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 Post subject: Protection equipment clearing times (t)
PostPosted: Wed Mar 27, 2013 6:58 am 
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Is it correct at all times to use the time/current characteristics for BS88 fuses of the upstream protector. For instance a MCC (switchpanel) with an incomming switched fuse of 400A feeding a mix of 30A,63A,100A final circuits will mean that the arc flash calculation for each final circuit is determined by the disconnection time of the 400A incoming fuse.
I can understand that using the upsteam fuse achieve a more conservative result but reading more deeply into IEEE1584 the suggestion is that determination should be made on which protective device is most likely to clear the fault.
Any advice.


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PostPosted: Fri Mar 29, 2013 1:44 pm 
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Not sure what a BS88 fuse is but the incident energy of an arc flash is determined by the protective trip device time at the arcing current. Therefore, one has to look at the arcing current on the protective device TCC. I doubt that a 30A circuit fuse or breaker would trip later than a 400A one.

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PostPosted: Tue Apr 02, 2013 11:57 pm 
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wbd wrote:
Not sure what a BS88 fuse is but the incident energy of an arc flash is determined by the protective trip device time at the arcing current. Therefore, one has to look at the arcing current on the protective device TCC. I doubt that a 30A circuit fuse or breaker would trip later than a 400A one.

To clarify a BS88 fuse is a HRC fuse. The question is about the suggestion in the "complete guide to arc flash hazard calculations studies" by Jim Phillips whereby he suggests that the clearance time t is determined by the first upstream protection device. In my case that would be a 400A HRC fuse. This has serious time implications for a 30A HRC fuse.


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PostPosted: Wed Apr 03, 2013 4:37 am 
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I have that book also. Could you cite the page number?
I would have to believe that what was meant is that the clearing time is determined by the first upstream protection device that will clear at that arcing fault current level based on the TCC. I find it very hard to be believe that if a 30A fuse is the first upstream device that it would not clear for a fault. If the arcing current is so small that the 30A does not blow, no way is a 400A going to blow.

Of course if this 30A is in a panel that is being analyzed because energized work takes place in it and the construction is such that credit can not be taken for it, with the next upstream device a 400A fuse, I could see it could possibly take a long time to blow.

Are you using a 2 second cutoff time?

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PostPosted: Thu Apr 04, 2013 5:32 am 
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wbd wrote:
I have that book also. Could you cite the page number?
I would have to believe that what was meant is that the clearing time is determined by the first upstream protection device that will clear at that arcing fault current level based on the TCC. I find it very hard to be believe that if a 30A fuse is the first upstream device that it would not clear for a fault. If the arcing current is so small that the 30A does not blow, no way is a 400A going to blow.

Of course if this 30A is in a panel that is being analyzed because energized work takes place in it and the construction is such that credit can not be taken for it, with the next upstream device a 400A fuse, I could see it could possibly take a long time to blow.

Are you using a 2 second cutoff time?

Try page 113 and figure 9-1


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PostPosted: Thu Apr 04, 2013 6:35 am 
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Thank you. Now that I have read the section, yes the 400A will be the device to use for the IE for the arc flash. My understanding is the smaller fuses you cite are either in fuse disconnects or inside a control panel type setup. If the fuse disconnect is operated and fails for whatever reason and there is an ensuing arc flash, the device clearing that will be the 400A as it is considered the arc flash is on the line side of the 30A fuses.

Are you using a 2 second cutoff time?

What voltage is this at?

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PostPosted: Mon Apr 29, 2013 7:50 am 
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Looks like an old message but thee are some caveats here.

This is all assuming that you don't have miscoordination. If an upstream device will trip before a downstream device, then obviously the upstream device may take precedence.

Second, you can get into a condition where you get two different trips. For instance when you have breakers that are protected by fuses, in some cases the melting electrochemical reaction is already in progress or the breaker's magnetic motion is already in progress but then the other device trips first, and sometimes this is all based on random chance so that either device may trip first. Again, miscoordination is the culprit.

Third, you can end up with "dynamic resistance". This allows for instance instantaneous trips on breakers to be coordinated in spite of the fact that the time coordination curves suggest that there is miscoordination.

Fourth, there is zone protection, partial bus differential protection, arc flash relay sensors, and other things that really make the "simple" concept of a coordination curve difficult.

And finally, the arcing fault / bolted fault current calculations are based on empirical data on arcing faults. Arcing current may be greater or less than predicted, though it won't exceed the bolted fault number. There are several methodologies (using various assumptions) for bolted fault calculations as well so again, there is no "one true" way to do the calculation. The result is that the arcing current may be significantly less or more than predicted by the IEEE 1584 calculation, which can result in incident energy values significantly different from predicted. As an example, the assymetrical fault current which is entirely dependent on phase angle and X/R ratio can make the arcing current vary by as much as 2:1 ratio. The standard gives a caveat that numerically, wearing arc flash PPE meeting ASTM 1959 testing requirements will result in adequate or more than adequate protection 90%+ of the time. Note that ASTM 1959 itself has some issues. Data from several tests is plotted on a curve and then the rated value (ATPV or E_BT) is calculated based on applying a sigmoidal shaped curve to binary (pass/fail) data. The sigmoidal curve is not really the best fit and it is clear from the data that even though it predicts failures of say 10% at around 0.5 cal/cm^2 lower than the rated value, most data shows that at this point all tests pass. Further the test is based on a sample material mounted at a 90 degree angle to the arc flash. In the real world, people are not flat polygons and thus the incident angle comes into play. Very little surface would actually reach this maximum value.

So if this last paragraph sounds terrifying because of the number of assumptions and possible ways that they might be invalid, look at it this way. In almost 15 years now of actual real world testing, no arc flashes have been reported where properly used PPE has "failed" using the IEEE 1584 standard for calculating the required PPE. If this were to happen it would be pretty widely reported.


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