It is currently Sun Oct 22, 2017 3:11 am



Post new topic Reply to topic
Author Message
ekstra   ara
 Post subject: Help with calculation
PostPosted: Fri Jan 02, 2009 6:24 pm 
Offline

Joined: Sun Dec 07, 2008 6:17 pm
Posts: 14
:confused: I have been trying to figure out if I am doing this right before I continue on. I pick one machine that is hook up to a 480v bus that is feed by a Pow-R-Way switchboard, It is feed by 2500kva transformer. There is no fuses or CB between the transformer and switchboard.


You do not have the required permissions to view the files attached to this post.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Jan 02, 2009 7:08 pm 
Offline
Sparks Level

Joined: Sun Dec 23, 2007 1:44 pm
Posts: 348
Location: Charlotte, NC
Help

I think that before anyone here can help, you must supply more info and actually ask a question.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Fri Jan 02, 2009 8:11 pm 
Offline
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
also give impedance of transformer, cable type and lengths, and if the panel is MLO or MCB.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Sat Jan 03, 2009 2:34 pm 
Offline

Joined: Sun Dec 07, 2008 6:17 pm
Posts: 14
Sorry my zip attachment was missing. There is a line drawing in a PDF format plus a word doc that has my questions.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Jan 05, 2009 1:26 pm 
Offline
Sparks Level

Joined: Thu Jul 19, 2007 7:54 am
Posts: 201
Location: St. Louis, MO
I haven't looked much into the rest of your calculations, but I question the 0.1 sec clearing time on bus one. If this is based on the fuse upstream of the main transformer, then the clearing time is likely to be much longer.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Jan 05, 2009 4:56 pm 
Offline

Joined: Sun Dec 07, 2008 6:17 pm
Posts: 14
Yes you are right the clearing time should be higher. I still cannot get any information from the utility company. So I just used 0.1 sec to start with.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Jan 05, 2009 9:22 pm 
Offline
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
Where are you getting your PPE Levels.
At the main bus you show an IE of 5.6 cals and a Class PPE of 1.
If Class is the same as Level, then it should be Level 2, (4-8 cals).
For IE under 4 cals you are showing Class 0. These should be Level 1.

Are you trying to indicate the NFPA PPE Level or something else.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jan 06, 2009 4:42 am 
Offline

Joined: Sun Dec 07, 2008 6:17 pm
Posts: 14
Did they remove the hazard/Risk category 0 in NFPA 70E 2009. I was try to find any information on this change in category.


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jan 06, 2009 5:26 am 
Offline
Arc Level
User avatar

Joined: Wed May 07, 2008 5:00 pm
Posts: 676
Location: Rutland, VT
Category 0 is still in Table 130.7(C)(11) in NFPA 70E-2009 and this would be applicable up to 1.2 cal/cm2. Above 1.2 cal/cm2, FR clothing is required as per Article 130.7(C)(5).

IMHO, I think Table 130.7(C)(11) should have 5 J/cm2 (1.2 cal/cm2) in the Required Minimum Arc Rating of PPE column, not N/A for Category 0. This would make the table consistent with the text in Article 130.7(C)(5).

_________________
Barry Donovan, P.E.
www.workplacesafetysolutions.com


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Jan 06, 2009 5:16 pm 
Offline

Joined: Sun Dec 07, 2008 6:17 pm
Posts: 14
I change the Category levels, I must have miss read or type them in wrong


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Mon Feb 09, 2009 11:34 am 
Offline

Joined: Thu Feb 05, 2009 8:06 pm
Posts: 1
Calculating Bolted Fault Current

I need some guidance on calculating BOLTED FAULT CURRENT.
I cannot find out how to get access to IEEE-1584 which I would assume would answer most or all of my questions. I would appreciate advice from anyone wiser than I.
Here is what I have assumed (please correct me where I am wrong).

I = E/R.
I = Bolted Fault Current
E = 460vac (in my case) but I have questions below.
R = ‘the big mystery’

I assume that R = the aggregate impedance (@60hz) of the following:

1. Secondary of my transformer.
2. Wiring impedance all the way to the flash point (as found by using info from my ‘Ugly’s’ book pg 66.)
3. Safety factors & K factors which I hope a wise person can inform me of.

Questions:
1. When figuring wiring impedance, do I use ‘distance to flash point’ + ‘distance back from flash point’ at 460vac OR ‘distance to flash point’ only @ 277vac OR some other method?
2. What are ‘standard safety factors’ and K factors commonly accepted (if any)?

A phone call would be welcomed as well.
417-208-1352
Tyler Bliss

Thanks


Top
 Profile Send private message  
Reply with quote  
 Post subject:
PostPosted: Tue Feb 10, 2009 6:12 pm 
Offline
Arc Level

Joined: Thu Jan 10, 2008 8:49 pm
Posts: 480
Location: New England
Calculating fault currents isn't difficult but it takes some industry understanding of what the numbers all mean.

Go to your transformer nameplate and find out the Impedance %, it should say something like 5.75%.

Contact your utility and ask for Fault MVA and X/R ration to your transformer.

Identify the type of cable from the transformer secondary to the point of the potential flash location under review. If you are going through multiple gear identify type and distance for each run, and type of raceway. You just need to list for one phase, the other phases are identical. So for example, from transformer to switchboard, 4 parallel 500 MCM copper in EMT x 80 ft. From switchboard to lighting panel (point of flash) 1 each 4/0 Copper in tray cable x 300 feet.

Estimate the total KVA motor load and identifier any synchronous motor or generators.

I take it you don't have software, and you don't need it for small sites. Most of us would probably do this with the 'per unit' method of calculation. But it sounds as if you might need something easier to start with. For arc flash calcs, which only looks at 3 phase bolted fault, I like the MVA method for calculating faults. It follows the V=IR type of equations you seem to prefer.

Go to arcadvisor.com and investigate the method. If you don't find it, contact the owner and ask him for the white paper on the MVA method. If you have a limited number of points, spend the $50 and sign up with him and use his on line software. But let me warn you its a bit quirky until you understand how to use it.

In the back of your NEC book find the table for Conductor Properties, look for the Z impedance values for you cables and type of raceway.

Report back with more info.


Top
 Profile Send private message  
Reply with quote  
Display posts from previous:  Sort by  
Post new topic Reply to topic  [ 12 posts ] 

All times are UTC - 7 hours


Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
cron
© 2017 Arcflash Forum / Brainfiller, Inc. | P.O. Box 12024 | Scottsdale, AZ 85267 USA | 800-874-8883