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 Post subject: Pressure impact on the enclosure
PostPosted: Tue Nov 09, 2010 9:11 am 
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Joined: Fri Oct 01, 2010 10:12 am
Posts: 4
Hi ,

1. Is there any way to calculate the Pressure generated by Arc inside enclosure during Fault?
2. Is there any way to calculate How much Pressure is going out of the Pressure Flaps or Plenum?

I believe:

Total Pressure impact on the enclosure=Arc Pressure during Fault - Pressure escaping from pressure flaps/Plenum.

Any links or suggestions will be appreciated.

Regards
RAJ


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PostPosted: Tue Nov 09, 2010 9:43 am 
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Joined: Mon Jun 02, 2008 11:58 am
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Location: Charlotte, NC
Here you go


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PostPosted: Tue Nov 09, 2010 12:02 pm 
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Hi Zog,

Thanks for the Document. It gives the pressure developed by an ARC against the distance from Arc center for the particular rated current. It does not give information on how much pressure will be carried away using Pressure flaps or Plenum so that there will be a some amount of reduction in Impact pressure on the enclosure. Is there any thumb rule that how much % of pressure will go away from the vent. Of course it depends on the number of vents & its dimension.

Thanks & Best Regards
RAJ


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PostPosted: Tue Nov 09, 2010 1:26 pm 
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Sorry that is all I have, one of these other guys may have more info for you.


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PostPosted: Tue Nov 09, 2010 1:36 pm 
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re: arc pressure

I too am interested in this topic. We have considered building vented enclosures around our indoor transformers (2000kva) and using cable limiters on the secondary feeds...to reduce the arc flash rating of the substation room. I'm curious if this is a feasible solution - particularly concerning the blast pressure. Thanks for bringing this up.


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PostPosted: Mon Nov 22, 2010 7:24 am 
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Pressure Generation

As you might imagine, the pressure within a cabinet follows very complex rules! We have done some arc containment studies that involved real arc testing by deliberately shorting out the 3-phase busbars within the cabinet under test with flashing wire, then closing a high fault level supply onto the bars.
The internal pressures were monitored as the arc established.
You are right, the pressure rises and causes the arc flaps to open, then there is a flow balance where gas and vapour expanded by the arc energy causes the pressure to rise, but the flow out of the arc flaps keeps that in check.
We engagaged a consultant, from Theta Engineering in Germany, to analyse the whole event. They offer a service of predicting the arc flash scenario with models they have created.
The tests we did showed areas where we needed to increase the strength of our cubicles to make sure the arc was contained.
The calculations are further complicated by the fact that the amount of free air in the cabinet can make a significant difference to the over-pressure event. The amount of free air determines how much "spring" there is in the air as it becomes compressed. The more air, the longer it takes for the pressure to rise and burst the arc flaps (or the cubicle!). The calculations therefore need to take account of this, as well as the fault level, the type of fault, the likely arc length (gap between busbars usually), the arc flap area and opening pressure, the clearance time etc. etc..

If you are designing equipment that could potentially suffer a high fault level arc, you need to ensure the enclosure is safe. Rules of thumb don't really do it. The best way is to conduct a real arc fault containment test combined with calculation such as Theta offer.

(Theta are a supplier to us - I have no affiliation to them by the way!).


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