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 Post subject: Current limiting fuses
PostPosted: Thu Apr 13, 2017 6:50 am 
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Joined: Mon Jul 26, 2010 10:55 am
Posts: 56
We have a situation where we have old gear, old fuses and about 37k of available S.S.C.. at and MDP. In the MDP I have a 200a fused switch that feeds a small power panel nearby. Our recent survey tells me with the current fuse I have about 23k of available S.S.C at this panel. To help lessen the S.S.C. at the panel I have asked why we can't use the current limitation aspects of something like and LPN-RK200 fuse. That suggestion led off into a series rating conversation that didn't make sense. "I'm not saying they were wrong. It just didn't make sense." What am I missing? What is the purpose of the current limitation curves on a fuse like this if it must be series rated?

Thanks in advance for making me less ignorant.


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 Post subject: Re: Current limiting fuses
PostPosted: Thu Apr 13, 2017 7:10 am 
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Location: Wisconsin
100questions wrote:
What is the purpose of the current limitation curves on a fuse like this if it must be series rated?


For the most part these curves are now little more than marketing fluff. At one time they were important, but over the past several decades our industry has learned a lot about device interaction and the need for actual testing of devices in series ratings.
Molded case circuits breaker (since roughly the 60's) and definitely since the mid 80's have the ability to react and begin opening in just a few milliseconds. Search for information on Dynamic Impedance if you are interested.


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 Post subject: Re: Current limiting fuses
PostPosted: Mon Apr 24, 2017 10:51 am 
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Location: Maple Valley, WA.
The current limiting fuse curves and the "Up over and Down" Method can be used for static devices such as bus bar bracings on Switchgear and Bus Ducts. As mentioned in the previous post, this method can not be applied to circuit breakers because modern circuit breakers operate too fast. The breakers are interrupting at the same time the fuse is going into current limiting mode. The dynamic impedance makes it impossible to accurately calculate the reduce short circuit current. This is one of the reasons that manufactures pay to have the series tests done.

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Robert Fuhr, P.E.; P.Eng.
PowerStudies


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 Post subject: Re: Current limiting fuses
PostPosted: Mon Apr 24, 2017 11:32 am 
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Robertefuhr wrote:
The current limiting fuse curves and the "Up over and Down" Method can be used for static devices such as bus bar bracings on Switchgear and Bus Ducts.


These are two equipment designs which are now rarely, if ever, performed by consulting engineers, end use customers and electricians. The method is important for equipment manufacturers that want to do custom designs. But seriously, probably 9999 out of 10000 people involved with breaker selection will actually be able to use it, so why is so much time spent teaching it? Shouldn't we focus on teaching series ratings instead, after all Modern molded case circuit breakers have been around for 50 years.


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