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 Post subject: CL Fuse Equations (Flash Protection Boundary)
PostPosted: Wed Apr 23, 2008 6:20 pm 
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The IEEE 1584 supplies us with Current Limiting Fuse Equations for determining Incident Energy. However, it does not supply us with information on how to determine the Flash Protection Boundary for these fuses.

The IEEE 1584 Excel Spreadsheet calculator, on the other hand does, and it does not seem to use the same methodology given in Equation (8) of Section 5.5 (the basic method for determining the Flash Protection Boundary).

I guess my question is: (1) what is the basis for the equations in the calculator? and (2) should we use these equations in the calculator if they are not backed up by any documentation?

:confused:

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PostPosted: Thu Apr 24, 2008 9:41 am 
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It looks to me that the same boundary equation if used for CL and for non-CL. This seems ok to me, because it would result in the same boundary for the same energy level, regardless of whether CL fuses are used or not.

Where I have issue, is that the same energy level results in different boundaries depending on voltage level. Switchgear type, I can understand, a little; you may have some directional component in the buckets, although I don't think this was the original purpose. I don't see how voltage level should affect the boundary for the same energy level, however.


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PostPosted: Fri Apr 25, 2008 7:54 am 
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Hi Michelle and WD
I believe the 1.5 calculation factor for the FPB was a "fudge factor" that was included because of how they test data worked out. I heard that statement a while ago during a 1584 meeting. I'm not sure of the details but perhaps it is due to the properties of the HV arc having more power.

The fuse issue bothers me although the original tests for 1584 were done with a very limited budget so I understand why the did it this way. Here is my concern. Earlier in my career I was involved with the UL tested series ratings being developed using current limiting fuses. This is where tests are performed to determine which upstream current limiting fuse can protect an under rated downstream circuit breaker. Since the actual let thru characteristics might vary from one fuse manufacturer to another, UL requires the use of "umbrella value" fuses. These are special fuses that have the highest possible let thru current and I2T energy that the UL standards allow for a particular fuse. All fuses have lower values than the umbrella values but this was a way to make sure all the bases are covered.

The other thing UL required was to test using RK5 fuses not RK1 because RK5 has a higher let thru energy (worst case) and there is no way to prevent someone from substituting RK5 for RK1. The IEEE 1584 tests were with RK1.

All of this could have a bearing on the incident energy and FPB so I am hopeful that the new round of testing will lean more towards what UL reqiures for the series tests so we keep it conservative.

Just stirring it up and giving more things to think about!

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PostPosted: Sun Apr 27, 2008 4:39 pm 
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still need help.

Which equation in the 1584 then are you referring to (for regular fuses). I do not see one, but maybe I am overlooking something. The general equation for FPB requires a "time" to be entered, therefore, it cannot be used with the CL fuse equations. Unless you use the TCC just for determining the FPB, which is not what the calculator does.

Confused!
Thanks!

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PostPosted: Mon Apr 28, 2008 5:33 am 
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Michelle,
I see it now. Could the (t/0.2) factor be a result of "normalizing" the equation (which I don't understand.)? As such, the time may be able to be removed from the distance equation, or accounted for in other way? I don't see it documented either, though.

Jim,
I understand the HV arc has more power, and the 1.5 in the energy equation. This often results in a higher IE for the same conditions. The problem I have, is that when you solve for the boundary, this results in a different boundary level than a LV arc with the same energy level at the same working distance. Are they saying that there is something inherent in the voltage level that causes one type of arc flash to dissipate quicker?


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PostPosted: Mon Apr 28, 2008 1:39 pm 
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Hi Gang,
Congratulations! You have uncovered more holes in the standards. Quit Digging! :D

First the fuses. As you probably have seen there are several equations for each fuse. If the fault current is low, you use the curves. As the fault current goes into the "current limiting" region of the fuse i.e. higher current drives the curve below the 0.01 second horizontal axis, you use various equations that give you the incident energy. Now the problem as everyone sees is what time do you use for the FPB when you are using the incident energy equations that don't require time? From what I understand, this was never determined. :eek: What some have been doing is using 0.01 Seconds for the time which is the bottom of the TCC. The CL fuse actually should clear between 1/4 and 1/2 cycle when it is current limiting so 0.01 is conservative. The more I dig into the original testing the more I find this standard was developed around minimal tests. Even though there are areas where it is lacking (and you are finding many of those areas) it is still the best that we have until the next wave of tests occur.

Now on to the 1.5 factor. From what I understand this is another area that is a hiccup. The 1.5 was included for incident energy (fudge factor) depending on voltage level as I mentioned but it does seem a little inconsistant as you point out. This was another area that as I understand was not resolved or thought about. Not much of answer but stay tuned!

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