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 Post subject: Trip & Opening Times
PostPosted: Mon Mar 15, 2010 7:37 am 

Joined: Mon Mar 15, 2010 7:00 am
Posts: 3
I noticed that the trip and opening times of the spreadsheet calculator has default values of 0.18 and 0.08, respectively. There are some engineers that seem to think this is somehow an accepted value for most conditions when you don't have the device and trip curves. It seems pretty clear to me that device curves are pretty much required unless you can utilize the Table 5 Equations that only require bolted fault current, I_bf. But, what I am experiencing is that many times I_bf is not in the instantanious trip band so the equations are not usable and the device must be known in order to look up the trip curves.

Bottom line is that I would like to get some feedback on these trip and opening time values that appear to be assumed and were left in the calculator for some reason.

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PostPosted: Mon Mar 15, 2010 3:24 pm 
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Joined: Mon Sep 17, 2007 5:00 pm
Posts: 1664
Location: Scottsdale, Arizona
Welcome to the forum!

Your concerns are well founded. You really need the devices' time current curves to provide a more accurate representation of the device clearing time. Even the bolted short circuit current can be misleading.

Using the bolted sc current with the TCC could give the impression that a device trips instantaneously when in fact it might not. The arc current calculation is important and even then, IEEE 1584 suggests also evaluating the clearing time at 85% of the arcing current as well just in case it falls below the instantaneous.

0.18 and 0.08 seem a bit odd. 0.0167 is one electrical cycle and is the shortest duration unless a current limiting device is used. Everything else should be a multplie of 0.0167 seconds.

I hope this helps a little bit. Please let me know if you need anything else.

Jim Phillips, P.E.

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