you add it should become obvious whether this happens or not. But consider my case of "simply" adding yet another 1500 HP, 4160 V motor wound with around 20 poles. Can you say lousy X/R? High subtransients and transients? Yes, that's exactly the problem. Simply adding another 1500 HP pump can and does raise the incident energy of not only the new pump but the old ones, too.

Granted again, there are a lot of variables. But watch out for this little "gotcha" to the idea that you can simply look at equipment in total isolation.


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PostPosted: Thu Apr 12, 2012 3:15 pm 

Joined: Wed Jan 26, 2011 12:04 pm
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Thanks for the response. Given what you said, what if the motor was, at most, 15 HP (208 3-ph)? It's an small parts washer. We'll have our outside electrical engineer evaluate it during our arc flash/coordination tune-up, but in the interim, if this small motor could take the hazard from its current HRC 0 (measured 80' at the distribution panel) to HRC 1 or more, we can at least warn our site personnel, who do nothing more than occasional troubleshooting, voltage-testing, and an occasional electrical lockout (as opposed to a machine lockout alone).

If this small motor shouldn't be of any concern, it will help us to know for the time being.


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