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 Post subject: Phase Gaps
PostPosted: Wed Dec 12, 2012 9:29 pm 
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Basic question perhaps, but I haven't found a definite answer yet ...

For arc flash studies on low voltage (415V in Aust), the measured gaps between buses and CB terminal gaps on the switchboard I am working on are different from the typcial gaps given in IEEE1584, sometimes significantly. For example, in a segregated bus zones, the buses are anywhere from 30mm, 70mm, 90mm, even 140mm apart in some areas.

Software allows me to put anything between the 13-152mm range stipulated in IEEE1584, but does that make the IEEE1584 calcs invalid? Must I stick with the 25mm/32mm typical values? The studies we have done so far show the larger gaps result in arcing currents below the protection short time settings, hence higher category ratings.

Reading I have done indicates there may be a question as to the sustainability of an arc at these higher gaps, but nothing definitive enough to depend on.


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PostPosted: Sat Dec 15, 2012 6:50 am 
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The default value of 25 mm is typical for 600 volt breakers, contactors, etc., without phase isolation. It would not work with isolation. Some solid dielectric gear is questionable if L-L faults are possible as with some outdoor utility gear.


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PostPosted: Sun Dec 16, 2012 2:33 pm 
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Thanks Paul, I'm a little confused as to what won't work. Are you saying that putting phase barriers between phases won't work with 25mm phase gaps as there isn't enough room? Or that a fault won't sustain?

In case there is any confusion, the 'segregated bus zones' are sections of the board housing the main buses segregated from other gear, the phases are not segregated from each other. Some CB's have phase barriers (ranging from well fitted to ineffective), some don't have any barriers.


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PostPosted: Mon Jan 07, 2013 3:05 pm 
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The gap distance form part of the calculation of the incident energy so it does affect your end results. Are you doing a manual calc or using PTW? If you are using PTW, you can unlink the gap column and key in custom gap distance yourself but be mindful this can be quite a lengthy exercise and I would only bother for buses which have higher than expect/desired level. The IEEE gaps are very conservative especially when most of our switchboards are custom built locally in Australia and not like the ABB etc type.


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PostPosted: Tue Jan 08, 2013 7:59 pm 
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What I meant was that some designs make the probability of particular failure modes highly unlikely or nonexistent. For instance metalclad gear if designed properly is unlikely to have a phase-phase fault. Resistance grounded systems severely limit phase-ground current. So sometimes to do a proper analysis you have to look at design. This will be an even more prevalent issue in the next edition of IEEE 1584.


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